[Bug middle-end/92307] New: missing -Wstringop-overflow on a memcpy into an array with out-of-bounds variable offset

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92307

            Bug ID: 92307
           Summary: missing -Wstringop-overflow on a memcpy into an array
                    with out-of-bounds variable offset
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Even with PR89427 resolved, GCC still fails to detect the invalid accesses in
the functions below (_FORTIFY_SOURCE doesn't help because it doesn't try to
detect sizes from pointers involving variable offsets):

$ cat x.c && gcc -O2 -S -Wall x.c
char a[2];

void f (int i, const char *s)
{
  if (i < 1 || 2 < i) i = 1;
  char *p = &a[i] - 9;
  __builtin_memcpy (p, s, 2);   // writing before the beginning of a
}

void g (int i, const char *s)
{
  if (i < 1 || 2 < i) i = 1;
  char *p = &a[i] + 9;
  __builtin_memcpy (p, s, 2);   // writing past the end of a
}