svn commit: r348843 - head/sys/vm

[Doug Moore]

-fwrapv concerns signed arithmetic.  This calculation is with unsigned
arithmetic, and the possibility of wrapping around to 0 is part of the
language.

Doug Moore

On 6/10/19 1:35 AM, Andriy Gapon wrote:
> On 10/06/2019 06:07, Doug Moore wrote:
>> Author: dougm
>> Date: Mon Jun 10 03:07:10 2019
>> New Revision: 348843
>> URL: https://svnweb.freebsd.org/changeset/base/348843
>>
>> Log:
>>   There are times when a len==0 parameter to mmap is okay. But on a
>>   32-bit machine, a len parameter just a few bytes short of 4G, rounded
>>   up to a page boundary and hitting zero then, is not okay. Return
>>   failure in that case.
>>   
>>   Reported by: pho
>>   Reviewed by: alc, kib (mentor)
>>   Tested by: pho
>>   Differential Revision: https://reviews.freebsd.org/D20580
>>
>> Modified:
>>   head/sys/vm/vm_mmap.c
>>
>> Modified: head/sys/vm/vm_mmap.c
>> ==============================================================================
>> --- head/sys/vm/vm_mmap.c	Sun Jun  9 22:55:21 2019	(r348842)
>> +++ head/sys/vm/vm_mmap.c	Mon Jun 10 03:07:10 2019	(r348843)
>> @@ -257,7 +257,10 @@ kern_mmap(struct thread *td, uintptr_t addr0, size_t s
>>  
>>  	/* Adjust size for rounding (on both ends). */
>>  	size += pageoff;			/* low end... */
>> -	size = (vm_size_t) round_page(size);	/* hi end */
>> +	/* Check for rounding up to zero. */
>> +	if (round_page(size) < size)
> Is this guaranteed to work with all compilers?
> I think that some smart compilers may think that this condition is impossible.
> Are we finally using -fwrapv or something like it for kernel builds?
>
>> +		return (EINVAL);
>> +	size = round_page(size);		/* hi end */
>>  
>>  	/* Ensure alignment is at least a page and fits in a pointer. */
>>  	align = flags & MAP_ALIGNMENT_MASK;
>>
>