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From: "dmalcolm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug analyzer/106000] RFE: -fanalyzer should complain about memory
 accesses that are definitely out-of-bounds
Date: Sat, 02 Jul 2022 10:07:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106000

--- Comment #4 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
For example, the "classic test" referred to in section 1.2 of
  https://open-std.org/JTC1/SC22/WG14/www/docs/n3005.pdf

has:

#include <stdio.h>
#include <string.h>
int y=3D2, x=3D1;
int main() {
  int *p =3D &x + 1;
  int *q =3D &y;
  printf("Addresses: p=3D%p q=3D%p\n" ,(void*)p,(void*)q);
  if (memcmp(&p, &q, sizeof(p)) =3D=3D 0) {
    *p =3D 11;   // does  this  have  undefined  behaviour?
    printf("x=3D%d y=3D%d *p=3D%d *q=3D%d\n",x,y,*p,*q);
  }
}

where N3005 notes that "the mere formation of the &x+1 one-past pointer is
explicitly permitted by the ISO standard".

I think -fanalyzer ought to complain with an definite-out-of-bounds warning=
 at
the *p dereference: assuming sizeof(int) =3D=3D 4, we'd have a decl_region =
of size
4, where only bytes 0 to 3 are validly accessible, whereas here the code
attempts to accessing bytes 4-7 of the decl_region for x, which is
out-of-bounds.

(I think the memcpy result would be a conjured_svalue, and hence we would
consider both true and false out-edges after the test; if the user is relyi=
ng
on the two vars to be next to each other in memory we ought to be warning t=
hem
about that)=