On Tue, Mar 17, 2020 at 9:40 AM Jakub Jelinek via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi!
>
> The gcc.dg/pr68785.c test which contains:
> int
> foo (void)
> {
> return *(int *) "";
> }
> has UB in the program if it is ever called, but causes UB in the compiler
> as well as at least in theory non-reproduceable code generation.
> The problem is that nbytes is in this case 4, prep is the
> TREE_STRING_POINTER of a "" string literal with TREE_STRING_LENGTH of 1 and
> we do:
> 4890 for (const char *p = prep; p != prep + nbytes; ++p)
> 4891 if (*p)
> 4892 {
> 4893 *allnul = false;
> 4894 break;
> 4895 }
> and so read the bytes after the STRING_CST payload, which can be random.
> I think we should just punt in this case.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Richard.
> 2020-03-16 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/94187
> * tree-ssa-strlen.c (count_nonzero_bytes): Punt if
> nchars - offset < nbytes.
>
> --- gcc/tree-ssa-strlen.c.jj 2020-03-14 08:14:47.034742349 +0100
> +++ gcc/tree-ssa-strlen.c 2020-03-16 12:23:57.523534887 +0100
> @@ -4822,6 +4822,8 @@ count_nonzero_bytes (tree exp, unsigned
> of the access), set it here to the size of the string, including
> all internal and trailing nuls if the string has any. */
> nbytes = nchars - offset;
> + else if (nchars - offset < nbytes)
> + return false;
>
> prep = TREE_STRING_POINTER (exp) + offset;
> }
>
>
> Jakub
>
