[Bug dynamic-link/21269] i386 sigaction sa_restorer handling is wrong

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https://sourceware.org/bugzilla/show_bug.cgi?id=21269

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=3241353ab20b4cc8798088d456ffa9aace1514de

commit 3241353ab20b4cc8798088d456ffa9aace1514de
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Fri Nov 17 16:04:29 2017 -0200

    i386: Fix i386 sigaction sa_restorer initialization (BZ#21269)

    This patch fixes the i386 sa_restorer field initialization for sigaction
    syscall for kernel with vDSO.  As described in bug report, i386 Linux
    (and compat on x86_64) interprets SA_RESTORER clear with nonzero
    sa_restorer as a request for stack switching if the SS segment is 'funny'.
    This means that anything that tries to mix glibc's signal handling with
    segmentation (for instance through modify_ldt syscall) is randomly broken
    depending on what values lands in sa_restorer.

    The testcase added  is based on Linux test
tools/testing/selftests/x86/ldt_gdt.c,
    more specifically in do_multicpu_tests function.  The main changes are:

      - C11 atomics instead of plain access.

      - Remove x86_64 support which simplifies the syscall handling and
fallbacks.

      - Replicate only the test required to trigger the issue.

    Checked on i686-linux-gnu.

        [BZ #21269]
        * sysdeps/unix/sysv/linux/i386/Makefile (tests): Add tst-bz21269.
        * sysdeps/unix/sysv/linux/i386/sigaction.c (SET_SA_RESTORER): Clear
        sa_restorer for vDSO case.
        * sysdeps/unix/sysv/linux/i386/tst-bz21269.c: New file.

    (cherry picked from commit 68448be208ee06e76665918b37b0a57e3e00c8b4)

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                                  |    8 +
 NEWS                                       |    1 +
 sysdeps/unix/sysv/linux/i386/Makefile      |    3 +
 sysdeps/unix/sysv/linux/i386/sigaction.c   |    3 +-
 sysdeps/unix/sysv/linux/i386/tst-bz21269.c |  233 ++++++++++++++++++++++++++++
 5 files changed, 247 insertions(+), 1 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/i386/tst-bz21269.c

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