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Re: GCC 4.9.2 -O3 gives a seg fault / GCC 4.8.2 -O3 works


On Tue, Jan 06, 2015 at 08:50:58AM -0500, Paul Smith wrote:
> On Tue, 2015-01-06 at 09:43 +0100, Jakub Jelinek wrote:
> > On Tue, Jan 06, 2015 at 03:18:48AM -0500, Paul Smith wrote:
> > > Hi all.  It's possible my code is doing something illegal, but it's also
> > > possible I've found a problem with -O3 optimization in GCC 4.9.2.  I've
> > > built this same code with GCC 4.8.2 -O3 on GNU/Linux and it works fine.
> > > It also works with GCC 4.9.2 with lower -O (-O2 for example).
> > 
> > Your testcase is invalid.
> > GCC trunk -fsanitize=undefined (in particular -fsanitize=nonnull-attribute)
> > diagnoses it:
> > /tmp/mystring.cpp:103:26: runtime error: null pointer passed as argument 2, which is declared to never be null
> > LD_PRELOAD=libmemstomp.so detects it too.
> > 
> > Calling memcpy (p, NULL, 0); is invalid according to C and C++
> > standards, you need to guard it, e.g. with if (data) memcpy (p, data, len1);
> > or if (len1) memcpy (p, data, len1);
> 
> Ah interesting.  You're right, this is definitely not correct.  But
> since len1 is 0 in this case, no implementation of memcpy() actually
> tried to dereference the data pointer and so there was no failure (we
> build and test with clang on OSX and MSVC on Windows, and run with
> valgrind and ASAN (clang)).
> 
> I'll have to look at other possible failure situations.

Note, it is even mentioned in GCC 4.9 porting to documentation:
https://gcc.gnu.org/gcc-4.9/porting_to.html

	Jakub


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