Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

From: Marco Elver
Date: Mon Oct 21 2019 - 11:54:51 EST

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On Mon, 21 Oct 2019 at 15:37, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> On Thu, Oct 17, 2019 at 4:13 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
> > kernel space. KCSAN is a sampling watchpoint-based data-race detector.
> > See the included Documentation/dev-tools/kcsan.rst for more details.
> >
> > This patch adds basic infrastructure, but does not yet enable KCSAN for
> > any architecture.
> >
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > ---
> > v2:
> > * Elaborate comment about instrumentation calls emitted by compilers.
> > * Replace kcsan_check_access(.., {true, false}) with
> > kcsan_check_{read,write} for improved readability.
> > * Change bug title of race of unknown origin to just say "data-race in".
> > * Refine "Key Properties" in kcsan.rst, and mention observed slow-down.
> > * Add comment about safety of find_watchpoint without user_access_save.
> > * Remove unnecessary preempt_disable/enable and elaborate on comment why
> > we want to disable interrupts and preemptions.
> > * Use common struct kcsan_ctx in task_struct and for per-CPU interrupt
> > contexts [Suggested by Mark Rutland].
> > ---
> > Documentation/dev-tools/kcsan.rst | 203 ++++++++++++++
> > MAINTAINERS | 11 +
> > Makefile | 3 +-
> > include/linux/compiler-clang.h | 9 +
> > include/linux/compiler-gcc.h | 7 +
> > include/linux/compiler.h | 35 ++-
> > include/linux/kcsan-checks.h | 147 ++++++++++
> > include/linux/kcsan.h | 108 ++++++++
> > include/linux/sched.h | 4 +
> > init/init_task.c | 8 +
> > init/main.c | 2 +
> > kernel/Makefile | 1 +
> > kernel/kcsan/Makefile | 14 +
> > kernel/kcsan/atomic.c | 21 ++
> > kernel/kcsan/core.c | 428 ++++++++++++++++++++++++++++++
> > kernel/kcsan/debugfs.c | 225 ++++++++++++++++
> > kernel/kcsan/encoding.h | 94 +++++++
> > kernel/kcsan/kcsan.c | 86 ++++++
> > kernel/kcsan/kcsan.h | 140 ++++++++++
> > kernel/kcsan/report.c | 306 +++++++++++++++++++++
> > kernel/kcsan/test.c | 117 ++++++++
> > lib/Kconfig.debug | 2 +
> > lib/Kconfig.kcsan | 88 ++++++
> > lib/Makefile | 3 +
> > scripts/Makefile.kcsan | 6 +
> > scripts/Makefile.lib | 10 +
> > 26 files changed, 2069 insertions(+), 9 deletions(-)
> > create mode 100644 Documentation/dev-tools/kcsan.rst
> > create mode 100644 include/linux/kcsan-checks.h
> > create mode 100644 include/linux/kcsan.h
> > create mode 100644 kernel/kcsan/Makefile
> > create mode 100644 kernel/kcsan/atomic.c
> > create mode 100644 kernel/kcsan/core.c
> > create mode 100644 kernel/kcsan/debugfs.c
> > create mode 100644 kernel/kcsan/encoding.h
> > create mode 100644 kernel/kcsan/kcsan.c
> > create mode 100644 kernel/kcsan/kcsan.h
> > create mode 100644 kernel/kcsan/report.c
> > create mode 100644 kernel/kcsan/test.c
> > create mode 100644 lib/Kconfig.kcsan
> > create mode 100644 scripts/Makefile.kcsan
> >
> > diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
> > new file mode 100644
> > index 000000000000..497b09e5cc96
> > --- /dev/null
> > +++ b/Documentation/dev-tools/kcsan.rst
> > @@ -0,0 +1,203 @@
> > +The Kernel Concurrency Sanitizer (KCSAN)
> > +========================================
> > +
> > +Overview
> > +--------
> > +
> > +*Kernel Concurrency Sanitizer (KCSAN)* is a dynamic data-race detector for
> > +kernel space. KCSAN is a sampling watchpoint-based data-race detector -- this
> > +is unlike Kernel Thread Sanitizer (KTSAN), which is a happens-before data-race
> > +detector. Key priorities in KCSAN's design are lack of false positives,
> > +scalability, and simplicity. More details can be found in `Implementation
> > +Details`_.
> > +
> > +KCSAN uses compile-time instrumentation to instrument memory accesses. KCSAN is
> > +supported in both GCC and Clang. With GCC it requires version 7.3.0 or later.
> > +With Clang it requires version 7.0.0 or later.
> > +
> > +Usage
> > +-----
> > +
> > +To enable KCSAN configure kernel with::
> > +
> > + CONFIG_KCSAN = y
> > +
> > +KCSAN provides several other configuration options to customize behaviour (see
> > +their respective help text for more info).
> > +
> > +debugfs
> > +~~~~~~~
> > +
> > +* The file ``/sys/kernel/debug/kcsan`` can be read to get stats.
> > +
> > +* KCSAN can be turned on or off by writing ``on`` or ``off`` to
> > + ``/sys/kernel/debug/kcsan``.
> > +
> > +* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
> > + ``some_func_name`` to the report filter list, which (by default) blacklists
> > + reporting data-races where either one of the top stackframes are a function
> > + in the list.
> > +
> > +* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
> > + changes the report filtering behaviour. For example, the blacklist feature
> > + can be used to silence frequently occurring data-races; the whitelist feature
> > + can help with reproduction and testing of fixes.
> > +
> > +Error reports
> > +~~~~~~~~~~~~~
> > +
> > +A typical data-race report looks like this::
> > +
> > + ==================================================================
> > + BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
> > +
> > + write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
> > + kernfs_refresh_inode+0x70/0x170
> > + kernfs_iop_permission+0x4f/0x90
> > + inode_permission+0x190/0x200
> > + link_path_walk.part.0+0x503/0x8e0
> > + path_lookupat.isra.0+0x69/0x4d0
> > + filename_lookup+0x136/0x280
> > + user_path_at_empty+0x47/0x60
> > + vfs_statx+0x9b/0x130
> > + __do_sys_newlstat+0x50/0xb0
> > + __x64_sys_newlstat+0x37/0x50
> > + do_syscall_64+0x85/0x260
> > + entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > +
> > + read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
> > + generic_permission+0x5b/0x2a0
> > + kernfs_iop_permission+0x66/0x90
> > + inode_permission+0x190/0x200
> > + link_path_walk.part.0+0x503/0x8e0
> > + path_lookupat.isra.0+0x69/0x4d0
> > + filename_lookup+0x136/0x280
> > + user_path_at_empty+0x47/0x60
> > + do_faccessat+0x11a/0x390
> > + __x64_sys_access+0x3c/0x50
> > + do_syscall_64+0x85/0x260
> > + entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > +
> > + Reported by Kernel Concurrency Sanitizer on:
> > + CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
> > + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > + ==================================================================
> > +
> > +The header of the report provides a short summary of the functions involved in
> > +the race. It is followed by the access types and stack traces of the 2 threads
> > +involved in the data-race.
> > +
> > +The other less common type of data-race report looks like this::
> > +
> > + ==================================================================
> > + BUG: KCSAN: data-race in e1000_clean_rx_irq+0x551/0xb10
> > +
> > + race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
> > + e1000_clean_rx_irq+0x551/0xb10
> > + e1000_clean+0x533/0xda0
> > + net_rx_action+0x329/0x900
> > + __do_softirq+0xdb/0x2db
> > + irq_exit+0x9b/0xa0
> > + do_IRQ+0x9c/0xf0
> > + ret_from_intr+0x0/0x18
> > + default_idle+0x3f/0x220
> > + arch_cpu_idle+0x21/0x30
> > + do_idle+0x1df/0x230
> > + cpu_startup_entry+0x14/0x20
> > + rest_init+0xc5/0xcb
> > + arch_call_rest_init+0x13/0x2b
> > + start_kernel+0x6db/0x700
> > +
> > + Reported by Kernel Concurrency Sanitizer on:
> > + CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
> > + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > + ==================================================================
> > +
> > +This report is generated where it was not possible to determine the other
> > +racing thread, but a race was inferred due to the data-value of the watched
> > +memory location having changed. These can occur either due to missing
> > +instrumentation or e.g. DMA accesses.
> > +
> > +Data-Races
> > +----------
> Nit: I was under the impression "data races" were commonly written
> without a hyphen. I may be mistaken.

Thanks. I've updated it everywhere except in bug titles, which should
remain as-is.

