[Bug tree-optimization/86688] New: missing -Wstringop-overflow using a non-string local array in strnlen with excessive bound

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86688

            Bug ID: 86688
           Summary: missing -Wstringop-overflow using a non-string local
                    array in strnlen with excessive bound
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

With the solution for pr86042 applied, GCC folds certain strnlen (and strlen)
calls into constants before the attribute nonstring checker has had a chance to
diagnose uses of nonstring arguments.  As a result, while the strnlen call in
function f() is diagnosed, the corresponding call in g() is not.  They should
both be diagnosed.

$ cat d.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout d.c
__attribute__ ((nonstring)) char a[3] = "123";

int f (void)
{
  return __builtin_strnlen (a, sizeof a + 1);
}

int g (void)
{
  __attribute__ ((nonstring)) char b[3];
  __builtin_memcpy (b, "123", 3);

  return __builtin_strnlen (b, sizeof b + 1);
}


;; Function f (f, funcdef_no=0, decl_uid=1899, cgraph_uid=1, symbol_order=1)

f ()
{
  long unsigned int _1;
  int _3;

  <bb 2> [local count: 1073741825]:
  _1 = __builtin_strnlen (&a, 4);
  _3 = (int) _1;
  return _3;

}


d.c: In function ‘f’:
d.c:5:10: warning: ‘__builtin_strnlen’ argument 1 declared attribute
‘nonstring’ is smaller than the specified bound 4 [-Wstringop-overflow=]
   return __builtin_strnlen (a, sizeof a + 1);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
d.c:1:34: note: argument ‘a’ declared here
 __attribute__ ((nonstring)) char a[3] = "123";
                                  ^

;; Function g (g, funcdef_no=1, decl_uid=1902, cgraph_uid=2, symbol_order=2)

g ()
{
  <bb 2> [local count: 1073741825]:
  return 3;

}