[PATCH v4 03/10] kcsan: Add Documentation entry in dev-tools

From: Marco Elver
Date: Thu Nov 14 2019 - 13:04:48 EST

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
v4:
* Update to performance numbers after optimizations: no measurable
change with default, and minor improvement when no watchpoints are set
up.
* Add section on race conditions vs. data races.
* Add section on selective analysis.

v3:
* Split Documentation into separate patch.
* Fix typos.
* Accuracy: refer to unsoundness/completeness.
* Update with new slow-down after optimizations.
* Add Alternatives Considered section and move KTSAN mentions there.
---
Documentation/dev-tools/index.rst | 1 +
Documentation/dev-tools/kcsan.rst | 256 ++++++++++++++++++++++++++++++
2 files changed, 257 insertions(+)
create mode 100644 Documentation/dev-tools/kcsan.rst

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index b0522a4dd107..1b756a7014e0 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -21,6 +21,7 @@ whole; patches welcome!
kasan
ubsan
kmemleak
+ kcsan
gdb-kernel-debugging
kgdb
kselftest
diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
new file mode 100644
index 000000000000..a6f4f92df2fa
--- /dev/null
+++ b/Documentation/dev-tools/kcsan.rst
@@ -0,0 +1,256 @@
+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. 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).
+
+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.
+
+Selective analysis
+~~~~~~~~~~~~~~~~~~
+
+To disable KCSAN data race detection for an entire subsystem, add to the
+respective ``Makefile``::
+
+ KCSAN_SANITIZE := n
+
+To disable KCSAN on a per-file basis, add to the ``Makefile``::
+
+ KCSAN_SANITIZE_file.o := n
+
+KCSAN also understands the ``data_race(expr)`` annotation, which tells KCSAN
+that any data races due to accesses in ``expr`` should be ignored and resulting
+behaviour when encountering a data race is deemed safe.
+
+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.
+
+Data Races
+----------
+
+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.
+
+Race conditions vs. data races
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Race conditions are logic bugs, where unexpected interleaving of racing
+concurrent operations result in an erroneous state.
+
+Data races on the other hand are defined at the *memory model/language level*.
+Many data races are also harmful race conditions, which a tool like KCSAN
+reports! However, not all data races are race conditions and vice-versa.
+KCSAN's intent is to report data races according to the LKMM. A data race
+detector can only work at the memory model/language level.
+
+Deeper analysis, to find high-level race conditions only, requires conveying
+the intended kernel logic to a tool. This requires (1) the developer writing a
+specification or model of their code, and then (2) the tool verifying that the
+implementation matches. This has been done for small bits of code using model
+checkers and other formal methods, but does not scale to the level of what can
+be covered with a dynamic analysis based data race detector such as KCSAN.
+
+For reasons outlined in this `article <https://lwn.net/Articles/793253/>`_,
+data races can be much more subtle, but can cause no less harm than high-level
+race conditions.
+
+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 for a small 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:** 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 on a system with 8 CPUs:
+
+ - 5.0x slow-down with the default KCSAN config;
+ - 2.8x slow-down from runtime fast-path overhead only (set very large
+ ``KCSAN_SKIP_WATCH`` and unset ``KCSAN_SKIP_WATCH_RANDOMIZE``).
+
+3. **Annotation Overheads:** Minimal annotations are required outside the KCSAN
+ runtime. As a result, maintenance overheads are minimal as the kernel
+ evolves.
+
+4. **Detects Racy Writes from Devices:** Due to checking data values upon
+ setting up watchpoints, racy writes from devices can also be detected.
+
+5. **Memory Ordering:** KCSAN is *not* explicitly aware of the LKMM's ordering
+ rules; this may result in missed data races (false negatives).
+
+6. **Analysis Accuracy:** For observed executions, due to using a sampling
+ strategy, the analysis is *unsound* (false negatives possible), but aims to
+ be complete (no false positives).
+
+Alternatives Considered
+-----------------------
+
+An alternative data race detection approach for the kernel can be found in
+`Kernel Thread Sanitizer (KTSAN) <https://github.com/google/ktsan/wiki>`_.
+KTSAN is a happens-before data race detector, which explicitly establishes the
+happens-before order between memory operations, which can then be used to
+determine data races as defined in `Data Races`_. To build a correct
+happens-before relation, KTSAN must be aware of all ordering rules of the LKMM
+and synchronization primitives. Unfortunately, any omission leads to false
+positives, which is especially important in the context of the kernel which
+includes numerous custom synchronization mechanisms. Furthermore, KTSAN's
+implementation requires metadata for each memory location (shadow memory);
+currently, for each page, KTSAN requires 4 pages of shadow memory.
--
2.24.0.rc1.363.gb1bccd3e3d-goog