> > +
> > +Informally, two operations *conflict* if they access the same memory location,
> > +and at least one of them is a write operation. In an execution, two memory
> > +operations from different threads form a **data-race** if they *conflict*, at
> > +least one of them is a *plain access* (non-atomic), and they are *unordered* in
> > +the "happens-before" order according to the `LKMM
> > +<../../tools/memory-model/Documentation/explanation.txt>`_.
> > +
> > +Relationship with the Linux Kernel Memory Model (LKMM)
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +The LKMM defines the propagation and ordering rules of various memory
> > +operations, which gives developers the ability to reason about concurrent code.
> > +Ultimately this allows to determine the possible executions of concurrent code,
> > +and if that code is free from data-races.
> > +
> > +KCSAN is aware of *atomic* accesses (``READ_ONCE``, ``WRITE_ONCE``,
> > +``atomic_*``, etc.), but is oblivious of any ordering guarantees. In other
> > +words, KCSAN assumes that as long as a plain access is not observed to race
> > +with another conflicting access, memory operations are correctly ordered.
> > +
> > +This means that KCSAN will not report *potential* data-races due to missing
> > +memory ordering. If, however, missing memory ordering (that is observable with
> > +a particular compiler and architecture) leads to an observable data-race (e.g.
> > +entering a critical section erroneously), KCSAN would report the resulting
> > +data-race.
> > +
> > +Implementation Details
> > +----------------------
> > +
> > +The general approach is inspired by `DataCollider
> > +<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
> > +Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
> > +relies on compiler instrumentation. Watchpoints are implemented using an
> > +efficient encoding that stores access type, size, and address in a long; the
> > +benefits of using "soft watchpoints" are portability and greater flexibility in
> > +limiting which accesses trigger a watchpoint.
> > +
> > +More specifically, KCSAN requires instrumenting plain (unmarked, non-atomic)
> > +memory operations; for each instrumented plain access:
> > +
> > +1. Check if a matching watchpoint exists; if yes, and at least one access is a
> > + write, then we encountered a racing access.
> > +
> > +2. Periodically, if no matching watchpoint exists, set up a watchpoint and
> > + stall some delay.
> > +
> > +3. Also check the data value before the delay, and re-check the data value
> > + after delay; if the values mismatch, we infer a race of unknown origin.
> > +
> > +To detect data-races between plain and atomic memory operations, KCSAN also
> > +annotates atomic accesses, but only to check if a watchpoint exists
> > +(``kcsan_check_atomic_*``); i.e. KCSAN never sets up a watchpoint on atomic
> > +accesses.
> > +
> > +Key Properties
> > +~~~~~~~~~~~~~~
> > +
> > +1. **Memory Overhead:** No shadow memory is required. The current
> > + implementation uses a small array of longs to encode watchpoint information,
> > + which is negligible.
> > +
> > +2. **Performance Overhead:** KCSAN's runtime aims to be minimal, using an
> > + efficient watchpoint encoding that does not require acquiring any shared
> > + locks in the fast-path. For kernel boot with a default config on a system
> > + where nproc=8 we measure a slow-down of 10-15x.
> > +
> > +3. **Memory Ordering:** KCSAN is *not* aware of the LKMM's ordering rules. This
> > + may result in missed data-races (false negatives), compared to a
> > + happens-before data-race detector.
> > +
> > +4. **Accuracy:** Imprecise, since it uses a sampling strategy.
> > +
> > +5. **Annotation Overheads:** Minimal annotation is required outside the KCSAN
> > + runtime. With a happens-before data-race detector, any omission leads to
> > + false positives, which is especially important in the context of the kernel
> > + which includes numerous custom synchronization mechanisms. With KCSAN, as a
> > + result, maintenance overheads are minimal as the kernel evolves.
> > +
> > +6. **Detects Racy Writes from Devices:** Due to checking data values upon
> > + setting up watchpoints, racy writes from devices can also be detected.
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 0154674cbad3..71f7fb625490 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -8847,6 +8847,17 @@ F: Documentation/kbuild/kconfig*
> > F: scripts/kconfig/
> > F: scripts/Kconfig.include
> >
> > +KCSAN
> > +M: Marco Elver <elver@xxxxxxxxxx>
> > +R: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > +L: kasan-dev@xxxxxxxxxxxxxxxx
> > +S: Maintained
> > +F: Documentation/dev-tools/kcsan.rst
> > +F: include/linux/kcsan*.h
> > +F: kernel/kcsan/
> > +F: lib/Kconfig.kcsan
> > +F: scripts/Makefile.kcsan
> > +
> > KDUMP
> > M: Dave Young <dyoung@xxxxxxxxxx>
> > M: Baoquan He <bhe@xxxxxxxxxx>
> > diff --git a/Makefile b/Makefile
> > index ffd7a912fc46..ad4729176252 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -478,7 +478,7 @@ export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
> >
> > export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
> > export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
> > -export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN
> > +export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN CFLAGS_KCSAN
> > export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
> > export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
> > export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
> > @@ -900,6 +900,7 @@ endif
> > include scripts/Makefile.kasan
> > include scripts/Makefile.extrawarn
> > include scripts/Makefile.ubsan
> > +include scripts/Makefile.kcsan
> >
> > # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
> > KBUILD_CPPFLAGS += $(KCPPFLAGS)
> > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> > index 333a6695a918..a213eb55e725 100644
> > --- a/include/linux/compiler-clang.h
> > +++ b/include/linux/compiler-clang.h
> > @@ -24,6 +24,15 @@
> > #define __no_sanitize_address
> > #endif
> >
> > +#if __has_feature(thread_sanitizer)
> > +/* emulate gcc's __SANITIZE_THREAD__ flag */
> > +#define __SANITIZE_THREAD__
> > +#define __no_sanitize_thread \
> > + __attribute__((no_sanitize("thread")))
> > +#else
> > +#define __no_sanitize_thread
> > +#endif
> > +
> > /*
> > * Not all versions of clang implement the the type-generic versions
> > * of the builtin overflow checkers. Fortunately, clang implements
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index d7ee4c6bad48..de105ca29282 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -145,6 +145,13 @@
> > #define __no_sanitize_address
> > #endif
> >
> > +#if __has_attribute(__no_sanitize_thread__) && defined(__SANITIZE_THREAD__)
> > +#define __no_sanitize_thread \
> > + __attribute__((__noinline__)) __attribute__((no_sanitize_thread))
> > +#else
> > +#define __no_sanitize_thread
> > +#endif
> > +
> > #if GCC_VERSION >= 50100
> > #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
> > #endif
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 5e88e7e33abe..350d80dbee4d 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -178,6 +178,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> > #endif
> >
> > #include <uapi/linux/types.h>
> > +#include <linux/kcsan-checks.h>
> >
> > #define __READ_ONCE_SIZE \
> > ({ \
> > @@ -193,12 +194,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> > } \
> > })
> >
> > -static __always_inline
> > -void __read_once_size(const volatile void *p, void *res, int size)
> > -{
> > - __READ_ONCE_SIZE;
> > -}
> > -
> > #ifdef CONFIG_KASAN
> > /*
> > * We can't declare function 'inline' because __no_sanitize_address confilcts
> > @@ -211,14 +206,38 @@ void __read_once_size(const volatile void *p, void *res, int size)
> > # define __no_kasan_or_inline __always_inline
> > #endif
> >
> > -static __no_kasan_or_inline
> > +#ifdef CONFIG_KCSAN
> > +# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
> > +#else
> > +# define __no_kcsan_or_inline __always_inline
> > +#endif
> > +
> > +#if defined(CONFIG_KASAN) || defined(CONFIG_KCSAN)
> > +/* Avoid any instrumentation or inline. */
> > +#define __no_sanitize_or_inline \
> > + __no_sanitize_address __no_sanitize_thread notrace __maybe_unused
> > +#else
> > +#define __no_sanitize_or_inline __always_inline
> > +#endif
> > +
> > +static __no_kcsan_or_inline
> > +void __read_once_size(const volatile void *p, void *res, int size)
