Re: rseq(2) man page

From: Alejandro Colomar
Date: Fri Jan 06 2023 - 17:58:00 EST

Hi Mathieu,

On 1/6/23 21:57, Mathieu Desnoyers wrote:
On 2023-01-06 12:50, Alejandro Colomar wrote:
.I cpu_id_start
Always-updated value of the CPU number on which the registered thread is
running. Its value is guaranteed to always be a possible CPU number,
even when rseq is not registered. Its value should always be confirmed by

rseq (and maybe other cases around too) should be formatted in italics, since it's a variable name (.I).

"rseq" here does not refer to a variable name, but rather to the system call. Should it be formatted in italics ? I format it as ".BR" elsewhere in the man page.

For referring to a function (or syscall), we use a bold name and non-bold (roman) trailing parens; empty if it's defined in the same page.

So, it would be:

.BR rseq ()

.TP
.B EFAULT
.I rseq
is an invalid address.

Doesn't this result in a SEGV?  It's trying to access invalid memory. We had some discussion about this in other syscalls, and concluded that that's undefined behavior, and a crash is valid behavior (and probably a good thing to do), right?  I'm just curious about the view from the kernel point of view.

If the registered rseq pointer / size points to invalid memory on rseq registration, the rseq registration system call will return -1, errno=EFAULT. If at some point _after_ registration the mapping becomes invalid (e.g. unmapped without prior unregistration), then a SIGSEGV can be triggered.

I was not aware of this discussion a regarding returning EFAULT errno vs SIGSEGV. If this is becoming a consensus across system calls to segfault  rather than return EFAULT errno, I'm open to improve sys_rseq accordingly.

The discussion was in libc-alpha:

<https://inbox.sourceware.org/libc-alpha/b55075c8-e55b-e492-7f4f-d6feb0ee2432@xxxxxxxxx/T/#u>

The reason is that, since holding a pointer to invalid memory is already undefined behavior (the standard defines that as overflow[1]), passing it to a syscall necessarily implies that you hold such a pointer, so UB was invoked prior to the call, and thus the call is already within UB.

[1]: <https://port70.net/%7Ensz/c/c11/n1570.html#6.5.6p8>

However, that discussion was in glibc; I don't know what kernel people might think of that. If you discuss that in the kernel, please link to that glibc thread, and CC the people from that thread.


Now I'll add some comments for v2.

Cheers,

Alex


--- v2:

'\" t

This is a comment for pages that use tables (tbl(1)), so that man(1) knows if it needs to run tbl(1) before roff(1). It seems that your page doesn't use tbl(1) features, so you should remove it.

.\" Copyright 2015-2023 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH rseq 2 (date) "Linux man-pages (unreleased)"
.SH NAME
rseq \- restartable sequences system call

I forgot; there's a new section: LIBRARY.

It specifies the library in which the function is defined, or in the case of syscalls, the wrapper (since we call it through syscall(2), it would be libc).

BTW, I wonder what librseq is. Is librseq something that users should care about?

.SH SYNOPSIS
.nf
.PP
.BR "#include <linux/rseq.h>" \
" /* Definition of " RSEQ_* " constants and rseq types */"

The line above goes beyond column 80 in formatted output. That's a hard limit for manual pages. If you add this page to the linux man-pages repo, and run the linters, you'll see a warning about that. In case you're interested in linting manual pages in the future, you can do something similar to what I do in the man-pages[2]

I decided to not include types in the comments. The reason, apart from having shorter comments, is that the man-pages now document types in their own pages in subsection 3type.

Since I guess you don't want to go so far to write a new manual page for the types (but if you want, feel free), since those types are specific to this call, what you can do is specify the include that provides the type, in the same piece of code where you document the type.

.BR "#include #include <sys/syscall.h>" " * Definition of " SYS_* " constants */"

The comment above seems broken. See '/*'.

