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

[Rémi Bernon]

On 3/30/21 8:33 PM, Alexandre Julliard wrote:
> Rémi Bernon <rbernon at codeweavers.com> 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 meant on the other side? But I guess the next part answers it, as I 
understand we have to keep a bit of space for anything that would need a 
stack while we clear the pages.

> 
>> 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.
> 
-- 
Rémi Bernon <rbernon at codeweavers.com>