[webkit-dev] stack alignment bug

Gavin Barraclough barraclough at apple.com
Thu Jun 4 04:13:10 PDT 2009

Hi Zoltan,

I'm a little confused – maybe I'm misunderstanding you, but the JIT  
does just subtract a fixed amount from the stack pointer on entry (28  
on x86, for a total frame size including return address, caller frame  
pointer and callee preserved registers of 48, a multiple of 16 to  
preserve stack alignment).  All JIT code then runs at the same stack  
depth.  The only pops in the JIT are simply removing the the return  
address implicitly pushed on x86, and the only pushes (bar a function  
call in put property access transition realloc) are restoring the  
return address prior to a return (or a tail call).  It is not clear to  
me what you're envisaging 'fake_sp' would be used for.

I've just landed a patch to move the pushes & pops in wrapper  
functions, and to switch put transition realloc to use a regular  
function call, hopefully this tidies things up a little.


On Jun 4, 2009, at 1:19 AM, Zoltan Herczeg wrote:

> Hi,
> actually there was a bug which took me a day to find out what  
> happened. It
> was somewhere deep in libc, called by a function in DateMath.cpp. It
> seemed that the stack was overwritten. By libc??? I can't belive it.
> Finally I realized that gcc's alloca realigned the stack (to 8 bytes  
> on
> ARM), so everything was in a wrong place (looked overwritten at first
> sight).
> My fake stack pointer idea:
> fake_sp: any non-volatile general purpose register
> JIT_entry:
>  mov fake_sp, sp
>  sub sp, sp, 32   ; I belive this is enough for the JIT,
>                   ; correct me if I am wrong
>  ; use fake_sp instead of sp for push/pops
> JIT_leava:
>  add sp, sp, 32
> I hope this even works for PPC (if someone ever wants to port the  
> JIT to
> old macs).
> Zoltan
>> Zoltan,
>> I filed a bug here: https://bugs.webkit.org/show_bug.cgi?id=26164
>> Stack is originally aligned then jit code destroys it; and, some data
>> structure or point to double is not aligned and I'm still trying to  
>> find
>> where they are.
>> I'm not sure how the fake stack would be, would you mind explains a  
>> bit
>> more?
>> Did you face same problem?
>> Thanks also for your articles that gives new ideas.
>> rgds
>> joe
>> --- On Wed, 6/3/09, Zoltan Herczeg <zherczeg at inf.u-szeged.hu> wrote:
>>> From: Zoltan Herczeg <zherczeg at inf.u-szeged.hu>
>>> Subject: Re: [webkit-dev] stack alignment bug
>>> To: "x yz" <lastguy at yahoo.com>
>>> Cc: webkit-dev at lists.webkit.org
>>> Date: Wednesday, June 3, 2009, 7:35 PM
>>> Hi,
>>> true, some architectures have strict policies for stack
>>> handling. Perhaps
>>> the worst one is PowerPC with its organized stack frame
>>> (back chains,
>>> pre-defined register save areas, etc). I think a fake stack
>>> pointer for
>>> JIT can solve the x86 compatibility problem.
>>> 1) allocate enough aligned stack space for the worst case
>>> when you enter
>>> to JIT
>>> 2) the fake stack pointer should use this pre-allocated
>>> stack frame.
>>> Zoltan
>>>> I don't know how to file bug so I posted here.
>>>> In privateCompileCTIMachineTrampolines() there are
>>> multiple align() to
>>>> align code on 16byte margin, yet, the stack can be put
>>> on 32bit margin
>>>> that causes crush.
>>>> Suppose original stack is aligned to 8/16bytes, the
>>> above function
>>>> frequently pop/push regT3 that makes stack
>>> mis-aligned. Then int to double
>>>> conversion uses stack that will fail.
>>>> rgds
>>>> joe
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