.B #include <unistd.h>
.PP
.BI "int syscall(SYS_rseq, struct rseq *_Nullable " rseq ", uint32_t " rseq_len \

What's the meaning for NULL? Does it have a valid sentinel meaning, or is it an invalid address? If it's just interpreted as an invalid address (for which from a user-space perspective a crash would be legitimate), then I'd remove _Nullable.

", int " flags ", uint32_t " sig ");

The last '"' is unmatched.

Also, this goes beyond 80-col. Please cut after rseq_len (my mailer will probably put the comma in a separate line; it should go at the end of the line, preceeded by a space):

.BI "int syscall(SYS_rseq, struct rseq *_Nullable " rseq ", uint32_t " rseq_len ,
.BI " int " flags ", uint32_t " sig );


.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR rseq (),
necessitating the use of
.BR syscall (2).
.SH DESCRIPTION
.PP

.PP after .SH or .SS is redundant, and should be removed.

The
.BR rseq ()
ABI accelerates specific user-space operations by registering a
per-thread data structure shared between kernel and user-space.
This data structure can be read from or written to by user-space to skip
otherwise expensive system calls.
.PP
A restartable sequence is a sequence of instructions guaranteed to be executed
atomically with respect to other threads and signal handlers on the current
CPU.

I'd break after "instructions", and after "respect to".

If its execution does not complete atomically, the kernel changes the

Breaking after commas is strongly preferred.

execution flow by jumping to an abort handler defined by user-space for
that restartable sequence.
.PP
Using restartable sequences requires to register a
rseq ABI per-thread data structure (

This produces a space after the '('.

.B struct rseq

Instead, use:

.RB ( "struct rseq" )

) through the
.BR rseq ()
system call.
Only one rseq ABI can be registered per thread, so user-space libraries
and applications must follow a user-space ABI defining how to share this
resource.
The ABI defining how to share this resource between applications and
libraries is defined by the C library.
Allocation of the per-thread rseq ABI and its registration to the kernel
is handled by glibc since version 2.35.
.PP
The rseq ABI per-thread data structure contains a
.I rseq_cs
field which points to the currently executing critical section.
For each thread, a single rseq critical section can run at any given
point.
Each critical section need to be implemented in assembly.
.PP
The
.BR rseq ()
ABI accelerates user-space operations on per-cpu data by defining a
shared data structure ABI between each user-space thread and the kernel.
.PP
It allows user-space to perform update operations on per-cpu data
without requiring heavy-weight atomic operations.
.PP
The term CPU used in this documentation refers to a hardware execution
context.
For instance, each CPU number returned by
.BR sched_getcpu ()
is a CPU.
The current CPU means to the CPU on which the registered thread is
running.
.PP
Restartable sequences are atomic with respect to preemption (making it
atomic with respect to other threads running on the same CPU),
as well as signal delivery (user-space execution contexts nested over
the same thread).
They either complete atomically with respect to preemption on the
current CPU and signal delivery, or they are aborted.
.PP
Restartable sequences are suited for update operations on per-cpu data.
.PP
Restartable sequences can be used on data structures shared between threads
within a process,
and on data structures shared between threads across different
processes.
.PP
Some examples of operations that can be accelerated or improved by this ABI:
.IP \(bu 3
Memory allocator per-cpu free-lists,
.IP \(bu 3
Querying the current CPU number,
.IP \(bu 3
Incrementing per-CPU counters,
.IP \(bu 3
Modifying data protected by per-CPU spinlocks,
.IP \(bu 3
Inserting/removing elements in per-CPU linked-lists,
.IP \(bu 3
Writing/reading per-CPU ring buffers content.
.IP \(bu 3
Accurately reading performance monitoring unit counters with respect to
thread migration.
.PP
Restartable sequences must not perform system calls.
Doing so may result in termination of the process by a segmentation
fault.
.PP
The
.I rseq
argument is a pointer to the thread-local rseq structure to be shared
between kernel and user-space.
.PP
The structure
.B struct rseq
is an extensible structure.
Additional feature fields can be added in future kernel versions.
Its layout is as follows:
.TP
.B Structure alignment
This structure is aligned on either 32-byte boundary,
or on the alignment value returned by
.I getauxval(
.B AT_RSEQ_ALIGN
)
if the structure size differs from 32 bytes.
.TP
.B Structure size
This structure size needs to be at least 32 bytes.
It can be either 32 bytes,
or it needs to be large enough to hold the result of
.I getauxval(
.B AT_RSEQ_FEATURE_SIZE
) .
Its size is passed as parameter to the rseq system call.
.RS

For code examples we use .in +4n rather than .RS. I don't remember the exact reason, but it had some glitches in some cases.

