Re: [RFC PATCH 2/2] linux-user/mmap: Fix Clang 'type-limit-compare' warning

[Thomas Huth <thuth@redhat.com>]

On 03/06/2020 20.01, Richard Henderson wrote:
> On 6/3/20 9:06 AM, Eric Blake wrote:
>> Instead of using #if, the following suffices to shut up clang:
>>
>> diff --git c/linux-user/mmap.c w/linux-user/mmap.c
>> index e37803379747..8d9ba201625d 100644
>> --- c/linux-user/mmap.c
>> +++ w/linux-user/mmap.c
>> @@ -715,7 +715,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
>>              host_addr = MAP_FAILED;
>>          }
>>          /* Check if address fits target address space */
>> -        if ((unsigned long)host_addr + new_size > (abi_ulong)-1) {
>> +        if ((unsigned long)host_addr > (abi_ulong)-1 - new_size) {
>>              /* Revert mremap() changes */
>>              host_addr = mremap(g2h(old_addr), new_size, old_size, flags);
>>              errno = ENOMEM;
>>
>>
>> That is, it is no longer a tautological type compare if you commute the
>> operations so that neither side is a compile-time constant.
> 
> To some extent the tautological compare is a hint to the compiler that the
> comparison may be optimized away.  If sizeof(abi_ulong) >= sizeof(unsigned
> long), then the host *cannot* produce an out-of-range target address.
> 
> We could add the sizeof test to the if, to preserve the optimization, but that
> by itself doesn't prevent the clang warning.
> 
> Which is why I have repeatedly suggested that we disable this warning globally.

I guess most people (like me) don't have a strong opinion about this. So
maybe simply suggest a patch to disable the warning? I don't think that
anybody will object.

 Thomas