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From: "siddhesh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/104964] Wrong *** buffer overflow detected
 ***: terminated - acl
Date: Fri, 25 Mar 2022 15:00:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104964

--- Comment #13 from Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> ---
It's not really a regression AFAICT, it's only more visible with __bdos bec=
ause
non-constant offsets don't stop it.  Also the problem is only with subobjec=
ts
(hence limited to _FORTIFY_SOURCE > 1 for strcpy) where the block in
addr_object_size that is supposed to deal with flex arrays at the end doesn=
't
quite do its job with nested structs.

The same reproducer tweaked a bit will crash even for __builtin_object_size:

struct __string_ext
{
  char s_str[0];
};

typedef struct
{
  int o_prefix;
  struct __string_ext i;
} string_obj;

#define SUFFIX ".suffix"

string_obj *
__acl_to_any_text (unsigned long n)
{
  unsigned long off =3D 0;
  unsigned long size =3D sizeof SUFFIX;

  string_obj *obj =3D __builtin_malloc (sizeof (string_obj) + size);

  if (n =3D=3D 0)
          __builtin_unreachable ();

  while (n-- !=3D 0)
    {
      if (off + 1 > size - sizeof SUFFIX)
        {
          size <<=3D 1;
          string_obj *tmp =3D __builtin_realloc (obj, sizeof (string_obj) +
                                               size);
          if (!tmp)
            __builtin_unreachable ();
          obj =3D tmp;
        }
      obj->i.s_str[off++] =3D 'A';
    }

  char *t =3D obj->i.s_str;
  __strcpy_chk (t, SUFFIX, __builtin_object_size (t, 1));

  return obj;
}

int
main ()
{
  string_obj *s =3D __acl_to_any_text (32);

  __builtin_printf ("%zu: %s\n", __builtin_strlen (s->i.s_str), s->i.s_str);
  return 0;
}


$ gcc/cc1 -g -o test.s -quiet -Wall -O3 fs3.c
fs3.c: In function =E2=80=98__acl_to_any_text=E2=80=99:
fs3.c:40:3: warning: =E2=80=98__builtin___memcpy_chk=E2=80=99 writing 8 byt=
es into a region of
size 0 overflows the destination [-Wstringop-overflow=3D]
   40 |   __strcpy_chk (t, SUFFIX, __builtin_object_size (t, 1));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~=