> > +{
> > + kcsan_check_atomic_read((const void *)p, size);
> > + __READ_ONCE_SIZE;
> > +}
> > +
> > +static __no_sanitize_or_inline
> > void __read_once_size_nocheck(const volatile void *p, void *res, int size)
> > {
> > __READ_ONCE_SIZE;
> > }
> >
> > -static __always_inline void __write_once_size(volatile void *p, void *res, int size)
> > +static __no_kcsan_or_inline
> > +void __write_once_size(volatile void *p, void *res, int size)
> > {
> > + kcsan_check_atomic_write((const void *)p, size);
> > +
> > switch (size) {
> > case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
> > case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
> > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > new file mode 100644
> > index 000000000000..4203603ae852
> > --- /dev/null
> > +++ b/include/linux/kcsan-checks.h
> > @@ -0,0 +1,147 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _LINUX_KCSAN_CHECKS_H
> > +#define _LINUX_KCSAN_CHECKS_H
> > +
> > +#include <linux/types.h>
> > +
> > +/*
> > + * __kcsan_*: Always available when KCSAN is enabled. This may be used
> > + * even in compilation units that selectively disable KCSAN, but must use KCSAN
> > + * to validate access to an address. Never use these in header files!
> > + */
> > +#ifdef CONFIG_KCSAN
> > +/**
> > + * __kcsan_check_watchpoint - check if a watchpoint exists
> > + *
> > + * Returns true if no race was detected, and we may then proceed to set up a
> > + * watchpoint after. Returns false if either KCSAN is disabled or a race was
> > + * encountered, and we may not set up a watchpoint after.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + * @is_write is access a write
> > + * @return true if no race was detected, false otherwise.
> > + */
> > +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write);
> I think the parameter indentations are a bit off here and below (I've
> also looked at the Github diff);
> have you considered running checkpatch.pl?

It was formatted with clang-format, it's correct with 8 space tabs.
checkpath.pl is happy.

> > +
> > +/**
> > + * __kcsan_setup_watchpoint - set up watchpoint and report data-races
> > + *
> > + * Sets up a watchpoint (if sampled), and if a racing access was observed,
> > + * reports the data-race.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + * @is_write is access a write
> > + */
> > +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write);
> > +#else
> > +static inline bool __kcsan_check_watchpoint(const volatile void *ptr,
> > + size_t size, bool is_write)
> > +{
> > + return true;
> > +}
> > +static inline void __kcsan_setup_watchpoint(const volatile void *ptr,
> > + size_t size, bool is_write)
> > +{
> > +}
> > +#endif
> > +
> > +/*
> > + * kcsan_*: Only available when the particular compilation unit has KCSAN
> > + * instrumentation enabled. May be used in header files.
> > + */
> > +#ifdef __SANITIZE_THREAD__
> > +#define kcsan_check_watchpoint __kcsan_check_watchpoint
> > +#define kcsan_setup_watchpoint __kcsan_setup_watchpoint
> > +#else
> > +static inline bool kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write)
> > +{
> > + return true;
> > +}
> > +static inline void kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write)
> > +{
> > +}
> > +#endif
> > +
> > +/**
> > + * __kcsan_check_read - check regular read access for data-races
> > + *
> > + * Full read access that checks watchpoint and sets up a watchpoint if this
> > + * access is sampled. Note that, setting up watchpoints for plain reads is
> > + * required to also detect data-races with atomic accesses.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +#define __kcsan_check_read(ptr, size) \
> > + do { \
> > + if (__kcsan_check_watchpoint(ptr, size, false)) \
> > + __kcsan_setup_watchpoint(ptr, size, false); \
> > + } while (0)
> > +
> > +/**
> > + * __kcsan_check_write - check regular write access for data-races
> > + *
> > + * Full write access that checks watchpoint and sets up a watchpoint if this
> > + * access is sampled.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +#define __kcsan_check_write(ptr, size) \
> > + do { \
> > + if (__kcsan_check_watchpoint(ptr, size, true) && \
> > + !IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE)) \
> > + __kcsan_setup_watchpoint(ptr, size, true); \
> > + } while (0)
> > +
> > +/**
> > + * kcsan_check_read - check regular read access for data-races
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +#define kcsan_check_read(ptr, size) \
> > + do { \
> > + if (kcsan_check_watchpoint(ptr, size, false)) \
> > + kcsan_setup_watchpoint(ptr, size, false); \
> > + } while (0)
> > +
> > +/**
> > + * kcsan_check_write - check regular write access for data-races
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +#define kcsan_check_write(ptr, size) \
> > + do { \
> > + if (kcsan_check_watchpoint(ptr, size, true) && \
> > + !IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE)) \
> > + kcsan_setup_watchpoint(ptr, size, true); \
> > + } while (0)
> > +
> > +/*
> > + * Check for atomic accesses: if atomic access are not ignored, this simply
> > + * aliases to kcsan_check_watchpoint, otherwise becomes a no-op.
> > + */
> > +#ifdef CONFIG_KCSAN_IGNORE_ATOMICS
> > +#define kcsan_check_atomic_read(...) \
> > + do { \
> > + } while (0)
> > +#define kcsan_check_atomic_write(...) \
> > + do { \
> > + } while (0)
> > +#else
> > +#define kcsan_check_atomic_read(ptr, size) \
> > + kcsan_check_watchpoint(ptr, size, false)
> > +#define kcsan_check_atomic_write(ptr, size) \
> > + kcsan_check_watchpoint(ptr, size, true)
> > +#endif
> > +
> > +#endif /* _LINUX_KCSAN_CHECKS_H */
> > diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
> > new file mode 100644
> > index 000000000000..fd5de2ba3a16
> > --- /dev/null
> > +++ b/include/linux/kcsan.h
> > @@ -0,0 +1,108 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _LINUX_KCSAN_H
> > +#define _LINUX_KCSAN_H
> > +
> > +#include <linux/types.h>
> > +#include <linux/kcsan-checks.h>
> > +
> > +#ifdef CONFIG_KCSAN
> > +
> > +/*
> > + * Context for each thread of execution: for tasks, this is stored in
> > + * task_struct, and interrupts access internal per-CPU storage.
> > + */
> > +struct kcsan_ctx {
> > + int disable; /* disable counter */
> > + int atomic_next; /* number of following atomic ops */
> > +
> > + /*
> > + * We use separate variables to store if we are in a nestable or flat
> > + * atomic region. This helps make sure that an atomic region with
> > + * nesting support is not suddenly aborted when a flat region is
> > + * contained within. Effectively this allows supporting nesting flat
> > + * atomic regions within an outer nestable atomic region. Support for
> > + * this is required as there are cases where a seqlock reader critical
> > + * section (flat atomic region) is contained within a seqlock writer
> > + * critical section (nestable atomic region), and the "mismatching
> > + * kcsan_end_atomic()" warning would trigger otherwise.
> > + */
> > + int atomic_region;
> > + bool atomic_region_flat;
> > +};
> > +
> > +/**
> > + * kcsan_init - initialize KCSAN runtime
> > + */
> > +void kcsan_init(void);
> > +
> > +/**
> > + * kcsan_disable_current - disable KCSAN for the current context
> > + *
> > + * Supports nesting.
> > + */
> > +void kcsan_disable_current(void);
> > +
> > +/**
> > + * kcsan_enable_current - re-enable KCSAN for the current context
> > + *
> > + * Supports nesting.
> > + */
> > +void kcsan_enable_current(void);
> > +
> > +/**
> > + * kcsan_begin_atomic - use to denote an atomic region
> > + *
> > + * Accesses within the atomic region may appear to race with other accesses but
> > + * should be considered atomic.
> > + *
> > + * @nest true if regions may be nested, or false for flat region
> > + */
> > +void kcsan_begin_atomic(bool nest);
> > +
> > +/**
> > + * kcsan_end_atomic - end atomic region
> > + *
> > + * @nest must match argument to kcsan_begin_atomic().
> > + */
> > +void kcsan_end_atomic(bool nest);
> > +
> > +/**
> > + * kcsan_atomic_next - consider following accesses as atomic
> > + *
> > + * Force treating the next n memory accesses for the current context as atomic
> > + * operations.
> > + *
> > + * @n number of following memory accesses to treat as atomic.
> > + */
> > +void kcsan_atomic_next(int n);
> > +
> > +#else /* CONFIG_KCSAN */
> > +
> > +static inline void kcsan_init(void)
> I think it should be ok to put {} on the same line with the function
> prototype here, see e.g. include/linux/kasan.h

