[Bug libc/28531] New: [x86] Cancelable syscall stubs fail to preserve 16-byte stack alignment

["schwab@linux-m68k.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28531

            Bug ID: 28531
           Summary: [x86] Cancelable syscall stubs fail to preserve
                    16-byte stack alignment
           Product: glibc
           Version: 2.22
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: schwab@linux-m68k.org
                CC: drepper.fsp at gmail dot com, hjl.tools at gmail dot com
  Target Milestone: ---
              Host: i?86-*-linux*

On function entry, stack alignment is 4 mod 16, and there are function calls
without realigning the stack.

00000000 <__GI___nanosleep>:
   0:   65 83 3d 0c 00 00 00    cmpl   $0x0,%gs:0xc
   7:   00 
   8:   75 20                   jne    2a <__nanosleep_nocancel+0x20>

0000000a <__nanosleep_nocancel>:
   a:   89 da                   mov    %ebx,%edx
   c:   8b 4c 24 08             mov    0x8(%esp),%ecx
  10:   8b 5c 24 04             mov    0x4(%esp),%ebx
  14:   b8 a2 00 00 00          mov    $0xa2,%eax
  19:   65 ff 15 10 00 00 00    call   *%gs:0x10
  20:   89 d3                   mov    %edx,%ebx
  22:   3d 01 f0 ff ff          cmp    $0xfffff001,%eax
  27:   73 30                   jae    59 <__nanosleep_nocancel+0x4f>
  29:   c3                      ret    
  2a:   e8 fc ff ff ff          call   2b <__nanosleep_nocancel+0x21>
                        2b: R_386_PC32  __libc_enable_asynccancel

This is dormant on master, since there are no longer any cancelable syscalls
that are implemented using the stubs.

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