[Bug stdio/27842] New: Race condition in _IO_FLAGS2_NEED_LOCK optimization

[Bug stdio/27842] New: Race condition in _IO_FLAGS2_NEED_LOCK optimization

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=27842

            Bug ID: 27842
           Summary: Race condition in _IO_FLAGS2_NEED_LOCK optimization
           Product: glibc
           Version: 2.34
            Status: NEW
          Severity: minor
          Priority: P3
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

flockfile uses a read-modify-write operation on _flags2 outside a lock region,
which can potentially clobber concurrent _flags2 updates performed under the
lock.

Found via code review, no reproducer known.

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[Bug stdio/27842] Race condition in _IO_FLAGS2_NEED_LOCK optimization

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27842

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Wilco Dijkstra <wilco@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=f107b7b30d815f191181617e2c14f21e6af4ddcf

commit f107b7b30d815f191181617e2c14f21e6af4ddcf
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Fri Jun 10 13:33:26 2022 +0100

    libio: Avoid RMW of flags2 outside lock (BZ #27842)

    Remove an unconditional RMW on flags2 in flockfile - we don't need to
change
    _IO_FLAGS2_NEED_LOCK since it isn't used in flockfile or funlockfile.
    This fixes BZ #27842.

    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

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