[Bug middle-end/92814] New: missing -Wstringop-overflow writing into a dynamically allocated flexible array member

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92814

            Bug ID: 92814
           Summary: missing -Wstringop-overflow writing into a dynamically
                    allocated flexible array member
           Product: gcc
           Version: 10.0
            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: ---

With the first patch for pr91582 applied
(https://gcc.gnu.org/ml/gcc-patches/2019-11/msg00652.html) and with
-Wno-array-bounds, GCC 10 issues -Wstringop-overflow for the buffer overflow in
g() but not in h().  Both should be diagnosed by -Wstringop-overflow.

$ cat t.c && gcc -O2 -S -Wall -Wno-array-bounds t.c
void f (void*);

struct S { int n; char ax[]; };

struct S s = { 3, { 1, 2, 3 } };

void g (void)
{
  __builtin_strcpy (s.ax, "123");   // -Wstringop-overflow (good)
}

void h (void)
{
  struct S *p = (struct S*)__builtin_malloc (sizeof *p + 3);

  __builtin_strcpy (p->ax, "123");   // no warning (w/o -Warray-bounds)

  f (p);
}
t.c: In function ‘g’:
t.c:9:3: warning: ‘__builtin_memcpy’ writing 4 bytes into a region of size 3
overflows the destination [-Wstringop-overflow=]
    9 |   __builtin_strcpy (s.ax, "123");   // -Wstringop-overflow (good)
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~