Done @ v3.

> > +{
> > +}
> > +
> > +static inline void kcsan_disable_current(void)
> > +{
> > +}
> > +
> > +static inline void kcsan_enable_current(void)
> > +{
> > +}
> > +
> > +static inline void kcsan_begin_atomic(bool nest)
> > +{
> > +}
> > +
> > +static inline void kcsan_end_atomic(bool nest)
> > +{
> > +}
> > +
> > +static inline void kcsan_atomic_next(int n)
> > +{
> > +}
> > +
> > +#endif /* CONFIG_KCSAN */
> > +
> > +#endif /* _LINUX_KCSAN_H */
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 2c2e56bd8913..9490e417bf4a 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -31,6 +31,7 @@
> > #include <linux/task_io_accounting.h>
> > #include <linux/posix-timers.h>
> > #include <linux/rseq.h>
> > +#include <linux/kcsan.h>
> >
> > /* task_struct member predeclarations (sorted alphabetically): */
> > struct audit_context;
> > @@ -1171,6 +1172,9 @@ struct task_struct {
> > #ifdef CONFIG_KASAN
> > unsigned int kasan_depth;
> > #endif
> > +#ifdef CONFIG_KCSAN
> > + struct kcsan_ctx kcsan_ctx;
> > +#endif
> >
> > #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > /* Index of current stored address in ret_stack: */
> > diff --git a/init/init_task.c b/init/init_task.c
> > index 9e5cbe5eab7b..e229416c3314 100644
> > --- a/init/init_task.c
> > +++ b/init/init_task.c
> > @@ -161,6 +161,14 @@ struct task_struct init_task
> > #ifdef CONFIG_KASAN
> > .kasan_depth = 1,
> > #endif
> > +#ifdef CONFIG_KCSAN
> > + .kcsan_ctx = {
> > + .disable = 1,
> > + .atomic_next = 0,
> > + .atomic_region = 0,
> > + .atomic_region_flat = 0,
> > + },
> > +#endif
> > #ifdef CONFIG_TRACE_IRQFLAGS
> > .softirqs_enabled = 1,
> > #endif
> > diff --git a/init/main.c b/init/main.c
> > index 91f6ebb30ef0..4d814de017ee 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -93,6 +93,7 @@
> > #include <linux/rodata_test.h>
> > #include <linux/jump_label.h>
> > #include <linux/mem_encrypt.h>
> > +#include <linux/kcsan.h>
> >
> > #include <asm/io.h>
> > #include <asm/bugs.h>
> > @@ -779,6 +780,7 @@ asmlinkage __visible void __init start_kernel(void)
> > acpi_subsystem_init();
> > arch_post_acpi_subsys_init();
> > sfi_init_late();
> > + kcsan_init();
> >
> > /* Do the rest non-__init'ed, we're now alive */
> > arch_call_rest_init();
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index daad787fb795..74ab46e2ebd1 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -102,6 +102,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
> > obj-$(CONFIG_IRQ_WORK) += irq_work.o
> > obj-$(CONFIG_CPU_PM) += cpu_pm.o
> > obj-$(CONFIG_BPF) += bpf/
> > +obj-$(CONFIG_KCSAN) += kcsan/
> >
> > obj-$(CONFIG_PERF_EVENTS) += events/
> >
> > diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> > new file mode 100644
> > index 000000000000..c25f07062d26
> > --- /dev/null
> > +++ b/kernel/kcsan/Makefile
> > @@ -0,0 +1,14 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +KCSAN_SANITIZE := n
> > +KCOV_INSTRUMENT := n
> > +
> > +CFLAGS_REMOVE_kcsan.o = $(CC_FLAGS_FTRACE)
> > +CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
> > +CFLAGS_REMOVE_atomic.o = $(CC_FLAGS_FTRACE)
> > +
> > +CFLAGS_kcsan.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> > +CFLAGS_core.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> > +CFLAGS_atomic.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> > +
> > +obj-y := kcsan.o core.o atomic.o debugfs.o report.o
> > +obj-$(CONFIG_KCSAN_SELFTEST) += test.o
> > diff --git a/kernel/kcsan/atomic.c b/kernel/kcsan/atomic.c
> > new file mode 100644
> > index 000000000000..dd44f7d9e491
> > --- /dev/null
> > +++ b/kernel/kcsan/atomic.c
> > @@ -0,0 +1,21 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/jiffies.h>
> > +
> > +#include "kcsan.h"
> > +
> > +/*
> > + * List all volatile globals that have been observed in races, to suppress
> > + * data-race reports between accesses to these variables.
> > + *
> > + * For now, we assume that volatile accesses of globals are as strong as atomic
> > + * accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
> > + * entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
> > + * than cast to volatile. Eventually, we hope to be able to remove this
> > + * function.
> > + */
> > +bool kcsan_is_atomic(const volatile void *ptr)
> > +{
> > + /* only jiffies for now */
> > + return ptr == &jiffies;
> > +}
> > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> > new file mode 100644
> > index 000000000000..bc8d60b129eb
> > --- /dev/null
> > +++ b/kernel/kcsan/core.c
> > @@ -0,0 +1,428 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/atomic.h>
> > +#include <linux/bug.h>
> > +#include <linux/delay.h>
> > +#include <linux/export.h>
> > +#include <linux/init.h>
> > +#include <linux/percpu.h>
> > +#include <linux/preempt.h>
> > +#include <linux/random.h>
> > +#include <linux/sched.h>
> > +#include <linux/uaccess.h>
> > +
> > +#include "kcsan.h"
> > +#include "encoding.h"
> > +
> > +/*
> > + * Helper macros to iterate slots, starting from address slot itself, followed
> > + * by the right and left slots.
> > + */
> > +#define CHECK_NUM_SLOTS (1 + 2 * KCSAN_CHECK_ADJACENT)
> > +#define SLOT_IDX(slot, i) \
> > + ((slot + (((i + KCSAN_CHECK_ADJACENT) % CHECK_NUM_SLOTS) - \
> > + KCSAN_CHECK_ADJACENT)) % \
> > + KCSAN_NUM_WATCHPOINTS)
> > +
> > +bool kcsan_enabled;
> > +
> > +/* Per-CPU kcsan_ctx for interrupts */
> > +static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
> > + .disable = 0,
> > + .atomic_next = 0,
> > + .atomic_region = 0,
> > + .atomic_region_flat = 0,
> > +};
> > +
> > +/*
> > + * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
> > + * able to safely update and access a watchpoint without introducing locking
> > + * overhead, we encode each watchpoint as a single atomic long. The initial
> > + * zero-initialized state matches INVALID_WATCHPOINT.
> > + */
> > +static atomic_long_t watchpoints[KCSAN_NUM_WATCHPOINTS];
> > +
> > +/*
> > + * Instructions skipped counter; see should_watch().
> > + */
> > +static DEFINE_PER_CPU(unsigned long, kcsan_skip);
> > +
> > +static inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size,
> > + bool expect_write,
> > + long *encoded_watchpoint)
> > +{
> > + const int slot = watchpoint_slot(addr);
> > + const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
> > + atomic_long_t *watchpoint;
> > + unsigned long wp_addr_masked;
> > + size_t wp_size;
> > + bool is_write;
> > + int i;
> > +
> > + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> > + watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> > + *encoded_watchpoint = atomic_long_read(watchpoint);
> > + if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
> > + &wp_size, &is_write))
> > + continue;
> > +
> > + if (expect_write && !is_write)
> > + continue;
> > +
> > + /* Check if the watchpoint matches the access. */
> > + if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
> > + return watchpoint;
> > + }
> > +
> > + return NULL;
> > +}
> > +
> > +static inline atomic_long_t *insert_watchpoint(unsigned long addr, size_t size,
> > + bool is_write)
> > +{
> > + const int slot = watchpoint_slot(addr);
> > + const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
> > + atomic_long_t *watchpoint;
> > + int i;
> > +
> > + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> > + long expect_val = INVALID_WATCHPOINT;
> > +
> > + /* Try to acquire this slot. */
> > + watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> > + if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val,
> > + encoded_watchpoint))
> > + return watchpoint;
> > + }
> > +
> > + return NULL;
> > +}
> > +
> > +/*
> > + * Return true if watchpoint was successfully consumed, false otherwise.
> > + *
> > + * This may return false if:
> > + *
> > + * 1. another thread already consumed the watchpoint;
> > + * 2. the thread that set up the watchpoint already removed it;
> > + * 3. the watchpoint was removed and then re-used.
> > + */
> > +static inline bool try_consume_watchpoint(atomic_long_t *watchpoint,
> > + long encoded_watchpoint)
> > +{
> > + return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint,
> > + CONSUMED_WATCHPOINT);
> > +}
> > +
> > +/*
> > + * Return true if watchpoint was not touched, false if consumed.
> > + */
> > +static inline bool remove_watchpoint(atomic_long_t *watchpoint)
> > +{
> > + return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) !=
> > + CONSUMED_WATCHPOINT;
> > +}
> > +
> > +static inline struct kcsan_ctx *get_ctx(void)
> > +{
> > + /*
> > + * In interrupt, use raw_cpu_ptr to avoid unnecessary checks, that would
> > + * also result in calls that generate warnings in uaccess regions.
> > + */
> > + return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
> > +}
> > +
> > +
> > +static inline bool is_atomic(const volatile void *ptr)
> > +{
> > + struct kcsan_ctx *ctx = get_ctx();
> > +
> > + if (unlikely(ctx->atomic_next > 0)) {
> > + --ctx->atomic_next;
> > + return true;
> > + }
> > + if (unlikely(ctx->atomic_region > 0 || ctx->atomic_region_flat))
> > + return true;
> Won't ctx->atomic_region suffice for both flat and non-flat regions?
> (Do we really need the flat ones?)

