This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: memset zero bytes at NULL - isolate-erroneous-paths
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Florian Weimer <fw at deneb dot enyo dot de>, Jeff Law <law at redhat dot com>, Jonathan Wakely <jwakely dot gcc at gmail dot com>, Dominic News <d dot newsgroups at googlemail dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Mon, 18 Nov 2013 12:13:58 +0100
- Subject: Re: memset zero bytes at NULL - isolate-erroneous-paths
- Authentication-results: sourceware.org; auth=none
- References: <CALx3oLxLBCYez=jUiD7sLE-ynVHZ0Dr9QfX3QeLqCP-1p2p1nw at mail dot gmail dot com> <CAH6eHdSS9q-McGntA1+61TSe8EU_GVZ67RiFdLkr2APWVT7Evg at mail dot gmail dot com> <87d2lyn0u8 dot fsf at mid dot deneb dot enyo dot de> <5289980B dot 4080309 at redhat dot com> <87habachfy dot fsf at mid dot deneb dot enyo dot de> <CAFiYyc1riMiOkoJWzdGU6rJe7p263K2A4noHw9-1doY0XT=7=g at mail dot gmail dot com>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Mon, Nov 18, 2013 at 12:08:27PM +0100, Richard Biener wrote:
> I'd say that turning memset (0, '\0', 0) into a trap is bad from a QOI
> perspective. Jeff, is there an easy way to avoid this? Testcase:
>
> void fn (void *addr, int a)
> {
> if (a == 0)
> addr = (void *)0;
> __builtin_memset (addr, '\0', a);
> }
>
> I wonder where in isolate-paths you check for builtins at all? ah,
> it's probably from the nonnull attribute on memset. Which also
> means that trying to catch this case reliably isn't going to work
> (you cannot prove the program has len == 0 in this case and
> conditionally not trapping would somewhat defeat the purpose
> of isolating this path)
Well, if some function has nonnull attribute on some argument, then that
argument shouldn't have NULL value even if some length argument is 0.
In the case of memset (and various other functions) C99 clearly says that
memset (NULL, 0, 0); is invalid, if there are some functions that have
a pointer, length argument pair and for length 0 pointer is allowed to be
NULL, then those functions shouldn't have nonnull attribute.
Jakub