[Qemu-devel] gcc auto-omit-frame-pointer vs msvc longjmp

Bob Breuer breuerr@mc.net
Tue Oct 18 04:20:00 GMT 2011


Kai Tietz wrote:
> 2011/10/17 Bob Breuer <breuerr@mc.net>:
>> Richard Henderson wrote:
>>> On 10/17/2011 07:09 AM, Bob Breuer wrote:
>>>> I don't think this is a free/g_free issue.  If I use the following
>>>> patch, then I at least get the openbios messages:
>>>>
>>>> diff --git a/cpu-exec.c b/cpu-exec.c
>>>> index a9fa608..dfbd6ea 100644
>>>> --- a/cpu-exec.c
>>>> +++ b/cpu-exec.c
>>>> @@ -180,6 +180,7 @@ static void cpu_handle_debug_exception(CPUState
>>>>  /* main execution loop */
>>>>
>>>>  volatile sig_atomic_t exit_request;
>>>> +register void *ebp asm("ebp");
>>>>
>>>>  int cpu_exec(CPUState *env)
>>>>  {
>>>> @@ -233,6 +234,8 @@ int cpu_exec(CPUState *env)
>>>>
>>>>      /* prepare setjmp context for exception handling */
>>>>      for(;;) {
>>>> +        int dummy = 0;
>>>> +        ebp = &dummy;
>>> See if
>>>
>>>   asm("" : : : "ebp");
>>>
>>> also solves the problem.
>> No, that doesn't fix it.
>>
>>>> Google finds a mention of longjmp failing with -fomit-frame-pointer:
>>>> http://lua-users.org/lists/lua-l/2005-02/msg00158.html
>>>>
>>>> Looks like gcc 4.6 turns on -fomit-frame-pointer by default.
>>> Hmm.  This is the first I've heard of a longjmp implementation
>>> failing without a frame pointer.  Presumably this is with the
>>> mingw i.e. msvc libc?
>> Yeah, mingw from www.mingw.org which I believe uses msvcrt.dll, package
>> gcc-core-4.6.1-2-mingw32-bin.
>>
>>> This is something that could be worked around in gcc, I suppose.
>>> We recognize longjmp for some things, we could force the use of
>>> a frame pointer for msvc targets too.
>>>
>>> For now it might be best to simply force -fno-omit-frame-pointer
>>> for mingw host in the configure script.
>> Here's a testcase that crashes on the longjmp:
>>
>> #include <stdio.h>
>> #include <setjmp.h>
>>
>> jmp_buf env;
>>
>> int test(void)
>> {
>>  int i;
>>
>>  asm("xor %%ebp,%%ebp" ::: "ebp");
>>
>>  i = setjmp(env);
>>  printf("i = %d\n", i);
>>
>>  if (i == 0)
>>    longjmp(env, 2);
>>
>>  return i;
>> }
>>
>> int main(void)
>> {
>>  return test();
>> }
>>
>> Remove the asm statement to make it not crash.  Obviously with
>> omit-frame-pointer, gcc can shove anything into ebp.
>>
>> Bob
> 
> This crash isn'r related to ebp existing, or not. The issue is the
> hidden argument of setjmp, which is missing.  If you can try the
> following at top of file after include section.
> 
> #define setjmp(BUF) _setjmpex((BUF), NULL)
> int __cdecl __attribute__ ((__nothrow__,__returns_twice__))
> _setjmp3(jmp_buf _Buf, void *_Ctx);
> ...

Did you mean _setjmp3 instead of _setjmpex?  With _setjmp3, it works
without the asm, but still crashes if I zero out ebp before the setjmp.
 Aren't the function arguments on the stack anyway?

> 
> This will work as expected with or without omit-frame-pointer.
> 
> The issue is that setjmp has a second (undocumented as usual)
> argument, which has a meaning.

So why does my testcase above fail with the asm, but work without the
asm statement?  Compile it with gcc -O2 and try it yourself.

> 
> Regards,
> Kai
> 
> PS:  _setjmp3 is an export from msvcrt.dll.  So if symbol is missing
> on link, simply specify msvcrt.dll as argument to link-line.




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