The comment in include/linux/kcsan.h explains:
/*
* We use separate variables to store if we are in a nestable or flat
* atomic region. This helps make sure that an atomic region with
* nesting support is not suddenly aborted when a flat region is
* contained within. Effectively this allows supporting nesting flat
* atomic regions within an outer nestable atomic region. Support for
* this is required as there are cases where a seqlock reader critical
* section (flat atomic region) is contained within a seqlock writer
* critical section (nestable atomic region), and the "mismatching
* kcsan_end_atomic()" warning would trigger otherwise.
*/

> > + return kcsan_is_atomic(ptr);
> > +}
> > +
> > +static inline bool should_watch(const volatile void *ptr)
> > +{
> > + /*
> > + * Never set up watchpoints when memory operations are atomic.
> > + *
> > + * We need to check this first, because: 1) atomics should not count
> > + * towards skipped instructions below, and 2) to actually decrement
> > + * kcsan_atomic_next for each atomic.
> > + */
> > + if (is_atomic(ptr))
> > + return false;
> > +
> > + /*
> > + * We use a per-CPU counter, to avoid excessive contention; there is
> > + * still enough non-determinism for the precise instructions that end up
> > + * being watched to be mostly unpredictable. Using a PRNG like
> > + * prandom_u32() turned out to be too slow.
> > + */
> > + return (this_cpu_inc_return(kcsan_skip) %
> > + CONFIG_KCSAN_WATCH_SKIP_INST) == 0;
> > +}
> > +
> > +static inline bool is_enabled(void)
> > +{
> > + return READ_ONCE(kcsan_enabled) && get_ctx()->disable == 0;
> > +}
> > +
> > +static inline unsigned int get_delay(void)
> > +{
> > + unsigned int max_delay = in_task() ? CONFIG_KCSAN_UDELAY_MAX_TASK :
> > + CONFIG_KCSAN_UDELAY_MAX_INTERRUPT;
> > + return IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
> > + ((prandom_u32() % max_delay) + 1) :
> > + max_delay;
> > +}
> > +
> > +/* === Public interface ===================================================== */
> > +
> > +void __init kcsan_init(void)
> > +{
> > + BUG_ON(!in_task());
> > +
> > + kcsan_debugfs_init();
> > + kcsan_enable_current();
> > +#ifdef CONFIG_KCSAN_EARLY_ENABLE
> > + /*
> > + * We are in the init task, and no other tasks should be running.
> > + */
> > + WRITE_ONCE(kcsan_enabled, true);
> > +#endif
> > +}
> > +
> > +/* === Exported interface =================================================== */
> > +
> > +void kcsan_disable_current(void)
> > +{
> > + ++get_ctx()->disable;
> > +}
> > +EXPORT_SYMBOL(kcsan_disable_current);
> > +
> > +void kcsan_enable_current(void)
> > +{
> > + if (get_ctx()->disable-- == 0) {
> > + kcsan_disable_current(); /* restore to 0 */
> > + kcsan_disable_current();
> > + WARN(1, "mismatching %s", __func__);
> > + kcsan_enable_current();
> > + }
> > +}
> > +EXPORT_SYMBOL(kcsan_enable_current);
> > +
> > +void kcsan_begin_atomic(bool nest)
> > +{
> > + if (nest)
> > + ++get_ctx()->atomic_region;
> > + else
> > + get_ctx()->atomic_region_flat = true;
> > +}
> > +EXPORT_SYMBOL(kcsan_begin_atomic);
> > +
> > +void kcsan_end_atomic(bool nest)
> > +{
> > + if (nest) {
> > + if (get_ctx()->atomic_region-- == 0) {
> > + kcsan_begin_atomic(true); /* restore to 0 */
> > + kcsan_disable_current();
> > + WARN(1, "mismatching %s", __func__);
> > + kcsan_enable_current();
> > + }
> > + } else {
> > + get_ctx()->atomic_region_flat = false;
> > + }
> > +}
> > +EXPORT_SYMBOL(kcsan_end_atomic);
> > +
> > +void kcsan_atomic_next(int n)
> > +{
> > + get_ctx()->atomic_next = n;
> > +}
> > +EXPORT_SYMBOL(kcsan_atomic_next);
> > +
> > +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write)
> > +{
> > + atomic_long_t *watchpoint;
> > + long encoded_watchpoint;
> > + unsigned long flags;
> > + enum kcsan_report_type report_type;
> > +
> > + if (unlikely(!is_enabled()))
> > + return false;
> > +
> > + /*
> > + * Avoid user_access_save in fast-path here: find_watchpoint is safe
> > + * without user_access_save, as the address that ptr points to is only
> > + * used to check if a watchpoint exists; ptr is never dereferenced.
> > + */
> > + watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> > + &encoded_watchpoint);
> > + if (watchpoint == NULL)
> > + return true;
> > +
> > + flags = user_access_save();
> > + if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> > + /*
> > + * The other thread may not print any diagnostics, as it has
> > + * already removed the watchpoint, or another thread consumed
> > + * the watchpoint before this thread.
> > + */
> > + kcsan_counter_inc(kcsan_counter_report_races);
> > + report_type = kcsan_report_race_check_race;
> > + } else {
> > + report_type = kcsan_report_race_check;
> > + }
> > +
> > + /* Encountered a data-race. */
> > + kcsan_counter_inc(kcsan_counter_data_races);
> > + kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> > +
> > + user_access_restore(flags);
> > + return false;
> > +}
> > +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> > +
> > +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> > + bool is_write)
> > +{
> > + atomic_long_t *watchpoint;
> > + union {
> > + u8 _1;
> > + u16 _2;
> > + u32 _4;
> > + u64 _8;
> > + } expect_value;
> > + bool is_expected = true;
> > + unsigned long ua_flags = user_access_save();
> > + unsigned long irq_flags;
> > +
> > + if (!should_watch(ptr))
> > + goto out;
> > +
> > + if (!check_encodable((unsigned long)ptr, size)) {
> > + kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> > + goto out;
> > + }
> > +
> > + /*
> > + * Disable interrupts & preemptions to avoid another thread on the same
> > + * CPU accessing memory locations for the set up watchpoint; this is to
> > + * avoid reporting races to e.g. CPU-local data.
> > + *
> > + * An alternative would be adding the source CPU to the watchpoint
> > + * encoding, and checking that watchpoint-CPU != this-CPU. There are
> > + * several problems with this:
> > + * 1. we should avoid stealing more bits from the watchpoint encoding
> > + * as it would affect accuracy, as well as increase performance
> > + * overhead in the fast-path;
> > + * 2. if we are preempted, but there *is* a genuine data-race, we
> > + * would *not* report it -- since this is the common case (vs.
> > + * CPU-local data accesses), it makes more sense (from a data-race
> > + * detection PoV) to simply disable preemptions to ensure as many
> > + * tasks as possible run on other CPUs.
> > + */
> > + local_irq_save(irq_flags);
> > +
> > + watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
> > + if (watchpoint == NULL) {
> > + /*
> > + * Out of capacity: the size of `watchpoints`, and the frequency
> > + * with which `should_watch()` returns true should be tweaked so
> > + * that this case happens very rarely.
> > + */
> > + kcsan_counter_inc(kcsan_counter_no_capacity);
> > + goto out_unlock;
> > + }
> > +
> > + kcsan_counter_inc(kcsan_counter_setup_watchpoints);
> > + kcsan_counter_inc(kcsan_counter_used_watchpoints);
> > +
> > + /*
> > + * Read the current value, to later check and infer a race if the data
> > + * was modified via a non-instrumented access, e.g. from a device.
> > + */
> > + switch (size) {
> > + case 1:
> > + expect_value._1 = READ_ONCE(*(const u8 *)ptr);
> > + break;
> > + case 2:
> > + expect_value._2 = READ_ONCE(*(const u16 *)ptr);
> > + break;
> > + case 4:
> > + expect_value._4 = READ_ONCE(*(const u32 *)ptr);
> > + break;
> > + case 8:
> > + expect_value._8 = READ_ONCE(*(const u64 *)ptr);
> > + break;
> > + default:
> > + break; /* ignore; we do not diff the values */
> > + }
> > +
> > +#ifdef CONFIG_KCSAN_DEBUG
> > + kcsan_disable_current();
> > + pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
> > + is_write ? "write" : "read", size, ptr,
> > + watchpoint_slot((unsigned long)ptr),
> > + encode_watchpoint((unsigned long)ptr, size, is_write));
> > + kcsan_enable_current();
> > +#endif
> > +
> > + /*
> > + * Delay this thread, to increase probability of observing a racy
> > + * conflicting access.
> > + */
> > + udelay(get_delay());
> > +
> > + /*
> > + * Re-read value, and check if it is as expected; if not, we infer a
> > + * racy access.
> > + */
> > + switch (size) {
> > + case 1:
> > + is_expected = expect_value._1 == READ_ONCE(*(const u8 *)ptr);
> > + break;
> > + case 2:
> > + is_expected = expect_value._2 == READ_ONCE(*(const u16 *)ptr);
> > + break;
> > + case 4:
> > + is_expected = expect_value._4 == READ_ONCE(*(const u32 *)ptr);
> > + break;
> > + case 8:
> > + is_expected = expect_value._8 == READ_ONCE(*(const u64 *)ptr);
> > + break;
> > + default:
> > + break; /* ignore; we do not diff the values */
> > + }
> > +
> > + /* Check if this access raced with another. */
> > + if (!remove_watchpoint(watchpoint)) {
> > + /*
> > + * No need to increment 'race' counter, as the racing thread
> > + * already did.
> > + */
> > + kcsan_report(ptr, size, is_write, smp_processor_id(),
> > + kcsan_report_race_setup);
> > + } else if (!is_expected) {
> > + /* Inferring a race, since the value should not have changed. */
> > + kcsan_counter_inc(kcsan_counter_races_unknown_origin);
> > +#ifdef CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> > + kcsan_report(ptr, size, is_write, smp_processor_id(),
> > + kcsan_report_race_unknown_origin);
> > +#endif
> > + }
> > +
> > + kcsan_counter_dec(kcsan_counter_used_watchpoints);
> > +out_unlock:
> > + local_irq_restore(irq_flags);
> > +out:
> > + user_access_restore(ua_flags);
> > +}
> > +EXPORT_SYMBOL(__kcsan_setup_watchpoint);
> > diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> > new file mode 100644
> > index 000000000000..6ddcbd185f3a
> > --- /dev/null
> > +++ b/kernel/kcsan/debugfs.c
> > @@ -0,0 +1,225 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/atomic.h>
> > +#include <linux/bsearch.h>
> > +#include <linux/bug.h>
> > +#include <linux/debugfs.h>
> > +#include <linux/init.h>
> > +#include <linux/kallsyms.h>
> > +#include <linux/mm.h>
> > +#include <linux/seq_file.h>
> > +#include <linux/sort.h>
> > +#include <linux/string.h>
> > +#include <linux/uaccess.h>
> > +
> > +#include "kcsan.h"
> > +
> > +/*
> > + * Statistics counters.
> > + */
> > +static atomic_long_t counters[kcsan_counter_count];
> > +
> > +/*
> > + * Addresses for filtering functions from reporting. This list can be used as a
> > + * whitelist or blacklist.
> > + */
> > +static struct {
> > + unsigned long *addrs; /* array of addresses */
> > + size_t size; /* current size */
> > + int used; /* number of elements used */
> > + bool sorted; /* if elements are sorted */
> > + bool whitelist; /* if list is a blacklist or whitelist */
> > +} report_filterlist = {
> > + .addrs = NULL,
> > + .size = 8, /* small initial size */
> > + .used = 0,
> > + .sorted = false,
> > + .whitelist = false, /* default is blacklist */
> > +};
> > +static DEFINE_SPINLOCK(report_filterlist_lock);
> > +
> > +static const char *counter_to_name(enum kcsan_counter_id id)
> > +{
> > + switch (id) {
> > + case kcsan_counter_used_watchpoints:
> > + return "used_watchpoints";
> > + case kcsan_counter_setup_watchpoints:
> > + return "setup_watchpoints";
> > + case kcsan_counter_data_races:
> > + return "data_races";
> > + case kcsan_counter_no_capacity:
> > + return "no_capacity";
> > + case kcsan_counter_report_races:
> > + return "report_races";
> > + case kcsan_counter_races_unknown_origin:
> > + return "races_unknown_origin";
> > + case kcsan_counter_unencodable_accesses:
> > + return "unencodable_accesses";
> > + case kcsan_counter_encoding_false_positives:
> > + return "encoding_false_positives";
> > + case kcsan_counter_count:
> > + BUG();
> > + }
> > + return NULL;
> > +}
> > +
> > +void kcsan_counter_inc(enum kcsan_counter_id id)
> > +{
> > + atomic_long_inc(&counters[id]);
> > +}
> > +
> > +void kcsan_counter_dec(enum kcsan_counter_id id)
> > +{
> > + atomic_long_dec(&counters[id]);
> > +}
> > +
> > +static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
> > +{
> > + const unsigned long a = *(const unsigned long *)rhs;
> > + const unsigned long b = *(const unsigned long *)lhs;
> > +
> > + return a < b ? -1 : a == b ? 0 : 1;
> > +}
> > +
> > +bool kcsan_skip_report(unsigned long func_addr)
> > +{
> > + unsigned long symbolsize, offset;
> > + unsigned long flags;
> > + bool ret = false;
> > +
> > + if (!kallsyms_lookup_size_offset(func_addr, &symbolsize, &offset))
> > + return false;
> > + func_addr -= offset; /* get function start */
> > +
> > + spin_lock_irqsave(&report_filterlist_lock, flags);
> > + if (report_filterlist.used == 0)
> > + goto out;
> > +
> > + /* Sort array if it is unsorted, and then do a binary search. */
> > + if (!report_filterlist.sorted) {
> > + sort(report_filterlist.addrs, report_filterlist.used,
> > + sizeof(unsigned long), cmp_filterlist_addrs, NULL);
> > + report_filterlist.sorted = true;
> > + }
> > + ret = !!bsearch(&func_addr, report_filterlist.addrs,
> > + report_filterlist.used, sizeof(unsigned long),
> > + cmp_filterlist_addrs);
> > + if (report_filterlist.whitelist)
> > + ret = !ret;
> > +
> > +out:
> > + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> > + return ret;
> > +}
> > +
> > +static void set_report_filterlist_whitelist(bool whitelist)
> > +{
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&report_filterlist_lock, flags);
> > + report_filterlist.whitelist = whitelist;
> > + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> > +}
> > +
> > +static void insert_report_filterlist(const char *func)
> > +{
> > + unsigned long flags;
> > + unsigned long addr = kallsyms_lookup_name(func);
> > +
> > + if (!addr) {
> > + pr_err("KCSAN: could not find function: '%s'\n", func);
> > + return;
> > + }
> > +
> > + spin_lock_irqsave(&report_filterlist_lock, flags);
> > +
> > + if (report_filterlist.addrs == NULL)
> > + report_filterlist.addrs = /* initial allocation */
> > + kvmalloc_array(report_filterlist.size,
> > + sizeof(unsigned long), GFP_KERNEL);
> You need to use braces in both branches here:
> https://www.kernel.org/doc/html/v4.10/process/coding-style.html#placing-braces-and-spaces