.PP

This should be .IP, since it's a continuation of the tagged paragraph (TP). IP continues the indentation, while PP start a clean paragraph that is not indented as the previous one.

.EX

You could add the #include <linux/rseq.h> here.

struct rseq {
__u32 cpu_id_start;
__u32 cpu_id;
union {
/* Edited out for conciseness. [...] */

You can simply say /* ... */

We use that in a few other places.

} rseq_cs;
__u32 flags;
__u32 node_id;
__u32 mm_cid;
} __attribute__((aligned(32)));
.EE
.RE
.TP
.B Fields
.RS

Here there should be another .TP

.I cpu_id_start
.RS

And that TP would make this RS unnecessary, since it would indent the paragraph.

Always-updated value of the CPU number on which the registered thread is
running.
Its value is guaranteed to always be a possible CPU number,
even when rseq is not registered.
Its value should always be confirmed by reading the cpu_id field before
user-space performs any side-effect
(e.g. storing to memory).
.PP

This should be a IP, to continue the indentation of the previous TP.

This field is always guaranteed to hold a valid CPU number in the range
[ 0 .. nr_possible_cpus - 1 ].
It can therefore be loaded by user-space and used as an offset in
per-cpu data structures without having to check whether its value is
within the valid bounds compared to the number of possible CPUs in the
system.

Having a word alone in a line is usually a hint that there exists a better point to break the sentence (except where .I or .B ask for it).

.PP
Initialized by user-space to a possible CPU number (e.g., 0),
updated by the kernel for threads registered with rseq.

rseq shoudl be formatted.

.PP
For user-space applications executed on a kernel without rseq support,
the cpu_id_start field stays initialized at 0, which is indeed a valid
CPU number.
It is therefore valid to use it as an offset in per-cpu data structures,
and only validate whether it's actually the current CPU number by
comparing it with the cpu_id field within the rseq critical section.
If the kernel does not provide rseq support, that cpu_id field stays
initialized at -1,
so the comparison always fails, as intended.
.PP
This field should only be read by the thread which registered this data
structure.
Aligned on 32-bit.
.PP
It is up to user-space to implement a fall-back mechanism for scenarios where

s/user-space/user space/

The - is used as in natural English, for form an adjective. Since here it acts as a subject, it goes with a space.

rseq is not available.
.RE
.PP
.I cpu_id
.RS
Always-updated value of the CPU number on which the registered thread is
running.
Initialized by user-space to -1,
updated by the kernel for threads registered with rseq.
.PP
This field should only be read by the thread which registered this data
structure.
Aligned on 32-bit.
.RE
.PP
.I rseq_cs
.RS
The rseq_cs field is a pointer to a
.B struct rseq_cs .
Is is NULL when no rseq assembly block critical section is active for
the registered thread.
Setting it to point to a critical section descriptor (
.B struct rseq_cs
) marks the beginning of the critical section.
.PP
Initialized by user-space to NULL.
.PP
Updated by user-space, which sets the address of the currently
active rseq_cs at the beginning of assembly instruction sequence
block,
and set to NULL by the kernel when it restarts an assembly instruction
sequence block,
as well as when the kernel detects that it is preempting or delivering a
signal outside of the range targeted by the rseq_cs.
Also needs to be set to NULL by user-space before reclaiming memory that
contains the targeted
.B struct rseq_cs .
.PP
Read and set by the kernel.
.PP
This field should only be updated by the thread which registered this
data structure.
Aligned on 64-bit.
.RE
.PP
.I flags
.RS
Flags indicating the restart behavior for the registered thread.
This is mainly used for debugging purposes.
Can be a combination of:
.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
Inhibit instruction sequence block restart on preemption for this
thread.
This flag is deprecated since kernel 6.1.

