Re: [PATCH 2/2] ntdll: Clear the first two stack pages in signal_start_thread.

[Alexandre Julliard]

Rémi Bernon  writes:
> > @@ -1540,10 +1540,8 @@ __ASM_GLOBAL_FUNC( signal_start_thread,
> >                      "movq %rcx,%rbx\n\t"        /* context */
> >                      /* clear the thread stack */
> >                      "andq $~0xfff,%rcx\n\t"     /* round down to page size */
> > -
> >               "movq %gs:0x30,%rax\n\t"
> > -
> >               "movq 0x10(%rax),%rdi\n\t"  /* NtCurrentTeb()->Tib.StackLimit */
> > -
> >               "addq $0x2000,%rdi\n\t"
> > -
> >               "movq %rdi,%rsp\n\t"
> > -
> >               "movq %rcx,%rdi\n\t"
> > -
> >               "subq $0x2000,%rdi\n\t"
> >                "subq %rdi,%rcx\n\t"
> >                "xorl %eax,%eax\n\t"
> >                "shrq $3,%rcx\n\t"
>
> Also out of curiosity, what was the reason not to clear the last (last
> two on x86_64) stack page(s) here?

There's already data there, like the initial context.
> I also removed the rsp stores, assuming it wasn't used for rep movs
> anyway but I'm now thinking that maybe all this was here for the case
> we get signaled while clearing the pages?

Yes, in general touching the stack below the stack pointer is not a good
idea.