On Tue, 2014-08-12 at 19:23 +0100, Ben Hutchings wrote:
> On Tue, 2014-08-12 at 09:05 -0700, Steve Langasek wrote:
> [...]
> > Matthias, could you please have a look at the below test case? We have a
> > regression in the latest nfs-kernel-server build, which appears to be caused
> > by a gcc-4.9 bug.
> >
> > Should I work around this in nfs-utils, or is a quick fix possible in
> > gcc-4.9?
> >
> > > char buf[100];
> > >
> > > void
> > > add_name(char *old)
> > > {
> > > char *cp = old;
> > >
> > > while (cp && *cp) {
> > > cp++;
> > > }
> > > __builtin_strncpy(buf, old, cp-old);
> [...]
>
> So far as I know (haven't checked the latest standard), pointer
> subtraction has undefined behaviour unless both operands point into (or
> one beyond) the same array. As this is not true of null pointers, the
> compiler may infer that old can't be null, so cp can't be null, so there
> is no need to check whether it is.
>
> I.e. this is a bug in nfs-utils, not the compiler.
By the way, in case there are similar bugs elsewhere in nfs-utils, it
may be sensible to work around this with the compiler option
-fno-delete-null-pointer-checks.
Ben.
--
Ben Hutchings
Humans are not rational beings; they are rationalising beings.
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