[Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94444

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-01
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |WAITING

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Can you include a small test case that reproduce the problem (and the full
command line used to reproduce it)?  I can't think of a reason why the
attribute would be completely ineffective with -Os and a simple test case
confirms it does the right thing even with with -fno-builtin-memcpy (if the
built-in happens to be disabled while building libc).  GCC recognizes
__builtin_memcpy as special (built-ins aren't decorated with the attribute in
GCC 10) so it should have the same detection as functions explicitly declared
with the attribute.

$ cat z.c && gcc -Os -S -fno-builtin-memcpy z.c
__attribute__ ((access (write_only, 1, 3), access (read_only, 2, 3)))
void* memcpy (void*, const void*, __SIZE_TYPE__);

char a[3];

void f (const void *s)
{
  memcpy (a, s, 5);
}

void g (const void *s)
{
  __builtin_memcpy (a, s, 5);
}

z.c: In function ‘f’:
z.c:8:3: warning: ‘memcpy’ writing 5 bytes into a region of size 3 overflows
the destination [-Wstringop-overflow=]
    8 |   memcpy (a, s, 5);
      |   ^~~~~~~~~~~~~~~~
z.c:2:7: note: in a call to function ‘memcpy’ declared with attribute
‘write_only (1, 3)’
    2 | void* memcpy (void*, const void*, __SIZE_TYPE__);
      |       ^~~~~~
z.c: In function ‘g’:
z.c:13:3: warning: ‘__builtin_memcpy’ writing 5 bytes into a region of size 3
overflows the destination [-Wstringop-overflow=]
   13 |   __builtin_memcpy (a, s, 5);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~