* [BUG] missing warning for pointer arithmetic out of bounds
@ 2022-12-13 19:08 Alejandro Colomar
2022-12-13 19:15 ` Alejandro Colomar
2022-12-13 19:22 ` Paul Koning
0 siblings, 2 replies; 8+ messages in thread
From: Alejandro Colomar @ 2022-12-13 19:08 UTC (permalink / raw)
To: gcc
[-- Attachment #1.1: Type: text/plain, Size: 946 bytes --]
Hi!
For the following program:
$ cat buf.c
#include <stdio.h>
int main(void)
{
char *p, buf[5];
p = buf + 6;
printf("%p\n", p);
}
There are no warnings in gcc, as I would expect:
$ gcc -Wall -Wextra buf.c -O0
Clang does warn, however:
$ clang -Weverything -Wall -Wextra buf.c -O0
buf.c:8:17: warning: format specifies type 'void *' but the argument has
type 'char *' [-Wformat-pedantic]
printf("%p\n", p);
~~ ^
%s
buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the
array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
p = buf + 6;
^ ~
buf.c:5:2: note: array 'buf' declared here
char *p, buf[5];
^
2 warnings generated.
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:08 [BUG] missing warning for pointer arithmetic out of bounds Alejandro Colomar
@ 2022-12-13 19:15 ` Alejandro Colomar
2022-12-13 19:18 ` Andrew Pinski
2022-12-13 19:22 ` [BUG] missing warning for pointer arithmetic out of bounds David Malcolm
2022-12-13 19:22 ` Paul Koning
1 sibling, 2 replies; 8+ messages in thread
From: Alejandro Colomar @ 2022-12-13 19:15 UTC (permalink / raw)
To: gcc
[-- Attachment #1.1: Type: text/plain, Size: 1299 bytes --]
On 12/13/22 20:08, Alejandro Colomar wrote:
> Hi!
>
> For the following program:
>
>
> $ cat buf.c
> #include <stdio.h>
>
> int main(void)
> {
> char *p, buf[5];
>
> p = buf + 6;
> printf("%p\n", p);
> }
>
>
> There are no warnings in gcc, as I would expect:
I just re-read my text, and it is ambiguous. I meant that I expect warnings.
>
> $ gcc -Wall -Wextra buf.c -O0
>
> Clang does warn, however:
>
> $ clang -Weverything -Wall -Wextra buf.c -O0
> buf.c:8:17: warning: format specifies type 'void *' but the argument has
> type 'char *' [-Wformat-pedantic]
> printf("%p\n", p);
> ~~ ^
> %s
> buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the
> array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> p = buf + 6;
> ^ ~
> buf.c:5:2: note: array 'buf' declared here
> char *p, buf[5];
> ^
> 2 warnings generated.
>
> Cheers,
>
> Alex
>
>
--
<http://www.alejandro-colomar.es/>
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:15 ` Alejandro Colomar
@ 2022-12-13 19:18 ` Andrew Pinski
2022-12-18 12:48 ` Optimization levels for getting all warnings (was: [BUG] missing warning for pointer arithmetic out of bounds) Alejandro Colomar
2022-12-13 19:22 ` [BUG] missing warning for pointer arithmetic out of bounds David Malcolm
1 sibling, 1 reply; 8+ messages in thread
From: Andrew Pinski @ 2022-12-13 19:18 UTC (permalink / raw)
To: Alejandro Colomar; +Cc: gcc
On Tue, Dec 13, 2022 at 11:16 AM Alejandro Colomar via Gcc
<gcc@gcc.gnu.org> wrote:
>
>
>
> On 12/13/22 20:08, Alejandro Colomar wrote:
> > Hi!
> >
> > For the following program:
> >
> >
> > $ cat buf.c
> > #include <stdio.h>
> >
> > int main(void)
> > {
> > char *p, buf[5];
> >
> > p = buf + 6;
> > printf("%p\n", p);
> > }
> >
> >
> > There are no warnings in gcc, as I would expect:
>
> I just re-read my text, and it is ambiguous. I meant that I expect warnings.
GCC only warns during VRP which is only enabled at -O2:
<source>:8:12: warning: array subscript 6 is outside array bounds of
'char[5]' [-Warray-bounds=]
8 | p = buf + 6;
| ~~^~~~~~~~~
<source>:6:19: note: at offset 6 into object 'buf' of size 5
6 | char *p, buf[5];
| ^~~
Thanks,
Andrew
>
>
> >
> > $ gcc -Wall -Wextra buf.c -O0
> >
> > Clang does warn, however:
> >
> > $ clang -Weverything -Wall -Wextra buf.c -O0
> > buf.c:8:17: warning: format specifies type 'void *' but the argument has
> > type 'char *' [-Wformat-pedantic]
> > printf("%p\n", p);
> > ~~ ^
> > %s
> > buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the
> > array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> > p = buf + 6;
> > ^ ~
> > buf.c:5:2: note: array 'buf' declared here
> > char *p, buf[5];
> > ^
> > 2 warnings generated.