Done @ v3.

> > + else if (report_filterlist.used == report_filterlist.size) {
> > + /* resize filterlist */
> > + unsigned long *new_addrs;
> > +
> > + report_filterlist.size *= 2;
> > + new_addrs = kvmalloc_array(report_filterlist.size,
> > + sizeof(unsigned long), GFP_KERNEL);
> > + memcpy(new_addrs, report_filterlist.addrs,
> > + report_filterlist.used * sizeof(unsigned long));
> > + kvfree(report_filterlist.addrs);
> > + report_filterlist.addrs = new_addrs;
> > + }
> > +
> > + /* Note: deduplicating should be done in userspace. */
> > + report_filterlist.addrs[report_filterlist.used++] =
> > + kallsyms_lookup_name(func);
> > + report_filterlist.sorted = false;
> > +
> > + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> > +}
> > +
> > +static int show_info(struct seq_file *file, void *v)
> > +{
> > + int i;
> > + unsigned long flags;
> > +
> > + /* show stats */
> > + seq_printf(file, "enabled: %i\n", READ_ONCE(kcsan_enabled));
> > + for (i = 0; i < kcsan_counter_count; ++i)
> > + seq_printf(file, "%s: %ld\n", counter_to_name(i),
> > + atomic_long_read(&counters[i]));
> > +
> > + /* show filter functions, and filter type */
> > + spin_lock_irqsave(&report_filterlist_lock, flags);
> > + seq_printf(file, "\n%s functions: %s\n",
> > + report_filterlist.whitelist ? "whitelisted" : "blacklisted",
> > + report_filterlist.used == 0 ? "none" : "");
> > + for (i = 0; i < report_filterlist.used; ++i)
> > + seq_printf(file, " %ps\n", (void *)report_filterlist.addrs[i]);
> > + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> > +
> > + return 0;
> > +}
> > +
> > +static int debugfs_open(struct inode *inode, struct file *file)
> > +{
> > + return single_open(file, show_info, NULL);
> > +}
> > +
> > +static ssize_t debugfs_write(struct file *file, const char __user *buf,
> > + size_t count, loff_t *off)
> > +{
> > + char kbuf[KSYM_NAME_LEN];
> > + char *arg;
> > + int read_len = count < (sizeof(kbuf) - 1) ? count : (sizeof(kbuf) - 1);
> > +
> > + if (copy_from_user(kbuf, buf, read_len))
> > + return -EINVAL;
> > + kbuf[read_len] = '\0';
> > + arg = strstrip(kbuf);
> > +
> > + if (!strncmp(arg, "on", sizeof("on") - 1))
> > + WRITE_ONCE(kcsan_enabled, true);
> > + else if (!strncmp(arg, "off", sizeof("off") - 1))
> > + WRITE_ONCE(kcsan_enabled, false);
> > + else if (!strncmp(arg, "whitelist", sizeof("whitelist") - 1))
> > + set_report_filterlist_whitelist(true);
> > + else if (!strncmp(arg, "blacklist", sizeof("blacklist") - 1))
> > + set_report_filterlist_whitelist(false);
> > + else if (arg[0] == '!')
> > + insert_report_filterlist(&arg[1]);
> > + else
> > + return -EINVAL;
> > +
> > + return count;
> > +}
> > +
> > +static const struct file_operations debugfs_ops = { .read = seq_read,
> > + .open = debugfs_open,
> > + .write = debugfs_write,
> > + .release = single_release };
> > +
> > +void __init kcsan_debugfs_init(void)
> > +{
> > + debugfs_create_file("kcsan", 0644, NULL, NULL, &debugfs_ops);
> > +}
> > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> > new file mode 100644
> > index 000000000000..8f9b1ce0e59f
> > --- /dev/null
> > +++ b/kernel/kcsan/encoding.h
> > @@ -0,0 +1,94 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _MM_KCSAN_ENCODING_H
> > +#define _MM_KCSAN_ENCODING_H
> > +
> > +#include <linux/bits.h>
> > +#include <linux/log2.h>
> > +#include <linux/mm.h>
> > +
> > +#include "kcsan.h"
> > +
> > +#define SLOT_RANGE PAGE_SIZE
> > +#define INVALID_WATCHPOINT 0
> > +#define CONSUMED_WATCHPOINT 1
> > +
> > +/*
> > + * The maximum useful size of accesses for which we set up watchpoints is the
> > + * max range of slots we check on an access.
> > + */
> > +#define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
> > +
> > +/*
> > + * Number of bits we use to store size info.
> > + */
> > +#define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
> > +/*
> > + * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
> > + * however, most 64-bit architectures do not use the full 64-bit address space.
> > + * Also, in order for a false positive to be observable 2 things need to happen:
> > + *
> > + * 1. different addresses but with the same encoded address race;
> > + * 2. and both map onto the same watchpoint slots;
> > + *
> > + * Both these are assumed to be very unlikely. However, in case it still happens
> > + * happens, the report logic will filter out the false positive (see report.c).
> > + */
> > +#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG - 1 - WATCHPOINT_SIZE_BITS)
> > +
> > +/*
> > + * Masks to set/retrieve the encoded data.
> > + */
> > +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG - 1)
> > +#define WATCHPOINT_SIZE_MASK \
> > + GENMASK(BITS_PER_LONG - 2, BITS_PER_LONG - 2 - WATCHPOINT_SIZE_BITS)
> > +#define WATCHPOINT_ADDR_MASK \
> > + GENMASK(BITS_PER_LONG - 3 - WATCHPOINT_SIZE_BITS, 0)
> > +
> > +static inline bool check_encodable(unsigned long addr, size_t size)
> > +{
> > + return size <= MAX_ENCODABLE_SIZE;
> > +}
> > +
> > +static inline long encode_watchpoint(unsigned long addr, size_t size,
> > + bool is_write)
> > +{
> > + return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
> > + (size << WATCHPOINT_ADDR_BITS) |
> > + (addr & WATCHPOINT_ADDR_MASK));
> > +}
> > +
> > +static inline bool decode_watchpoint(long watchpoint,
> > + unsigned long *addr_masked, size_t *size,
> > + bool *is_write)
> > +{
> > + if (watchpoint == INVALID_WATCHPOINT ||
> > + watchpoint == CONSUMED_WATCHPOINT)
> > + return false;
> > +
> > + *addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
> > + *size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >>
> > + WATCHPOINT_ADDR_BITS;
> > + *is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
> > +
> > + return true;
> > +}
> > +
> > +/*
> > + * Return watchpoint slot for an address.
> > + */
> > +static inline int watchpoint_slot(unsigned long addr)
> > +{
> > + return (addr / PAGE_SIZE) % KCSAN_NUM_WATCHPOINTS;
> > +}
> > +
> > +static inline bool matching_access(unsigned long addr1, size_t size1,
> > + unsigned long addr2, size_t size2)
> > +{
> > + unsigned long end_range1 = addr1 + size1 - 1;
> > + unsigned long end_range2 = addr2 + size2 - 1;
> > +
> > + return addr1 <= end_range2 && addr2 <= end_range1;
> > +}
> > +
> > +#endif /* _MM_KCSAN_ENCODING_H */
> > diff --git a/kernel/kcsan/kcsan.c b/kernel/kcsan/kcsan.c
> > new file mode 100644
> > index 000000000000..45cf2fffd8a0
> > --- /dev/null
> > +++ b/kernel/kcsan/kcsan.c
> > @@ -0,0 +1,86 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +/*
> > + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> > + * see Documentation/dev-tools/kcsan.rst.
> > + */
> > +
> > +#include <linux/export.h>
> > +
> > +#include "kcsan.h"
> > +
> > +/*
> > + * KCSAN uses the same instrumentation that is emitted by supported compilers
> > + * for Thread Sanitizer (TSAN).
> > + *
> > + * When enabled, the compiler emits instrumentation calls (the functions
> > + * prefixed with "__tsan" below) for all loads and stores that it generated;
> > + * inline asm is not instrumented.
> > + */
> > +
> > +#define DEFINE_TSAN_READ_WRITE(size) \
> > + void __tsan_read##size(void *ptr) \
> > + { \
> > + __kcsan_check_read(ptr, size); \
> > + } \
> > + EXPORT_SYMBOL(__tsan_read##size); \
> > + void __tsan_write##size(void *ptr) \
> > + { \
> > + __kcsan_check_write(ptr, size); \
> > + } \
> > + EXPORT_SYMBOL(__tsan_write##size)
> > +
> > +DEFINE_TSAN_READ_WRITE(1);
> > +DEFINE_TSAN_READ_WRITE(2);
> > +DEFINE_TSAN_READ_WRITE(4);
> > +DEFINE_TSAN_READ_WRITE(8);
> > +DEFINE_TSAN_READ_WRITE(16);
> > +
> > +/*
> > + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> > + * but e.g. recent versions of Clang do.
> > + */
> > +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size) \
> > + void __tsan_unaligned_read##size(void *ptr) \
> > + { \
> > + __kcsan_check_read(ptr, size); \
> > + } \
> > + EXPORT_SYMBOL(__tsan_unaligned_read##size); \
> > + void __tsan_unaligned_write##size(void *ptr) \
> > + { \
> > + __kcsan_check_write(ptr, size); \
> > + } \
> > + EXPORT_SYMBOL(__tsan_unaligned_write##size)
> > +
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> > +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> > +
> > +void __tsan_read_range(void *ptr, size_t size)
> > +{
> > + __kcsan_check_read(ptr, size);
> > +}
> > +EXPORT_SYMBOL(__tsan_read_range);
> > +
> > +void __tsan_write_range(void *ptr, size_t size)
> > +{
> > + __kcsan_check_write(ptr, size);
> > +}
> > +EXPORT_SYMBOL(__tsan_write_range);
> > +
> > +/*
> > + * The below are not required KCSAN, but can still be emitted by the compiler.
> > + */
> > +void __tsan_func_entry(void *call_pc)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_func_entry);
> > +void __tsan_func_exit(void)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_func_exit);
> > +void __tsan_init(void)
> > +{
> > +}
> > +EXPORT_SYMBOL(__tsan_init);
> > diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
> > new file mode 100644
> > index 000000000000..429479b3041d
> > --- /dev/null
> > +++ b/kernel/kcsan/kcsan.h
> > @@ -0,0 +1,140 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _MM_KCSAN_KCSAN_H
> > +#define _MM_KCSAN_KCSAN_H
> > +
> > +#include <linux/kcsan.h>
> > +
> > +/*
> > + * Total number of watchpoints. An address range maps into a specific slot as
> > + * specified in `encoding.h`. Although larger number of watchpoints may not even
> > + * be usable due to limited thread count, a larger value will improve
> > + * performance due to reducing cache-line contention.
> > + */
> > +#define KCSAN_NUM_WATCHPOINTS 64
> > +
> > +/*
> > + * The number of adjacent watchpoints to check; the purpose is 2-fold:
> > + *
> > + * 1. the address slot is already occupied, check if any adjacent slots are
> > + * free;
> > + * 2. accesses that straddle a slot boundary due to size that exceeds a
> > + * slot's range may check adjacent slots if any watchpoint matches.
> > + *
> > + * Note that accesses with very large size may still miss a watchpoint; however,
> > + * given this should be rare, this is a reasonable trade-off to make, since this
> > + * will avoid:
> > + *
> > + * 1. excessive contention between watchpoint checks and setup;
> > + * 2. larger number of simultaneous watchpoints without sacrificing
> > + * performance.
> > + */
> > +#define KCSAN_CHECK_ADJACENT 1
> > +
> > +/*
> > + * Globally enable and disable KCSAN.
> > + */
> > +extern bool kcsan_enabled;
> > +
> > +/*
> > + * Helper that returns true if access to ptr should be considered as an atomic
> > + * access, even though it is not explicitly atomic.
> > + */
> > +bool kcsan_is_atomic(const volatile void *ptr);
> > +
> > +/*
> > + * Initialize debugfs file.
> > + */
> > +void kcsan_debugfs_init(void);
> > +
> > +enum kcsan_counter_id {
> Labels in enums should be capitalized:
> https://www.kernel.org/doc/html/v4.10/process/coding-style.html#macros-enums-and-rtl

Done @ v3.

