Re: [Xen-devel] [PATCH for-4.10] string: fix memmove when size is 0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.10.17 at 14:52, <roger.pau@xxxxxxxxxx> wrote:
> On Tue, Oct 17, 2017 at 01:41:35PM +0100, Andrew Cooper wrote:
>> There are many passed values which could trigger this warning.  Does
>> 
>> diff --git a/xen/arch/x86/string.c b/xen/arch/x86/string.c
>> index cd85a38..4f55856 100644
>> --- a/xen/arch/x86/string.c
>> +++ b/xen/arch/x86/string.c
>> @@ -47,7 +47,7 @@ void *(memmove)(void *dest, const void *src, size_t n)
>>          "   rep movsb   ; "
>>          "   cld           "
>>          : "=&c" (d0), "=&S" (d1), "=&D" (d2)
>> -        : "0" (n), "1" (n-1+(const char *)src), "2" (n-1+(char *)dest)
>> +        : "0" (n), "1" ((uintptr_t)src + n - 1), "2" ((uintptr_t)dest + n - 
>> 1)
>>          : "memory");
>>  
>>      return dest;
>> 
>> work any better?
> 
> That does indeed work, but I'm not sure if it would mask legitimate
> pointer overflows by casting them into integers.

It certainly would, as the tool can't possibly know that the asm()
itself then effectively casts the integers back to pointers (i.e. it
has no basis to try to "look through" the cast and continue analysis).

Jan


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