I did some consistency fixes regarding linux versions. Now we always use "Linux x.y", rather than "Linux kernel x.y", or "kernel x.y". That helps grepping.

.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
Inhibit instruction sequence block restart on signal delivery for this
thread.
This flag is deprecated since kernel 6.1.
.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
Inhibit instruction sequence block restart on migration for this thread.
This flag is deprecated since kernel 6.1.
.PP
Initialized by user-space, used by the kernel.
.RE
.PP
.I node_id
.RS
Always-updated value of the current NUMA node ID.
.PP
Initialized by user-space to 0.
.PP
Updated by the kernel.
Read by user-space with single-copy atomicity semantics.
This field should only be read by the thread which registered
this data structure.
Aligned on 32-bit.
.RE
.PP
.I mm_cid
.RS
Contains the current thread's concurrency ID
(allocated uniquely within a memory map).
.PP
Updated by the kernel.
Read by user-space with single-copy atomicity semantics.
This field should only be read by the thread which registered this data
structure.
Aligned on 32-bit.
.PP
This concurrency ID is within the possible cpus range,
and is temporarily (and uniquely) assigned while threads are actively
running within a memory map.
If a memory map has fewer threads than cores,
or is limited to run on few cores concurrently through sched affinity or
cgroup cpusets,
the concurrency IDs will be values close to 0,
thus allowing efficient use of user-space memory for per-cpu data
structures.
.RE
.RE
.RE
.PP
The layout of
.B struct rseq_cs
version 0 is as follows:
.TP
.B Structure alignment
This structure is aligned on 32-byte boundary.
.TP
.B Structure size
This structure has a fixed size of 32 bytes.
.RS
.EX
struct rseq_cs {
__u32 version;
__u32 flags;
__u64 start_ip;
__u64 post_commit_offset;
__u64 abort_ip;
} __attribute__((aligned(32)));
.EE
.RE
.PP
.B Fields
.RS
.I version
.RS
Version of this structure.
Should be initialized to 0.
.RE
.PP
.I flags
.RS
Flags indicating the restart behavior of this structure.
Can be a combination of:
.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
Inhibit instruction sequence block restart on preemption for this
critical section.
This flag is deprecated since kernel 6.1.
.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
Inhibit instruction sequence block restart on signal delivery for this
critical section.
This flag is deprecated since kernel 6.1.
.TP
.B RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
Inhibit instruction sequence block restart on migration for this
critical section.
This flag is deprecated since kernel 6.1.
.RE
.PP
.I start_ip
.RS
Instruction pointer address of the first instruction of the sequence of
consecutive assembly instructions.
.RE
.PP
.I post_commit_offset
.RS
Offset (from start_ip address) of the address after the last instruction
of the sequence of consecutive assembly instructions.
.RE
.PP
.I abort_ip
.RS
Instruction pointer address where to move the execution flow in case of
abort of the sequence of consecutive assembly instructions.
.RE
.RE
.PP
The
.I rseq_len
argument is the size of the
.B struct rseq
to register.
.PP
The
.I flags
argument is 0 for registration, and
.B RSEQ_FLAG_UNREGISTER
for unregistration.
.PP
The
.I sig
argument is the 32-bit signature to be expected before the abort
handler code.
.PP
A single library per process should keep the rseq structure in a
per-thread data structure.
The
.I cpu_id
field should be initialized to -1, and the
.I cpu_id_start
field should be initialized to a possible CPU value (typically 0).
.PP
Each thread is responsible for registering and unregistering its rseq
structure.
No more than one rseq structure address can be registered per thread at
a given time.
.PP
Reclaim of rseq object's memory must only be done after either an
explicit rseq unregistration is performed or after the thread exits.
.PP
In a typical usage scenario, the thread registering the rseq
structure will be performing loads and stores from/to that structure.
It is however also allowed to read that structure from other threads.
The rseq field updates performed by the kernel provide relaxed atomicity
semantics (atomic store, without memory ordering),
which guarantee that other threads performing relaxed atomic reads
(atomic load, without memory ordering) of the cpu number fields will
always observe a consistent value.
.PP
.SH RETURN VALUE
A return value of 0 indicates success.
On error, \-1 is returned, and
.I errno
is set appropriately.
.PP
.SH ERRORS
.TP
.B EINVAL
Either
.I flags
contains an invalid value, or
.I rseq
contains an address which is not appropriately aligned, or
.I rseq_len
contains an incorrect size.
.TP
.B ENOSYS
The
.BR rseq ()
system call is not implemented by this kernel.
.TP
.B EFAULT
.I rseq
is an invalid address.
.TP
.B EBUSY
Restartable sequence is already registered for this thread.
.TP
.B EPERM
The
.I sig
argument on unregistration does not match the signature received
on registration.
.PP
.SH VERSIONS
The
.BR rseq ()
system call was added in Linux 4.18.
.PP
.SH STANDARDS
.BR rseq ()
is Linux-specific.
.PP
.SH SEE ALSO
.BR sched_getcpu (3) ,
.BR membarrier (2) ,
.BR getauxval (3)



