[Bug string/28475] Incorrect access attribute on memfrob

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28475

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Joseph Myers <jsm28@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=2c6cabb3a442f467e78c702cffbd5f003f87f44f

commit 2c6cabb3a442f467e78c702cffbd5f003f87f44f
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Oct 20 13:38:50 2021 +0000

    Correct access attribute on memfrob (bug 28475)

    As noted in bug 28475, the access attribute on memfrob in <string.h>
    is incorrect: the function both reads and writes the memory pointed to
    by its argument, so it needs to use __read_write__, not
    __write_only__.  This incorrect attribute results in a build failure
    for accessing uninitialized memory for s390x-linux-gnu-O3 with
    build-many-glibcs.py using GCC mainline.

    Correct the attribute.  Fixing this shows up that some calls to
    memfrob in elf/ tests are reading uninitialized memory; I'm not
    entirely sure of the purpose of those calls, but guess they are about
    ensuring that the stack space is indeed allocated at that point in the
    function, and so it matters that they are calling a function whose
    semantics are unknown to the compiler.  Thus, change the first memfrob
    call in those tests to use explicit_bzero instead, as suggested by
    Florian in
    <https://sourceware.org/pipermail/libc-alpha/2021-October/132119.html>,
    to avoid the use of uninitialized memory.

    Tested for x86_64, and with build-many-glibcs.py (GCC mainline) for
    s390x-linux-gnu-O3.

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