Re: [PATCH v4 4/4] i386: Fix i386 sigaction sa_restorer initialization (BZ#21269)

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Feb 12, 2018 at 4:42 AM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> This patch fixes the i386 sa_restorer field initialization for sigaction
> syscall for kernel with vDSO.  As described in bug report, x86_32 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.
>
> (I added some comments of my understanding of how the testcase is triggering
> the issue, so if someone with more x86 knowledge could check if I get this
> right I will be grateful)
>
> 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.
>
> Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

LGTM.

Thanks.

-- 
H.J.