Re: undefined-behavior obstack.c:139

[Jeffrey Walton]

On Fri, Dec 1, 2023 at 8:15 AM Bruno Haible <bruno@clisp.org> wrote:
>
> [CCing bug-gnulib because obstack.c comes from gnulib.]
>
> Alexey Palienko <Alexey.Palienko@cma.se> wrote in
> <https://lists.gnu.org/archive/html/bug-m4/2023-02/msg00000.html>:
>
> > It has been built by clang 13 with "-g -fsanitize=address,undefined 
> > -fno-omit-frame-pointer -fsanitize-address-use-after-scope 
> > -fno-sanitize-recover=all"
> > And we have an error:
> >
> > # 
> > /home/docker/.conan/data/m4/latest/_/_/package/3421fde5744f1eadef515027cbcbb9a8fbcd667c/bin/m4
> > obstack.c:139:35: runtime error: applying non-zero offset 107820858999056 
> > to null pointer
> > SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior obstack.c:139:35 in
>
> I reproduce the issue with the newest m4 snapshot and with clang 17.
>
> However, it's not a bug in m4, but rather a false alarm from clang's
> UndefinedBehaviorSanitizer.
>
> Namely, when I use the modified CC value
>   CC="clang 
> -fsanitize=address,undefined,signed-integer-overflow,shift,integer-divide-by-zero
>  -fno-sanitize=pointer-overflow -fsanitize-address-use-after-scope 
> -fno-sanitize-recover=all"
> there are no issues.
>
> The 'pointer-overflow' sanitizer considers adding NULL + 0 as undefined:
> ========================
> #include <stdlib.h>
> int main ()
> {
>   char *p = NULL;
>   char *q = p + 0;
>   return 0;
> }
> ========================
>
> I don't see wording to this effect in ISO C 23 § 6.5.6.(9).
> Many programs use NULL + 0 in some cases, because it avoids a gratuitous
> test against NULL.
>
> So, I recommend turning off the 'pointer-overflow' sanitizer.
>
> Bruno
>
> [1] https://gitlab.com/gnu-m4/ci-distcheck

I think that's a valid finding. NULL is not a valid address. You can't
add anything to it.

Jeff