> >
> > Cheers,
> >
> > Alex
> >
> >
>
> --
> <http://www.alejandro-colomar.es/>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:15 ` Alejandro Colomar
2022-12-13 19:18 ` Andrew Pinski
@ 2022-12-13 19:22 ` David Malcolm
1 sibling, 0 replies; 8+ messages in thread
From: David Malcolm @ 2022-12-13 19:22 UTC (permalink / raw)
To: Alejandro Colomar, gcc
On Tue, 2022-12-13 at 20:15 +0100, Alejandro Colomar via Gcc wrote:
>
>
> On 12/13/22 20:08, Alejandro Colomar wrote:
> > Hi!
> >
> > For the following program:
> >
> >
> > $ cat buf.c
> > #include <stdio.h>
> >
> > int main(void)
> > {
> > char *p, buf[5];
> >
> > p = buf + 6;
> > printf("%p\n", p);
> > }
> >
> >
> > There are no warnings in gcc, as I would expect:
>
> I just re-read my text, and it is ambiguous. I meant that I expect
> warnings.
Yeah, it would be good to warn about such code.
Looking in bugzilla, I see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81172
which seems to talk about this issue, but that bug was closed as
resolved on 2020-04-13; I'm not sure what happened here.
Dave
>
>
> >
> > $ gcc -Wall -Wextra buf.c -O0
> >
> > Clang does warn, however:
> >
> > $ clang -Weverything -Wall -Wextra buf.c -O0
> > buf.c:8:17: warning: format specifies type 'void *' but the
> > argument has
> > type 'char *' [-Wformat-pedantic]
> > printf("%p\n", p);
> > ~~ ^
> > %s
> > buf.c:7:6: warning: the pointer incremented by 6 refers past
> > the end of the
> > array (that contains 5 elements) [-Warray-bounds-pointer-
> > arithmetic]
> > p = buf + 6;
> > ^ ~
> > buf.c:5:2: note: array 'buf' declared here
> > char *p, buf[5];
> > ^
> > 2 warnings generated.
> >
> > Cheers,
> >
> > Alex
> >
> >
>
> --
> <http://www.alejandro-colomar.es/>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:08 [BUG] missing warning for pointer arithmetic out of bounds Alejandro Colomar
2022-12-13 19:15 ` Alejandro Colomar
@ 2022-12-13 19:22 ` Paul Koning
2022-12-13 19:24 ` Alejandro Colomar
2022-12-13 20:45 ` Jonathan Wakely
1 sibling, 2 replies; 8+ messages in thread
From: Paul Koning @ 2022-12-13 19:22 UTC (permalink / raw)
To: Alejandro Colomar; +Cc: gcc
> On Dec 13, 2022, at 2:08 PM, Alejandro Colomar via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi!
>
> For the following program:
>
>
> $ cat buf.c
> #include <stdio.h>
>
> int main(void)
> {
> char *p, buf[5];
>
> p = buf + 6;
> printf("%p\n", p);
> }
>
>
> There are no warnings in gcc, as I would expect:
>
> $ gcc -Wall -Wextra buf.c -O0
>
> Clang does warn, however:
>
> $ clang -Weverything -Wall -Wextra buf.c -O0
> buf.c:8:17: warning: format specifies type 'void *' but the argument has type 'char *' [-Wformat-pedantic]
> printf("%p\n", p);
> ~~ ^
> %s
> buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> p = buf + 6;
> ^ ~
I thought void * is a generic pointer that accepts any pointer argument. So a warning about char* being passed in seems to be flat out wrong.
> buf.c:5:2: note: array 'buf' declared here
> char *p, buf[5];
> ^
> 2 warnings generated.
That was discussed just days ago: C says that a pointer one past the end of the array is legal. So here too it looks like Clang is wrong and GCC is right.
paul
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:22 ` Paul Koning
@ 2022-12-13 19:24 ` Alejandro Colomar
2022-12-13 20:45 ` Jonathan Wakely
1 sibling, 0 replies; 8+ messages in thread
From: Alejandro Colomar @ 2022-12-13 19:24 UTC (permalink / raw)
To: Paul Koning; +Cc: gcc
[-- Attachment #1.1: Type: text/plain, Size: 1611 bytes --]
Hi Paul,
On 12/13/22 20:22, Paul Koning wrote:
>
>
>> On Dec 13, 2022, at 2:08 PM, Alejandro Colomar via Gcc <gcc@gcc.gnu.org> wrote:
>>
>> Hi!
>>
>> For the following program:
>>
>>
>> $ cat buf.c
>> #include <stdio.h>
>>
>> int main(void)
>> {
>> char *p, buf[5];
>>
>> p = buf + 6;
>> printf("%p\n", p);
>> }
>>
>>
>> There are no warnings in gcc, as I would expect:
>>
>> $ gcc -Wall -Wextra buf.c -O0
>>
>> Clang does warn, however:
>>
>> $ clang -Weverything -Wall -Wextra buf.c -O0
>> buf.c:8:17: warning: format specifies type 'void *' but the argument has type 'char *' [-Wformat-pedantic]
>> printf("%p\n", p);
>> ~~ ^
>> %s
>> buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
>> p = buf + 6;
>> ^ ~
>
> I thought void * is a generic pointer that accepts any pointer argument. So a warning about char* being passed in seems to be flat out wrong.
