Re: [PATCH v3 7/7] xen/memory: Fix mapping grant tables with XENMEM_acquire_resource

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.01.2021 00:44, Andrew Cooper wrote:
> On 15/01/2021 16:12, Jan Beulich wrote:
>> On 12.01.2021 20:48, Andrew Cooper wrote:
>>> --- a/xen/arch/x86/mm.c
>>> +++ b/xen/arch/x86/mm.c
>>> @@ -4628,7 +4628,6 @@ int arch_acquire_resource(struct domain *d, unsigned 
>>> int type,
>>>          if ( id != (unsigned int)ioservid )
>>>              break;
>>>  
>>> -        rc = 0;
>>>          for ( i = 0; i < nr_frames; i++ )
>>>          {
>>>              mfn_t mfn;
>> How "good" are our chances that older gcc won't recognize that
>> without this initialization ...
>>
>>> @@ -4639,6 +4638,9 @@ int arch_acquire_resource(struct domain *d, unsigned 
>>> int type,
>>>  
>>>              mfn_list[i] = mfn_x(mfn);
>>>          }
>>> +        if ( i == nr_frames )
>>> +            /* Success.  Passed nr_frames back to the caller. */
>>> +            rc = nr_frames;
>> ... rc will nevertheless not remain uninitialized when nr_frames
>> is zero?
> 
> I don't anticipate this function getting complicated enough for us to
> need to rely on tricks like that to spot bugs.
> 
> AFAICT, it would take a rather larger diffstat to make it "uninitialised
> clean" again.

Well, we'll see how it goes then. We still allow rather ancient
gcc, and I will eventually run into a build issue here once
having rebased accordingly, if such old gcc won't cope.

Jan