Re: [PATCH 2/4] lib/scatterlist: Avoid potential scatterlist entry overflow

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

Hi David,

Chris noticed your "scatterlist: don't overflow length field" patch and 
pinged me, so I am copying you on another thread which tries to solve 
the same problem.

My latest series is here: 
https://patchwork.freedesktop.org/series/18062/, but it has been going 
from some time November last year.

I like your BUILD_BUG_ON safety, but otherwise our patches are pretty 
similar.

i915 driver also benefits from the ability to create large sg chunks 
which saves us a few megabytes of RAM at runtime, but we do have to 
degrade to smaller chunks when running under a hypervisor. For that we 
are using the swiotlb_max_segment API Konrad recently added for this 
purpose.

So what I did in addition to fixing the overflow is exported a new 
flavour of sg_alloc_table_from_pages which allows you to control the 
maximum chunk.

Maybe you can have a look at my series and see if it would work for you? 
I've been trying to gain some traction for it for some months now.

Regards,

Tvrtko

On 07/03/2017 08:58, Tvrtko Ursulin wrote:
>
> Hi,
>
> On 16/01/2017 14:12, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> 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.
>>
>> v2: Drop reference to future use. Use UINT_MAX.
>> v3: max_segment must be page aligned.
>> v4: Do not rely on compiler to optimise out the rounddown.
>>     (Joonas Lahtinen)
>> v5: Simplified loops and use post-increments rather than
>>     pre-increments. Use PAGE_MASK and fix comment typo.
>>     (Andy Shevchenko)
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>> Cc: linux-kernel@vger.kernel.org
>> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>> Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
>
> Anyone in the mood for reviewing from here to the end of the series?
>
> Regards,
>
> Tvrtko
>
>> ---
>>  include/linux/scatterlist.h |  6 ++++++
>>  lib/scatterlist.c           | 31 ++++++++++++++++++++-----------
>>  2 files changed, 26 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
>> index c981bee1a3ae..4768eeeb7054 100644
>> --- a/include/linux/scatterlist.h
>> +++ b/include/linux/scatterlist.h
>> @@ -21,6 +21,12 @@ struct scatterlist {
>>  };
>>
>>  /*
>> + * Since the above length field is an unsigned int, below we define
>> the maximum
>> + * length in bytes that can be stored in one scatterlist entry.
>> + */
>> +#define SCATTERLIST_MAX_SEGMENT (UINT_MAX & PAGE_MASK)
>> +
>> +/*
>>   * These macros should be used after a dma_map_sg call has been done
>>   * to get bus addresses of each of the SG entries and their lengths.
>>   * You should only work with the number of sg entries dma_map_sg
>> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
>> index e05e7fc98892..65f375645df5 100644
>> --- a/lib/scatterlist.c
>> +++ b/lib/scatterlist.c
>> @@ -394,17 +394,22 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
>>      unsigned int offset, unsigned long size,
>>      gfp_t gfp_mask)
>>  {
>> -    unsigned int chunks;
>> -    unsigned int i;
>> -    unsigned int cur_page;
>> +    const unsigned int max_segment = SCATTERLIST_MAX_SEGMENT;
>> +    unsigned int chunks, cur_page, seg_len, i;
>>      int ret;
>>      struct scatterlist *s;
>>
>>      /* 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)
>> -            ++chunks;
>> +    seg_len = 0;
>> +    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 = 0;
>> +        }
>> +    }
>>
>>      ret = sg_alloc_table(sgt, chunks, gfp_mask);
>>      if (unlikely(ret))
>> @@ -413,17 +418,21 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
>>      /* merging chunks and putting them into the scatterlist */
>>      cur_page = 0;
>>      for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
>> -        unsigned long chunk_size;
>> -        unsigned int j;
>> +        unsigned int j, chunk_size;
>>
>>          /* look for the end of the current chunk */
>> -        for (j = cur_page + 1; j < n_pages; ++j)
>> -            if (page_to_pfn(pages[j]) !=
>> +        seg_len = 0;
>> +        for (j = cur_page + 1; j < n_pages; j++) {
>> +            seg_len += PAGE_SIZE;
>> +            if (seg_len >= max_segment ||
>> +                page_to_pfn(pages[j]) !=
>>                  page_to_pfn(pages[j - 1]) + 1)
>>                  break;
>> +        }
>>
>>          chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
>> -        sg_set_page(s, pages[cur_page], min(size, chunk_size), offset);
>> +        sg_set_page(s, pages[cur_page],
>> +                min_t(unsigned long, size, chunk_size), offset);
>>          size -= chunk_size;
>>          offset = 0;
>>          cur_page = j;
>>
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