Re: [PATCH v3] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.08.2022 14:10, Juergen Gross wrote:
> On 25.08.22 13:58, Jan Beulich wrote:
>> On 25.08.2022 13:40, Juergen Gross wrote:
>>> --- a/drivers/xen/privcmd.c
>>> +++ b/drivers/xen/privcmd.c
>>> @@ -581,7 +581,7 @@ static int lock_pages(
>>>     struct privcmd_dm_op_buf kbufs[], unsigned int num,
>>>     struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
>>>   {
>>> -   unsigned int i;
>>> +   unsigned int i, off = 0;
>>>   
>>>     for (i = 0; i < num; i++) {
>>>             unsigned int requested;
>>> @@ -589,19 +589,23 @@ static int lock_pages(
>>>   
>>>             requested = DIV_ROUND_UP(
>>>                     offset_in_page(kbufs[i].uptr) + kbufs[i].size,
>>> -                   PAGE_SIZE);
>>> +                   PAGE_SIZE) - off;
>>>             if (requested > nr_pages)
>>>                     return -ENOSPC;
>>>   
>>>             page_count = pin_user_pages_fast(
>>> -                   (unsigned long) kbufs[i].uptr,
>>> +                   (unsigned long)kbufs[i].uptr + off * PAGE_SIZE,
>>>                     requested, FOLL_WRITE, pages);
>>> -           if (page_count < 0)
>>> -                   return page_count;
>>> +           if (page_count <= 0)
>>> +                   return page_count ? : -EFAULT;
>>>   
>>>             *pinned += page_count;
>>>             nr_pages -= page_count;
>>>             pages += page_count;
>>> +
>>> +           off = requested - page_count;
>>> +           if (off)
>>> +                   i--;
>>>     }
>>
>> Initially I thought this would go wrong only on the 3rd iteration, but
>> meanwhile I think it's wrong already on the 2nd. What I think you need
>> is
>>
>>              if (page_count < requested)
>>                      i--;
>>              off += page_count;
>>
>> or with the i++ from the loop header absorbed here
>>
>>              if (page_count == requested)
>>                      i++;
>>              off += page_count;
>>
>> Plus of course off needs resetting to zero whenever i advances. I.e.
>>
>>              if (page_count == requested) {
>>                      i++;
>>                      off = 0;
>>              } else {
>>                      off += page_count;
>>              }
> 
> Yeah, or:
> 
>               off = (page_count == requested) ? 0 : off + page_count;
>               i += !off;

I wasn't daring to suggest something like that ;-)

Jan