GCC stack protector causes ntdll exception test to crash

Brendan Shanks bshanks at codeweavers.com
Fri Nov 8 19:16:12 CST 2019

While trying to run ntdll’s exception tests in 32-bit mode, I found that one test was causing Wine itself to segfault. It’s protection fault test #18, from dlls/ntdll/tests/exception.c:250:

/* test accessing a zero selector */
{ { 0x0f, 0xa8, 0x31, 0xc0, 0x8e, 0xe8, 0x65, 0xa1, 0, 0, 0, 0, 0x0f, 0xa9, 0xc3 },
  /* push %gs; xor %eax,%eax; mov %ax,%gs; mov %gs:(0),%ax; pop %gs; ret */
  6, 6, FALSE, STATUS_ACCESS_VIOLATION, 2, { 0, 0xffffffff } },

That test runs and triggers a segfault as expected, but another segfault is triggered in the prologue of segv_handler when it’s trying to copy the stack canary from TLS:

   0x7bcbb728:	push   %ebp
   0x7bcbb729:	mov    %esp,%ebp
   0x7bcbb72b:	sub    $0x48,%esp
   0x7bcbb72e:	mov    0xc(%ebp),%eax
   0x7bcbb731:	mov    %eax,-0x3c(%ebp)
   0x7bcbb734:	mov    0x10(%ebp),%eax
   0x7bcbb737:	mov    %eax,-0x40(%ebp)
=> 0x7bcbb73a:	mov    %gs:0x14,%eax
   0x7bcbb740:	mov    %eax,-0xc(%ebp)
   0x7bcbb743:	xor    %eax,%eax

Obviously %gs does not point to the TLS area and that causes the crash. More detail on how the stack protector works: https://www.elttam.com.au/blog/playing-with-canaries/

I’m using Gentoo, which builds gcc (8.2.0 in my case) with ‘—enable-default-ssp’, enabling '-fstack-protector-strong’ by default for all compiles. I believe Arch also builds gcc and clang with this flag (https://www.reddit.com/r/archlinux/comments/6n5tkp/arch_now_enables_pie_and_ssp_by_default_in_gcc/).

After rebuilding Wine with CFLAGS="-fno-stack-protector", the test passes.
I’m not sure what the best fix is:
- always build Wine with “-fno-stack-protector” added to CFLAGS if the compiler supports it?
- use pragmas to disable stack protection just for signal handlers that might get called with a trashed %gs?
- if it’s possible, add an assembly wrapper for segv_handler to set the correct %gs?
- or, is this test (and setting %gs) even representative of something 32-bit Windows apps do?


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