Re: [BUG] missing warning for pointer arithmetic out of bounds

[Alejandro Colomar <alx.manpages@gmail.com>]

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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/>

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