sprintf warning on overlapping output

Jeff Law law@redhat.com
Mon Sep 26 15:34:00 GMT 2016

On 09/26/2016 09:20 AM, Florian Weimer wrote:
> * Jeff Law:
>> On 09/25/2016 03:46 AM, Bernd Edlinger wrote:
>>> Hi Martin,
>>> in the past I have seen (and fixed) code like
>>> sprintf(buf, "%s %d", buf, x);
>>> that may possibly work by chance, but usually
>>> produces undefined results.
>>> Do you see a way to enhance the warning for cases
>>> where the output buffer overlaps an input buffer?
>> ISTM you really need strong PTA analysis here to nail down the
>> pointers to a single object, then you can query their ranges and look
>> for overlap.
> We could detect this at run time in glibc with reasonable cost, I
> think.  We should probably introduce new symbol versions if we do
> that, to avoid breaking existing applications needlessly.
Probably not a bad idea.  memstomp doesn't catch it for sprintf & 
friends -- it was deemed too painful to detect :-)

> Or we could make this well-defined because it is such a useful
> extension.
That just encourages developers to write non-portable code.  I'd rather 
see this kind of thing halt the program in its tracks before wandering 
into the realm of undefined or implementation defined behavior.


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