* [Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os
2020-04-01 15:31 [Bug c/94444] New: __attribute__((access(...))) ignored for memcpy when compiling with -Os felix-gcc at fefe dot de
@ 2020-04-01 15:49 ` jakub at gcc dot gnu.org
2020-04-01 16:05 ` msebor at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2020-04-01 15:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94444
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That makes no sense, access attribute can help for functions which GCC doesn't
know in detail what they do, but e.g. memcpy is something GCC has better
understanding of compared to what access attribute can even describe, and uses
it in various optimization passes.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os
2020-04-01 15:31 [Bug c/94444] New: __attribute__((access(...))) ignored for memcpy when compiling with -Os felix-gcc at fefe dot de
2020-04-01 15:49 ` [Bug c/94444] " jakub at gcc dot gnu.org
@ 2020-04-01 16:05 ` msebor at gcc dot gnu.org
2020-04-01 17:04 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: msebor at gcc dot gnu.org @ 2020-04-01 16:05 UTC (permalink / raw)
To: gcc-bugs
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);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os
2020-04-01 15:31 [Bug c/94444] New: __attribute__((access(...))) ignored for memcpy when compiling with -Os felix-gcc at fefe dot de
2020-04-01 15:49 ` [Bug c/94444] " jakub at gcc dot gnu.org
2020-04-01 16:05 ` msebor at gcc dot gnu.org
@ 2020-04-01 17:04 ` rguenth at gcc dot gnu.org
2020-04-01 17:17 ` felix-gcc at fefe dot de
2020-04-01 19:46 ` msebor at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-04-01 17:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94444
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note the access attribute is only for diagnostics, not for optimization.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os
2020-04-01 15:31 [Bug c/94444] New: __attribute__((access(...))) ignored for memcpy when compiling with -Os felix-gcc at fefe dot de
` (2 preceding siblings ...)
2020-04-01 17:04 ` rguenth at gcc dot gnu.org
@ 2020-04-01 17:17 ` felix-gcc at fefe dot de
2020-04-01 19:46 ` msebor at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: felix-gcc at fefe dot de @ 2020-04-01 17:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94444
--- Comment #4 from felix-gcc at fefe dot de ---
Sure, here's a test case:
#include <stddef.h>
__attribute__((access(read_only,2,3), access(write_only,1,3)))
extern void* memcpy(void* dest, const void* src, size_t len);
int main() {
char buf[10];
memcpy(buf,"fnordfnord",11); // should reject or at least warn
}
$ gcc -c t.c
t.c: In function ‘main’:
t.c:8:3: warning: ‘memcpy’ writing 11 bytes into a region of size 10 overflows
the destination [-Wstringop-overflow=]
8 | memcpy(buf,"fnordfnord",11); // should reject or at least warn
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:4:14: note: in a call to function ‘memcpy’ declared with attribute
‘write_only (1, 3)’
4 | extern void* memcpy(void* dest, const void* src, size_t len);
| ^~~~~~
$ gcc -c t.c -Os
$ gcc -v
[...]
gcc version 10.0.1 20200401 (experimental) (GCC)
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug c/94444] __attribute__((access(...))) ignored for memcpy when compiling with -Os
2020-04-01 15:31 [Bug c/94444] New: __attribute__((access(...))) ignored for memcpy when compiling with -Os felix-gcc at fefe dot de
` (3 preceding siblings ...)
2020-04-01 17:17 ` felix-gcc at fefe dot de
@ 2020-04-01 19:46 ` msebor at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: msebor at gcc dot gnu.org @ 2020-04-01 19:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94444
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |RESOLVED
Resolution|--- |WONTFIX
Keywords| |diagnostic
--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
Attribute access has an effect only on calls present in the IL after all
optimizations have run. In the test case, because the local variable isn't
used after the copy, the call is removed and there is no warning. (The warning
comes back if the buffer is used or made extern or static.) This is by design,
and expected not just of the attribute but of most late warnings (including,
for example, -Warray-bounds). Doing otherwise and processing the attribute
much earlier would lead to warnings for unreachable code that most users view
as false positives. We have no plans to change it.
^ permalink raw reply [flat|nested] 6+ messages in thread