Re: [patch] Use unsigned constants for ICMP6 filters [BZ #22489]

[DJ Delorie <dj@redhat.com>]

Andreas Schwab <schwab@linux-m68k.org> writes:
>> GCC complains that -65 doesn't fit in "uint32_t" and was converted to
>> (unsigned) 0xffffffbf.
>
> -65 fits very well in uint32_t, this is a perfectly defined operation.
> The bug is an undefined shift (which can produce an arbitrary value).

The left shift is limited to "1" shifted by 0..31 bits, which is well
defined.  A shift of 31 bits doesn't result in a warning, either,
because the resulting ~bit pattern indicates a positive value.  The
right shift might produce an array overflow, but that would happen even
for unsigned values, and the warning happens for constants that are
known to fit.  GCC indicates the warning is with the &= operator:

$ gcc -c -Wsign-conversion icmp6_test.c
icmp6_test.c: In function ‘main’:
icmp6_test.c:14:7: warning: unsigned conversion from ‘int’ to ‘uint32_t’ {aka ‘unsigned int’} changes value from ‘-65’ to ‘4294967231’ [-Wsign-conversion]
   14 |       &= ~(1 <<
      |       ^~


#include <stdlib.h>                                                           
#include <netinet/icmp6.h>                                                    

#undef ICMP6_FILTER_SETPASS

#define ICMP6_FILTER_SETPASS(type, filterp) \
  ((((filterp)->icmp6_filt[(type) >> 5]) &= ~(1 << ((type) & 31))))
                                                                           
int main()                                                                    
{                                                                             
    struct icmp6_filter filter;                                               
                                                                              
    (((filter.icmp6_filt[(134) >> 5])
      &= ~(1 <<
	   ((134) & 31))));
                                                                              
    return EXIT_SUCCESS;                                                      
}