[2]: (linting manual pages)


If you add a new manual page to the man-pages repo, it will automatically be included in the build system, so you'll be able to lint it, as described in the CONTRIBUTING file (tl;dr: `make lint`). But if you want to add something similar to your build system, you can inspect the following commands being run:


alx@asus5775:~/src/linux/man-pages/man-pages/main$ make lint -ij >/dev/null 2>/dev/null
alx@asus5775:~/src/linux/man-pages/man-pages/main$ touch man2/rseq.2
alx@asus5775:~/src/linux/man-pages/man-pages/main$ make lint -k
LINT (groff) tmp/lint/man2/rseq.2.lint-man.groff.touch
an.tmac:man2/rseq.2:297: style: unbalanced .RE
found style problems; aborting
man2/rseq.2:7: #include <linux/rseq.h> /* Definition of RSEQ_* constants and rseq types */
man2/rseq.2:11: int syscall(SYS_rseq, struct rseq *_Nullable rseq, uint32_t rseq_len, int flags, uint32_t sig);
make: *** [lib/lint-man.mk:78: tmp/lint/man2/rseq.2.lint-man.groff.touch] Error 1
LINT (mandoc) tmp/lint/man2/rseq.2.lint-man.mandoc.touch
mandoc: man2/rseq.2:18:114: STYLE: unterminated quoted argument
mandoc: man2/rseq.2:297:2: ERROR: skipping end of block that is not open: RE
mandoc: man2/rseq.2:409:2: WARNING: skipping paragraph macro: PP empty
mandoc: man2/rseq.2:27:2: WARNING: skipping paragraph macro: PP after SH
mandoc: man2/rseq.2:415:2: WARNING: skipping paragraph macro: PP empty
mandoc: man2/rseq.2:444:2: WARNING: skipping paragraph macro: PP empty
mandoc: man2/rseq.2:449:2: WARNING: skipping paragraph macro: PP empty
mandoc: man2/rseq.2:453:2: WARNING: skipping paragraph macro: PP empty
make: *** [lib/lint-man.mk:88: tmp/lint/man2/rseq.2.lint-man.mandoc.touch] Error 1
LINT (tbl) tmp/lint/man2/rseq.2.lint-man.tbl.touch
man2/rseq.2:1: spurious '\" t' comment:
'\" t
make: *** [lib/lint-man.mk:101: tmp/lint/man2/rseq.2.lint-man.tbl.touch] Error 1
make: Target 'lint' not remade because of errors.


If you want to see the commands being run, you can use `make lint V=1`. Feel free to incorporate any of that to your Makefile.


--
<http://www.alejandro-colomar.es/>

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