[Bug middle-end/79016] missing -Wstringop-overflow= overflowing allocated buffers

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79016

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
This is also affects overflowing buffers allocated by a user-defined function
declared with attribute alloc_size.

$ cat t.c && gcc -D_FORTIFY_SOURCE=2 -O2 -S -Wall
-fdump-tree-optimized=/dev/stdout t.c
#include <string.h>

int* __attribute__ ((alloc_size (1)))
f (int);

void* g (const void *s)
{
  void *p = f (8);
  __builtin_memcpy (p, s, 9);   // missing -Wstringop-overflow
  return p;
}

void* h (const void *s)
{
  void *p = f (8);
  memcpy (p, s, 9);   // -Wstringop-overflow with _FORTIFY_SOURCE
  return p;
}

;; Function g (g, funcdef_no=14, decl_uid=2244, cgraph_uid=14, symbol_order=14)

g (const void * s)
{
  void * p;

  <bb 2> [local count: 1073741825]:
  p_3 = f (8);
  __builtin_memcpy (p_3, s_4(D), 9);
  return p_3;

}



;; Function h (h, funcdef_no=15, decl_uid=2248, cgraph_uid=15, symbol_order=15)

h (const void * s)
{
  void * p;

  <bb 2> [local count: 1073741825]:
  p_3 = f (8);
  __builtin___memcpy_chk (p_3, s_4(D), 9, 8);
  return p_3;

}


In file included from /usr/include/string.h:635,
                 from t.c:1:
In function ‘memcpy’,
    inlined from ‘h’ at t.c:16:3:
/usr/include/bits/string3.h:53:10: warning: ‘__builtin___memcpy_chk’ writing 9
bytes into a region of size 8 overflows the destination [-Wstringop-overflow=]
   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~