On 05/27/2015 04:45 AM, Herbert Xu wrote:
On Wed, May 27, 2015 at 05:43:05PM +0800, Herbert Xu wrote:
Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
Scatter gather lists can be created with more available entries than are
actually used (e.g. using sg_init_table() to reserve a specific number
of sg entries, but in actuality using something less than that based on
the data length). The caller sometimes fails to mark the last entry
with sg_mark_end(). In these cases, sg_nents() will return the original
size of the sg list as opposed to the actual number of sg entries that
contain valid data.
On arm64, if the sg_nents() value is used in a call to dma_map_sg() in
this situation, then it causes a BUG_ON in lib/swiotlb.c because an
"empty" sg list entry results in dma_capable() returning false and
swiotlb trying to create a bounce buffer of size 0. This occurred in
the userspace crypto interface before being fixed by
0f477b655a52 ("crypto: algif - Mark sgl end at the end of data")
Protect against this in the future by counting the number of sg entries
needed to meet the length requirement and supplying that value to
dma_map_sg().
Is this needed for any reason other than this bug that's already
been fixed?
Could this be needed if you have a properly marked SG list say of
100 bytes but len is only 10 bytes?
I don't think that situation matters because the DMA mapping should
succeed just fine at 100 bytes even if only needing/using 10 bytes.
Thanks,
Tom
Cheers,
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