Accessing non-existing varargs argument

Florian Weimer fweimer@redhat.com
Fri Dec 12 11:51:00 GMT 2014


On 12/12/2014 12:30 PM, Andrew Haley wrote:
> On 12/12/2014 10:20 AM, Florian Weimer wrote:
>> open is declared as:
>>
>>     int open(const char *pathname, int flags, ...);
>>
>> The intent is to have two overloaded variants:
>>
>>     int open(const char *pathname, int flags);
>>     int open(const char *pathname, int flags, mode_t mode);
>>
>> The presence of the mode argument depends on the flags specified.  The
>> set of such flags is known by the kernel.  I wish to avoid to maintain
>> this implementation detail in libc as well, so I want to always read the
>> mode argument, possibly passing a garbage value to the kernel.
>>
>> Is there GCC-portable way to achieve this?
>
> You're assuming that for all targets the varargs calling convention is
> the same as the non-varargs calling convention, at least for integer
> registers.  I don't think this is true for all targets, although it
> might be true for the targets GCC supports today.

No, I would be perfectly satisfied with calling va_arg unconditionally 
to read a garbage argument if that can be made GCC-portable in some way. 
  The implementation can still be vararg.

(And I think we had a related conversation about this in the context of 
the ppc64le bring-up, with the result that we practically need matching 
calling conventions. :-/)

-- 
Florian Weimer / Red Hat Product Security



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