From: Zebediah Figura Subject: Re: [PATCH v2] ntdll: Force align the stack in RtlRaiseException() on i386. Message-Id: <1cf7e06d-3344-d75d-a640-12aaeaeaec18@gmail.com> Date: Sat, 11 Jan 2020 23:04:12 -0600 In-Reply-To: References: <2a432543-68a5-9d73-5bf3-5df6ae0bccdf@gmail.com> <20200111225140.6126-1-cdavis@codeweavers.com> On 1/11/20 8:38 PM, Chip Davis wrote: > January 11, 2020 5:34 PM, "Zebediah Figura" 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 >>> --- >>> >>> 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. Huh, I'm still confused about how we can get from RtlRaiseException() to send_debug_event() without going through a WINAPI function (which should end up realigning the stack). Am I missing something? > >> >>> + "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 >