Re: [Intel-gfx] [PATCH v4] lib/scatterlist: Avoid potential scatterlist entry overflow

[Tvrtko Ursulin]

Hi,

On 11/01/2017 23:59, Andy Shevchenko wrote:
> On Wed, Jan 11, 2017 at 1:58 PM, Tvrtko Ursulin <tursulin@xxxxxxxxxxx> wrote:
> > Since the scatterlist length field is an unsigned int, make
> > sure that sg_alloc_table_from_pages does not overflow it while
> > coallescing pages to a single entry.
>
>
> >  /*
> > + * Since the above length field is an unsigned int, below we define the maximum
> > + * lenght in bytes that can be stored in one scatterlist entry.
>
> length
>
> > + */
> > +#define SCATTERLIST_MAX_SEGMENT (0xfffff000)
>
> Shouldn't be calculated from PAGE_SIZE (PAGE bits, etc)?

Yep, and at the same time I would potentially change the name to be 
consistent with the other defines in the file as Joonas suggested. 
Something like:

#define SG_MAX_SEGMENT (UINT_MAX & PAGE_MASK)

But would need a better name since SG_MAX_SEGMENT*S* already exists and 
means something else. If we can't come up with a better name then leave 
it as SCATTERLIST_MAX_SEGMENT?

> > --- a/lib/scatterlist.c
> > +++ b/lib/scatterlist.c
>
> > @@ -402,9 +403,16 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
> >
> >         /* compute number of contiguous chunks */
> >         chunks = 1;
> > -       for (i = 1; i < n_pages; ++i)
> > -               if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1)
> > +       seg_len = PAGE_SIZE;
> > +       for (i = 1; i < n_pages; ++i) {
> > +               if (seg_len >= max_segment ||
> > +                   page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
> >                         ++chunks;
> > +                       seg_len = PAGE_SIZE;
> > +               } else {
> > +                       seg_len += PAGE_SIZE;
> > +               }
> > +       }
>
> Wouldn't be following looks more readable?
>
> seg_len = 0;
> // Are compilers so stupid doing calculation per iteration in for-conditional?
> // for (i = 0; i + 1 < n_pages; i++) ?

I didn't get what you meant here?

> for (i = 1; i < n_pages; ++i) {
>   seg_len += PAGE_SIZE;
>   if (seg_len >= max_segment || page_to_pfn(pages[i]) !=
> page_to_pfn(pages[i - 1]) + 1) {
>     ++chunks;
>     seg_len = PAGE_SIZE;
>   }
> }

Tried it in my unit tester but it doesn't work for all scenarios, guess 
there is a subtle bug somewhere. I don't find it that unreadable so 
would prefer to leave it since it works.

> Perhaps while() or do-while() will increase readability even more, but
> I didn't check.
>
> >                 /* look for the end of the current chunk */
> > +               seg_len = PAGE_SIZE;
> >                 for (j = cur_page + 1; j < n_pages; ++j)
> > -                       if (page_to_pfn(pages[j]) !=
> > +                       if (seg_len >= max_segment ||
> > +                           page_to_pfn(pages[j]) !=
> >                             page_to_pfn(pages[j - 1]) + 1)
> >                                 break;
> > +                       else
> > +                               seg_len += PAGE_SIZE;
>
> Something similar here (didn't you get warning from checkpath about
> curly braces?).

It didn't, but agreed that I should have added them. Will do.

Regards,

Tvrtko