[Bug libc/31479] New: Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[Bug libc/31479] New: Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31479

            Bug ID: 31479
           Summary: Missing #include <sys/rseq.h> in sched_getcpu.c may
                    result in a loss of rseq acceleration
           Product: glibc
           Version: 2.35
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

This commit removed #include <sys/rseq.h> from nptl/descr.h:

commit 2c6b4b272e6b4d07303af25709051c3e96288f2d
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Jul 21 16:18:18 2023 +0200

    nptl: Unconditionally use a 32-byte rseq area

    If the kernel headers provide a larger struct rseq, we used that
    size as the argument to the rseq system call.  As a result,
    rseq registration would fail on older kernels which only accept
    size 32.

As a result, <sys/rseq.h> may or may not be implicitly included when
sysdeps/unix/sysv/linux/sched_getcpu.c is built. If <sys/rseq.h> is not
included, RSEQ_SIG is not defined, and we do not get rseq acceleration even if
supported on the architecture.

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[Bug libc/31479] Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31479

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at sourceware dot org   |fweimer at redhat dot com
              Flags|                            |security-

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[Bug libc/31479] Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31479

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
Patch posted:

[PATCH] linux: Add #include <sys/rseq.h> to sched_getcpu.c
<https://inbox.sourceware.org/libc-alpha/871q8es6yh.fsf@oldenburg.str.redhat.com/T/>

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[Bug libc/31479] Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31479

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://issues.redhat.com/b
                   |                            |rowse/RHEL-28119

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[Bug libc/31479] Missing #include <sys/rseq.h> in sched_getcpu.c may result in a loss of rseq acceleration

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31479

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |2.40
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
Fixed for 2.40 via:

commit 7a76f218677d149d8b7875b336722108239f7ee9
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Mar 15 19:08:24 2024 +0100

    linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

    Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
    in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("nptl:
    Unconditionally use a 32-byte rseq area").  After that, it was
    not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
    compilation that provided a definition.  This commit always checks
    the rseq area for CPU number information before using the other
    approaches.

    This adds an unnecessary (but well-predictable) branch on
    architectures which do not define RSEQ_SIG, but its cost is small
    compared to the system call.  Most architectures that have vDSO
    acceleration for getcpu also have rseq support.

    Fixes: 2c6b4b272e6b4d07303af25709051c3e96288f2d
    Fixes: 1d350aa06091211863e41169729cee1bca39f72f
    Reviewed-by: Arjun Shankar <arjun@redhat.com>

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