[llvm-dev] asan for allocas on powerpc64

[Maxim Ostapenko via llvm-dev]

Hi!

Sorry for delay, just returned from vacation.

On 12/11/15 23:44, Kostya Serebryany via llvm-dev wrote:
> +Maxim and Yuri, as I think this is their code.
>
> On Thu, Nov 12, 2015 at 3:02 AM, Jay Foad <jay.foad at gmail.com 
> <mailto:jay.foad at gmail.com>> wrote:
>
>     (Resending with the correct mailing list address.)
>
>     Hi,
>
>     Currently test/asan/TestCases/alloca_vla_interact.cc is XFAILed for
>     powerpc64. I've had a look at why it doesn't work. I think the only
>     problem is in the call to __asan_allocas_unpoison that is inserted at
>     the end of the "for" loop (just before a stackrestore instruction).
>
>     The call function is created something like this (paraphrasing from
>     lib/Transfoms/Instrumentation/AddressSanitizer.cpp):
>
>       // call __asan_allocas_unpoison(uptr top, uptr bottom);
>       // NB "top" here means lowest address and "bottom" means highest!
>
>       IRB.CreateCall(
>         AsanAllocasUnpoisonFunc,
>         {
>           IRB.CreateLoad(DynamicAllocaLayout),
>     IRB.CreatePointerToInt(SaveRestoreInst->getOperand(0), IntptrTy)
>         }
>       );
>
>     I think the problem is that the operand to stackrestore is the new
>     native sp register value to restore, and this code is assuming that
>     that will be a higher address than all the allocas that are being
>     unallocated. But on PowerPC64, the native sp is always lower than the
>     address of the most recent alloca by MaxCallFrameSize bytes, to leave
>     space for outgoing call arguments. So I think the second argument to
>     __asan_allocas_unpoison needs to be SaveRestoreInst->getOperand(0) +
>     MaxCallFrameSize, but I don't know how to implement that.
>
>     Thoughts?
>

Yeah, you are right, we rely on stackrestore to get "bottom" parameter 
for __asan_allocas_unpoison and indeed PowerPC64 seems to be special here.
However, right now I don't see a suitable way how to get 
MaxCallFrameSize from LLVM backend in ASan pass, investigation is in 
progress.

Thank you again for spending your time on this!

-Maxim

>     Thanks,
>     Jay.
>
>
>
>
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