[PATCH v2] ntdll: Force align the stack in RtlRaiseException() on i386.

[Chip Davis]

January 11, 2020 5:34 PM, "Zebediah Figura" <z.figura12 at gmail.com> wrote:

> Hello Chip,
> 
> On 1/11/20 4:51 PM, Chip Davis wrote:
> 
>> This is an assembly function, so we have to do it manually.
>> 
>> Signed-off-by: Chip Davis <cdavis at codeweavers.com>
>> ---
>> 
>> Notes:
>> v2: Fix saving EAX. Unfortunately, I need a register because of the 'subl'
>> instruction.
>> 
>> dlls/ntdll/signal_i386.c | 10 +++++++++-
>> 1 file changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/dlls/ntdll/signal_i386.c b/dlls/ntdll/signal_i386.c
>> index e9dd0de2fc3c..099e10a3ebf2 100644
>> --- a/dlls/ntdll/signal_i386.c
>> +++ b/dlls/ntdll/signal_i386.c
>> @@ -2548,14 +2548,22 @@ __ASM_STDCALL_FUNC( RtlRaiseException, 4,
>> "movl %esp,%ebp\n\t"
>> __ASM_CFI(".cfi_def_cfa_register %ebp\n\t")
>> "leal -0x2cc(%esp),%esp\n\t" /* sizeof(CONTEXT) */
>> - "pushl %esp\n\t" /* context */
>> + "andl $-16,%esp\n\t"
>> + "subl $16,%esp\n\t"
>> + "movl %eax,(%esp)\n\t"
>> + "movl %esp,%eax\n\t"
>> + "subl $12,%esp\n\t" /* stack alignment */
>> + "pushl %eax\n\t" /* context */
> 
> Out of curiosity, why do you need to align the stack again before
> calling RtlCaptureContext()? I don't see anything in our implementation
> that I'm aware relies on an aligned stack.
> 
> And if so, perhaps it'd be better to align the stack inside
> RtlCaptureContext() instead of here?

It isn't needed for RtlCaptureContext(). It's hand-hacked assembly and doesn't have any instructions in it that assume a 16-byte-aligned stack.

I was seeing crashes in send_debug_event(), which I tracked down to the compiler generating an aligned SSE instruction (movaps) on the assumption that the stack was 16-byte aligned. For some reason, adding this here made the crashes go away. These embedded aligned SSE instructions are part of why we compile exported functions with force_align_arg_pointer in the first place. Here we have to do it manually, because of the hand-hacked assembly.

> 
>> + "movl 16(%esp),%eax\n\t"
>> "call " __ASM_STDCALL("RtlCaptureContext",4) "\n\t"
>> + "addl $12,%esp\n\t"
>> "movl 4(%ebp),%eax\n\t" /* return address */
>> "movl 8(%ebp),%ecx\n\t" /* rec */
>> "movl %eax,12(%ecx)\n\t" /* rec->ExceptionAddress */
>> "leal 12(%ebp),%eax\n\t"
>> "movl %eax,0xc4(%esp)\n\t" /* context->Esp */
>> "movl %esp,%eax\n\t"
>> + "subl $8,%esp\n\t" /* stack alignment */
>> "pushl %eax\n\t"
>> "pushl %ecx\n\t"
>> "call " __ASM_NAME("raise_exception_full_context") "\n\t"


Chip