> > + /*
> > + * Number of watchpoints currently in use.
> > + */
> > + kcsan_counter_used_watchpoints,
> > +
> > + /*
> > + * Total number of watchpoints set up.
> > + */
> > + kcsan_counter_setup_watchpoints,
> > +
> > + /*
> > + * Total number of data-races.
> > + */
> > + kcsan_counter_data_races,
> > +
> > + /*
> > + * Number of times no watchpoints were available.
> > + */
> > + kcsan_counter_no_capacity,
> > +
> > + /*
> > + * A thread checking a watchpoint raced with another checking thread;
> > + * only one will be reported.
> > + */
> > + kcsan_counter_report_races,
> > +
> > + /*
> > + * Observed data value change, but writer thread unknown.
> > + */
> > + kcsan_counter_races_unknown_origin,
> > +
> > + /*
> > + * The access cannot be encoded to a valid watchpoint.
> > + */
> > + kcsan_counter_unencodable_accesses,
> > +
> > + /*
> > + * Watchpoint encoding caused a watchpoint to fire on mismatching
> > + * accesses.
> > + */
> > + kcsan_counter_encoding_false_positives,
> > +
> > + kcsan_counter_count, /* number of counters */
> > +};
> > +
> > +/*
> > + * Increment/decrement counter with given id; avoid calling these in fast-path.
> > + */
> > +void kcsan_counter_inc(enum kcsan_counter_id id);
> > +void kcsan_counter_dec(enum kcsan_counter_id id);
> > +
> > +/*
> > + * Returns true if data-races in the function symbol that maps to addr (offsets
> > + * are ignored) should *not* be reported.
> > + */
> > +bool kcsan_skip_report(unsigned long func_addr);
> > +
> > +enum kcsan_report_type {
> > + /*
> > + * The thread that set up the watchpoint and briefly stalled was
> > + * signalled that another thread triggered the watchpoint, and thus a
> > + * race was encountered.
> > + */
> > + kcsan_report_race_setup,
> > +
> > + /*
> > + * A thread encountered a watchpoint for the access, therefore a race
> > + * was encountered.
> > + */
> > + kcsan_report_race_check,
> > +
> > + /*
> > + * A thread encountered a watchpoint for the access, but the other
> > + * racing thread can no longer be signaled that a race occurred.
> > + */
> > + kcsan_report_race_check_race,
> > +
> > + /*
> > + * No other thread was observed to race with the access, but the data
> > + * value before and after the stall differs.
> > + */
> > + kcsan_report_race_unknown_origin,
> > +};
> > +/*
> > + * Print a race report from thread that encountered the race.
> > + */
> > +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> > + int cpu_id, enum kcsan_report_type type);
> > +
> > +#endif /* _MM_KCSAN_KCSAN_H */
> > diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
> > new file mode 100644
> > index 000000000000..517db539e4e7
> > --- /dev/null
> > +++ b/kernel/kcsan/report.c
> > @@ -0,0 +1,306 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/kernel.h>
> > +#include <linux/preempt.h>
> > +#include <linux/printk.h>
> > +#include <linux/sched.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/stacktrace.h>
> > +
> > +#include "kcsan.h"
> > +#include "encoding.h"
> > +
> > +/*
> > + * Max. number of stack entries to show in the report.
> > + */
> > +#define NUM_STACK_ENTRIES 16
> > +
> > +/*
> > + * Other thread info: communicated from other racing thread to thread that set
> > + * up the watchpoint, which then prints the complete report atomically. Only
> > + * need one struct, as all threads should to be serialized regardless to print
> > + * the reports, with reporting being in the slow-path.
> > + */
> > +static struct {
> > + const volatile void *ptr;
> > + size_t size;
> > + bool is_write;
> > + int task_pid;
> > + int cpu_id;
> > + unsigned long stack_entries[NUM_STACK_ENTRIES];
> > + int num_stack_entries;
> > +} other_info = { .ptr = NULL };
> > +
> > +static DEFINE_SPINLOCK(other_info_lock);
> > +static DEFINE_SPINLOCK(report_lock);
> > +
> > +static bool set_or_lock_other_info(unsigned long *flags,
> > + const volatile void *ptr, size_t size,
> > + bool is_write, int cpu_id,
> > + enum kcsan_report_type type)
> > +{
> > + if (type != kcsan_report_race_check && type != kcsan_report_race_setup)
> > + return true;
> > +
> > + for (;;) {
> > + spin_lock_irqsave(&other_info_lock, *flags);
> > +
> > + switch (type) {
> > + case kcsan_report_race_check:
> > + if (other_info.ptr != NULL) {
> > + /* still in use, retry */
> > + break;
> > + }
> > + other_info.ptr = ptr;
> > + other_info.size = size;
> > + other_info.is_write = is_write;
> > + other_info.task_pid =
> > + in_task() ? task_pid_nr(current) : -1;
> > + other_info.cpu_id = cpu_id;
> > + other_info.num_stack_entries = stack_trace_save(
> > + other_info.stack_entries, NUM_STACK_ENTRIES, 1);
> > + /*
> > + * other_info may now be consumed by thread we raced
> > + * with.
> > + */
> > + spin_unlock_irqrestore(&other_info_lock, *flags);
> > + return false;
> > +
> > + case kcsan_report_race_setup:
> > + if (other_info.ptr == NULL)
> > + break; /* no data available yet, retry */
> > +
> > + /*
> > + * First check if matching based on how watchpoint was
> > + * encoded.
> > + */
> > + if (!matching_access((unsigned long)other_info.ptr &
> > + WATCHPOINT_ADDR_MASK,
> > + other_info.size,
> > + (unsigned long)ptr &
> > + WATCHPOINT_ADDR_MASK,
> > + size))
> > + break; /* mismatching access, retry */
> > +
> > + if (!matching_access((unsigned long)other_info.ptr,
> > + other_info.size,
> > + (unsigned long)ptr, size)) {
> > + /*
> > + * If the actual accesses to not match, this was
> > + * a false positive due to watchpoint encoding.
> > + */
> > + other_info.ptr = NULL; /* mark for reuse */
> > + kcsan_counter_inc(
> > + kcsan_counter_encoding_false_positives);
> > + spin_unlock_irqrestore(&other_info_lock,
> > + *flags);
> > + return false;
> > + }
> > +
> > + /*
> > + * Matching access: keep other_info locked, as this
> > + * thread uses it to print the full report; unlocked in
> > + * end_report.
> > + */
> > + return true;
> > +
> > + default:
> > + BUG();
> > + }
> > +
> > + spin_unlock_irqrestore(&other_info_lock, *flags);
> > + }
> > +}
> > +
> > +static void start_report(unsigned long *flags, enum kcsan_report_type type)
> > +{
> > + switch (type) {
> > + case kcsan_report_race_setup:
> > + /* irqsaved already via other_info_lock */
> > + spin_lock(&report_lock);
> > + break;
> > +
> > + case kcsan_report_race_unknown_origin:
> > + spin_lock_irqsave(&report_lock, *flags);
> > + break;
> > +
> > + default:
> > + BUG();
> > + }
> > +}
> > +
> > +static void end_report(unsigned long *flags, enum kcsan_report_type type)
> > +{
> > + switch (type) {
> > + case kcsan_report_race_setup:
> > + other_info.ptr = NULL; /* mark for reuse */
> > + spin_unlock(&report_lock);
> > + spin_unlock_irqrestore(&other_info_lock, *flags);
> > + break;
> > +
> > + case kcsan_report_race_unknown_origin:
> > + spin_unlock_irqrestore(&report_lock, *flags);
> > + break;
> > +
> > + default:
> > + BUG();
> > + }
> > +}
> > +
> > +static const char *get_access_type(bool is_write)
> > +{
> > + return is_write ? "write" : "read";
> > +}
> > +
> > +/* Return thread description: in task or interrupt. */
> > +static const char *get_thread_desc(int task_id)
> > +{
> > + if (task_id != -1) {
> > + static char buf[32]; /* safe: protected by report_lock */
> > +
> > + snprintf(buf, sizeof(buf), "task %i", task_id);
> > + return buf;
> > + }
> > + return in_nmi() ? "NMI" : "interrupt";
> > +}
> > +
> > +/* Helper to skip KCSAN-related functions in stack-trace. */
> > +static int get_stack_skipnr(unsigned long stack_entries[], int num_entries)
> > +{
> > + char buf[64];
> > + int skip = 0;
> > +
> > + for (; skip < num_entries; ++skip) {
> > + snprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
> > + if (!strnstr(buf, "csan_", sizeof(buf)) &&
> > + !strnstr(buf, "tsan_", sizeof(buf)) &&
> > + !strnstr(buf, "_once_size", sizeof(buf))) {
> > + break;
> > + }
> > + }
> > + return skip;
> > +}
> FWIW another option is to put all KCSAN-related functions in a
> separate code section and check if the function addresses are in the
> address range belonging to that section.
> This will work even with non-symbolized stacks.

Thanks for the suggestion. Is it worth it, i.e. will it simplify the
design and code? If it simplifies the design (or makes the fast-path
significantly faster), then yes, but otherwise I prefer the simplest
possible solution here. AFAIK, it will not make it simpler nor faster.
Using non-symbolized stacks should not be the common use-case (how to
usefully debug any data-race?).

> > +/* Compares symbolized strings of addr1 and addr2. */
> > +static int sym_strcmp(void *addr1, void *addr2)
> > +{
> > + char buf1[64];
> > + char buf2[64];
> > +
> > + snprintf(buf1, sizeof(buf1), "%pS", addr1);
> > + snprintf(buf2, sizeof(buf2), "%pS", addr2);
> > + return strncmp(buf1, buf2, sizeof(buf1));
> > +}
> > +
> > +/*
> > + * Returns true if a report was generated, false otherwise.
> > + */
> > +static bool print_summary(const volatile void *ptr, size_t size, bool is_write,
> > + int cpu_id, enum kcsan_report_type type)
> > +{
> > + unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
> > + int num_stack_entries =
> > + stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
> > + int skipnr = get_stack_skipnr(stack_entries, num_stack_entries);
> > + int other_skipnr;
> > +
> > + /* Check if the top stackframe is in a blacklisted function. */
> > + if (kcsan_skip_report(stack_entries[skipnr]))
> > + return false;
> > + if (type == kcsan_report_race_setup) {
> > + other_skipnr = get_stack_skipnr(other_info.stack_entries,
> > + other_info.num_stack_entries);
> > + if (kcsan_skip_report(other_info.stack_entries[other_skipnr]))
> > + return false;
> > + }
> > +
> > + /* Print report header. */
> > + pr_err("==================================================================\n");
> > + switch (type) {
> > + case kcsan_report_race_setup: {
> > + void *this_fn = (void *)stack_entries[skipnr];
> > + void *other_fn = (void *)other_info.stack_entries[other_skipnr];
> > + int cmp;
> > +
> > + /*
> > + * Order functions lexographically for consistent bug titles.
> > + * Do not print offset of functions to keep title short.
> > + */
> > + cmp = sym_strcmp(other_fn, this_fn);
> > + pr_err("BUG: KCSAN: data-race in %ps / %ps\n",
> > + cmp < 0 ? other_fn : this_fn,
> > + cmp < 0 ? this_fn : other_fn);
> > + } break;
> > +
> > + case kcsan_report_race_unknown_origin:
> > + pr_err("BUG: KCSAN: data-race in %pS\n",
> > + (void *)stack_entries[skipnr]);
> > + break;
> > +
> > + default:
> > + BUG();
> > + }
> > +
> > + pr_err("\n");
> > +
> > + /* Print information about the racing accesses. */
> > + switch (type) {
> > + case kcsan_report_race_setup:
> > + pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> > + get_access_type(other_info.is_write), other_info.ptr,
> > + other_info.size, get_thread_desc(other_info.task_pid),
> > + other_info.cpu_id);
> > +
> > + /* Print the other thread's stack trace. */
> > + stack_trace_print(other_info.stack_entries + other_skipnr,
> > + other_info.num_stack_entries - other_skipnr,
> > + 0);
> > +
> > + pr_err("\n");
> > + pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> > + get_access_type(is_write), ptr, size,
> > + get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> > + cpu_id);
> > + break;
> > +
> > + case kcsan_report_race_unknown_origin:
> > + pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
> > + get_access_type(is_write), ptr, size,
> > + get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> > + cpu_id);
> > + break;
> > +
> > + default:
> > + BUG();
> > + }
> > + /* Print stack trace of this thread. */
> > + stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
> > + 0);
> > +
> > + /* Print report footer. */
> > + pr_err("\n");
> > + pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
> > + dump_stack_print_info(KERN_DEFAULT);
> > + pr_err("==================================================================\n");
> > +
> > + return true;
> > +}
> > +
> > +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> > + int cpu_id, enum kcsan_report_type type)
> > +{
> > + unsigned long flags = 0;
> > +
> > + if (type == kcsan_report_race_check_race)
> > + return;
> > +
> > + kcsan_disable_current();
> > + if (set_or_lock_other_info(&flags, ptr, size, is_write, cpu_id, type)) {
> > + start_report(&flags, type);
> > + if (print_summary(ptr, size, is_write, cpu_id, type) &&
> > + panic_on_warn)
> > + panic("panic_on_warn set ...\n");
> > +
> > + end_report(&flags, type);
> > + }
> > + kcsan_enable_current();
> > +}
> > diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
> > new file mode 100644
> > index 000000000000..68c896a24529
> > --- /dev/null
> > +++ b/kernel/kcsan/test.c
> > @@ -0,0 +1,117 @@
> > +// SPDX-License-Identifier: GPL-2.0
> IIRC checkpatch.pl requires all SPDX headers to look like this one
> (C++-style, not C-style).
> Please double check and fix the headers in other files if necessary.

