vsnprintf broken (was: Re: gcc update)

[Simon 'corecode' Schubert]

Simon 'corecode' Schubert wrote:
> Johannes Hofmann wrote:
>> Hello,
>>
>> I see crashes with a string handling library on DragonFly.
>> The problem can be reduced to the test program below. It crashes on
>> DragonFly when compiled with "gcc -O2 -o foo foo.c". Without -O2 it 
>> runs fine. No problems on Linux with or without -O2.
>> Can anyone spot the problem? I think its related to the use of
>> va_copy().
> 
> No, the problem is that gcc uses %ebx after a function call, which it is
> not allowed to do:
[snip]
> Or does the ABI dictate that %ebx needs to be restored?  Seems that
> linux/glibc doesn't clobber ebx.

okay, I am wrong here.  %ebx is supposed to be saved and is also being
saved by vsnprinf.  gcc is good.

So this is actually a case of stack smashing.  Have fun finding the bug
in vsnprintf or in your code :)

> cheers
>   simon


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