>
>> buf.c:5:2: note: array 'buf' declared here
>> char *p, buf[5];
>> ^
>> 2 warnings generated.
>
> That was discussed just days ago: C says that a pointer one past the end of the array is legal. So here too it looks like Clang is wrong and GCC is right.
Look again:
char *p, buf[5];
p = buf + 6;
That's a pointer two-past-the-end; not one.
That's UB.
Cheers,
Alex
>
> paul
>
--
<http://www.alejandro-colomar.es/>
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [BUG] missing warning for pointer arithmetic out of bounds
2022-12-13 19:22 ` Paul Koning
2022-12-13 19:24 ` Alejandro Colomar
@ 2022-12-13 20:45 ` Jonathan Wakely
1 sibling, 0 replies; 8+ messages in thread
From: Jonathan Wakely @ 2022-12-13 20:45 UTC (permalink / raw)
To: Paul Koning; +Cc: Alejandro Colomar, gcc
On Tue, 13 Dec 2022, 19:23 Paul Koning via Gcc, <gcc@gcc.gnu.org> wrote:
>
>
>
> > On Dec 13, 2022, at 2:08 PM, Alejandro Colomar via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Hi!
> >
> > For the following program:
> >
> >
> > $ cat buf.c
> > #include <stdio.h>
> >
> > int main(void)
> > {
> > char *p, buf[5];
> >
> > p = buf + 6;
> > printf("%p\n", p);
> > }
> >
> >
> > There are no warnings in gcc, as I would expect:
> >
> > $ gcc -Wall -Wextra buf.c -O0
> >
> > Clang does warn, however:
> >
> > $ clang -Weverything -Wall -Wextra buf.c -O0
> > buf.c:8:17: warning: format specifies type 'void *' but the argument has type 'char *' [-Wformat-pedantic]
> > printf("%p\n", p);
> > ~~ ^
> > %s
> > buf.c:7:6: warning: the pointer incremented by 6 refers past the end of the array (that contains 5 elements) [-Warray-bounds-pointer-arithmetic]
> > p = buf + 6;
> > ^ ~
>
> I thought void * is a generic pointer that accepts any pointer argument.
Yes, but printf doesn't take a void* argument.
>
> So a warning about char* being passed in seems to be flat out wrong.
I wouldn't say flat out wrong. It's wrong because the C standard
guarantees that void* and char* have the same representation and
alignment requirements, so no conversion is needed to pass a char*
through a varargs ellipsis where a void* is expected. But if it was
int* or struct X* then the warning would be correct (but not a problem
in practice on common arches, which is why Clang only enabels it with
-Wformat-pedantic).
>
> > buf.c:5:2: note: array 'buf' declared here
> > char *p, buf[5];
> > ^
> > 2 warnings generated.
>
> That was discussed just days ago: C says that a pointer one past the end of the array is legal. So here too it looks like Clang is wrong and GCC is right.
No, clang is totally right here.
^ permalink raw reply [flat|nested] 8+ messages in thread
* Optimization levels for getting all warnings (was: [BUG] missing warning for pointer arithmetic out of bounds)
2022-12-13 19:18 ` Andrew Pinski
@ 2022-12-18 12:48 ` Alejandro Colomar
0 siblings, 0 replies; 8+ messages in thread
From: Alejandro Colomar @ 2022-12-18 12:48 UTC (permalink / raw)
To: Andrew Pinski; +Cc: gcc
[-- Attachment #1.1: Type: text/plain, Size: 896 bytes --]
Hi Andrew
On 12/13/22 20:18, Andrew Pinski wrote:
[...]
>
> GCC only warns during VRP which is only enabled at -O2:
>
> <source>:8:12: warning: array subscript 6 is outside array bounds of
> 'char[5]' [-Warray-bounds=]
> 8 | p = buf + 6;
> | ~~^~~~~~~~~
> <source>:6:19: note: at offset 6 into object 'buf' of size 5
> 6 | char *p, buf[5];
> | ^~~
>
> Thanks,
> Andrew
>
Makes sense. I've seen some warnings that are disabled by optimization, and
others that are enabled by it.
So, if I want to get all warnings that GCC can emit for any given software, is
there a list of optimization levels on which I should compile? Is -O0 and -O3
enough for everything? Or should I also build with all intermediate
optimization levels?
Thanks
Alex
--
<http://www.alejandro-colomar.es/>
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2022-12-18 12:48 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-12-13 19:08 [BUG] missing warning for pointer arithmetic out of bounds Alejandro Colomar
2022-12-13 19:15 ` Alejandro Colomar
2022-12-13 19:18 ` Andrew Pinski
2022-12-18 12:48 ` Optimization levels for getting all warnings (was: [BUG] missing warning for pointer arithmetic out of bounds) Alejandro Colomar
2022-12-13 19:22 ` [BUG] missing warning for pointer arithmetic out of bounds David Malcolm
2022-12-13 19:22 ` Paul Koning
2022-12-13 19:24 ` Alejandro Colomar
2022-12-13 20:45 ` Jonathan Wakely
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).