Checkpatch is happy. // for .c, and /**/ for .h.

> This file might also use some comments, now it's not easy to
> understand what it's testing.

Done @ v3.

> > +
> > +#include <linux/init.h>
> > +#include <linux/kernel.h>
> > +#include <linux/printk.h>
> > +#include <linux/random.h>
> > +#include <linux/types.h>
> > +
> > +#include "encoding.h"
> > +
> > +#define ITERS_PER_TEST 2000
> > +
> > +/* Test requirements. */
> > +static bool test_requires(void)
> > +{
> > + /* random should be initialized */
> > + return prandom_u32() + prandom_u32() != 0;
> > +}
> > +
> > +/* Test watchpoint encode and decode. */
> > +static bool test_encode_decode(void)
> > +{
> > + int i;
> > +
> > + for (i = 0; i < ITERS_PER_TEST; ++i) {
> > + size_t size = prandom_u32() % MAX_ENCODABLE_SIZE + 1;
> > + bool is_write = prandom_u32() % 2;
> > + unsigned long addr;
> > +
> > + prandom_bytes(&addr, sizeof(addr));
> > + if (WARN_ON(!check_encodable(addr, size)))
> > + return false;
> > +
> > + /* encode and decode */
> > + {
> > + const long encoded_watchpoint =
> > + encode_watchpoint(addr, size, is_write);
> > + unsigned long verif_masked_addr;
> > + size_t verif_size;
> > + bool verif_is_write;
> > +
> > + /* check special watchpoints */
> > + if (WARN_ON(decode_watchpoint(
> > + INVALID_WATCHPOINT, &verif_masked_addr,
> > + &verif_size, &verif_is_write)))
> > + return false;
> > + if (WARN_ON(decode_watchpoint(
> > + CONSUMED_WATCHPOINT, &verif_masked_addr,
> > + &verif_size, &verif_is_write)))
> > + return false;
> > +
> > + /* check decoding watchpoint returns same data */
> > + if (WARN_ON(!decode_watchpoint(
> > + encoded_watchpoint, &verif_masked_addr,
> > + &verif_size, &verif_is_write)))
> > + return false;
> > + if (WARN_ON(verif_masked_addr !=
> > + (addr & WATCHPOINT_ADDR_MASK)))
> > + goto fail;
> > + if (WARN_ON(verif_size != size))
> > + goto fail;
> > + if (WARN_ON(is_write != verif_is_write))
> > + goto fail;
> > +
> > + continue;
> > +fail:
> > + pr_err("%s fail: %s %zu bytes @ %lx -> encoded: %lx -> %s %zu bytes @ %lx\n",
> > + __func__, is_write ? "write" : "read", size,
> > + addr, encoded_watchpoint,
> > + verif_is_write ? "write" : "read", verif_size,
> > + verif_masked_addr);
> > + return false;
> > + }
> > + }
> > +
> > + return true;
> > +}
> > +
> > +static bool test_matching_access(void)
> > +{
> > + if (WARN_ON(!matching_access(10, 1, 10, 1)))
> > + return false;
> > + if (WARN_ON(!matching_access(10, 2, 11, 1)))
> > + return false;
> > + if (WARN_ON(!matching_access(10, 1, 9, 2)))
> > + return false;
> > + if (WARN_ON(matching_access(10, 1, 11, 1)))
> > + return false;
> > + if (WARN_ON(matching_access(9, 1, 10, 1)))
> > + return false;
> > + return true;
> > +}
> > +
> > +static int __init kcsan_selftest(void)
> > +{
> > + int passed = 0;
> > + int total = 0;
> > +
> > +#define RUN_TEST(do_test) \
> > + do { \
> > + ++total; \
> > + if (do_test()) \
> > + ++passed; \
> > + else \
> > + pr_err("KCSAN selftest: " #do_test " failed"); \
> > + } while (0)
> > +
> > + RUN_TEST(test_requires);
> > + RUN_TEST(test_encode_decode);
> > + RUN_TEST(test_matching_access);
> > +
> > + pr_info("KCSAN selftest: %d/%d tests passed\n", passed, total);
> > + if (passed != total)
> > + panic("KCSAN selftests failed");
> > + return 0;
> > +}
> > +postcore_initcall(kcsan_selftest);
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 93d97f9b0157..35accd1d93de 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -2086,6 +2086,8 @@ source "lib/Kconfig.kgdb"
> >
> > source "lib/Kconfig.ubsan"
> >
> > +source "lib/Kconfig.kcsan"
> > +
> > config ARCH_HAS_DEVMEM_IS_ALLOWED
> > bool
> >
> > diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> > new file mode 100644
> > index 000000000000..3e1f1acfb24b
> > --- /dev/null
> > +++ b/lib/Kconfig.kcsan
> > @@ -0,0 +1,88 @@
> > +# SPDX-License-Identifier: GPL-2.0-only
> > +
> > +config HAVE_ARCH_KCSAN
> > + bool
> > +
> > +menuconfig KCSAN
> > + bool "KCSAN: watchpoint-based dynamic data-race detector"
> > + depends on HAVE_ARCH_KCSAN && !KASAN && STACKTRACE
> > + default n
> > + help
> > + Kernel Concurrency Sanitizer is a dynamic data-race detector, which
> > + uses a watchpoint-based sampling approach to detect races.
> > +
> > +if KCSAN
> > +
> > +config KCSAN_SELFTEST
> > + bool "KCSAN: perform short selftests on boot"
> > + default y
> > + help
> > + Run KCSAN selftests on boot. On test failure, causes kernel to panic.
> > +
> > +config KCSAN_EARLY_ENABLE
> > + bool "KCSAN: early enable"
> > + default y
> > + help
> > + If KCSAN should be enabled globally as soon as possible. KCSAN can
> > + later be enabled/disabled via debugfs.
> > +
> > +config KCSAN_UDELAY_MAX_TASK
> > + int "KCSAN: maximum delay in microseconds (for tasks)"
> > + default 80
> > + help
> > + For tasks, the max. microsecond delay after setting up a watchpoint.
> > +
> > +config KCSAN_UDELAY_MAX_INTERRUPT
> > + int "KCSAN: maximum delay in microseconds (for interrupts)"
> > + default 20
> > + help
> > + For interrupts, the max. microsecond delay after setting up a watchpoint.
> > +
> > +config KCSAN_DELAY_RANDOMIZE
> > + bool "KCSAN: randomize delays"
> > + default y
> > + help
> > + If delays should be randomized; if false, the chosen delay is simply
> > + the maximum values defined above.
> > +
> > +config KCSAN_WATCH_SKIP_INST
> > + int "KCSAN: watchpoint instruction skip"
> > + default 2000
> > + help
> > + The number of per-CPU memory operations to skip watching, before
> > + another watchpoint is set up; in other words, 1 in
> > + KCSAN_WATCH_SKIP_INST per-CPU memory operations are used to set up a
> > + watchpoint. A smaller value results in more aggressive race
> > + detection, whereas a larger value improves system performance at the
> > + cost of missing some races.
> > +
> > +config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> > + bool "KCSAN: report races of unknown origin"
> > + default y
> > + help
> > + If KCSAN should report races where only one access is known, and the
> > + conflicting access is of unknown origin. This type of race is
> > + reported if it was only possible to infer a race due to a data-value
> > + change while an access is being delayed on a watchpoint.
> > +
> > +config KCSAN_IGNORE_ATOMICS
> > + bool "KCSAN: do not instrument marked atomic accesses"
> > + default n
> > + help
> > + If enabled, never instruments marked atomic accesses. This results in
> > + not reporting data-races where one access is atomic and the other is
> > + a plain access.
> > +
> Isn't it better to decide at runtime, whether we want to ignore atomics or not?

See below.

> > +config KCSAN_PLAIN_WRITE_PRETEND_ONCE
> > + bool "KCSAN: pretend plain writes are WRITE_ONCE"
> > + default n
> > + help
> > + This option makes KCSAN pretend that all plain writes are WRITE_ONCE.
> > + This option should only be used to prune initial data-races found in
> > + existing code.
> Overall, I think it's better to make most of these configs boot-time flags.
> This way one won't need to rebuild the kernel every time they want to
> turn some option on or off.

>From a design point of view, this complicates things on several
fronts. For one I would prefer having config options in one place,
however, most of these were added to "tame" syzbot, and keep reporting
volume initially low. I do not expect these to be switched frequently,
and for simplicity sake and to optimize for the common use-case, it'll
be better to keep it as-is. Eventually, these might even go away
completely.

I will add a comment to that effect above these options for v3.

> > +config KCSAN_DEBUG
> > + bool "Debugging of KCSAN internals"
> > + default n
> > +
> > +endif # KCSAN
> > diff --git a/lib/Makefile b/lib/Makefile
> > index c5892807e06f..778ab704e3ad 100644
> > --- a/lib/Makefile
> > +++ b/lib/Makefile
> > @@ -24,6 +24,9 @@ KASAN_SANITIZE_string.o := n
> > CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
> > endif
> >
> > +# Used by KCSAN while enabled, avoid recursion.
> > +KCSAN_SANITIZE_random32.o := n
> > +
> > lib-y := ctype.o string.o vsprintf.o cmdline.o \
> > rbtree.o radix-tree.o timerqueue.o xarray.o \
> > idr.o extable.o \
> > diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
> > new file mode 100644
> > index 000000000000..caf1111a28ae
> > --- /dev/null
> > +++ b/scripts/Makefile.kcsan
> > @@ -0,0 +1,6 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +ifdef CONFIG_KCSAN
> > +
> > +CFLAGS_KCSAN := -fsanitize=thread
> > +
> > +endif # CONFIG_KCSAN
> > diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> > index 179d55af5852..0e78abab7d83 100644
> > --- a/scripts/Makefile.lib
> > +++ b/scripts/Makefile.lib
> > @@ -152,6 +152,16 @@ _c_flags += $(if $(patsubst n%,, \
> > $(CFLAGS_KCOV))
> > endif
> >
> > +#
> > +# Enable ConcurrencySanitizer flags for kernel except some files or directories
> "KernelConcurrencySanitizer" or "Kernel Concurrency Sanitizer", maybe?

Done @ v3.

> > +# we don't want to check (depends on variables KCSAN_SANITIZE_obj.o, KCSAN_SANITIZE)
> > +#
> > +ifeq ($(CONFIG_KCSAN),y)
> > +_c_flags += $(if $(patsubst n%,, \
> > + $(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
> > + $(CFLAGS_KCSAN))
> > +endif
> > +
> > # $(srctree)/$(src) for including checkin headers from generated source files
> > # $(objtree)/$(obj) for including generated headers from checkin source files
> > ifeq ($(KBUILD_EXTMOD),)
> > --
> > 2.23.0.866.gb869b98d4c-goog
> >

Thanks for your comments!
-- Marco

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