Re: [PATCH] SPARC64: fix iommu sg chaining

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Oct 17 2007, David Miller wrote:
> From: Jens Axboe <jens.axboe@oracle.com>
> Date: Wed, 17 Oct 2007 10:45:28 +0200
> 
> > Righto, it's invalid to call sg_next() on the last entry!
> 
> Unfortunately, that's what the sparc64 code wanted to do, this
> transformation in the sparc64 sg chaining patch is not equilavent:
> 
> -	struct scatterlist *sg_end = sg + nelems;
> +	struct scatterlist *sg_end = sg_last(sg, nelems);
>  ...
> -			while (sg < sg_end &&
> +			while (sg != sg_end &&

Auch indeed. That'd probably be better as a

        do {
                ...
        } while (sg != sg_end);

> No, that's not what the code was doing.  The while loop
> has to process the last entry in the list,
> 
> We really needed "sg_end" to be "one past the last element",
> rather than "the last element".
> 
> Since you say that sg_next() on the last entry is illegal,
> and that's what this code would have done to try and reach
> loop termination (it doesn't actually derefrence that
> "end plus one" scatterlist entry) I'll try to code this up
> some other way.
> 
> Besides, sg_last() is so absurdly expensive, it has to walk the entire
> chain in the chaining case.  So better to implement this without it.

It is, sg_last() should really not be used a lot since it'll leaf
through the entire sg list. People should either keep count of the
number of entries so that they know when they are dealing with the last
valid entry. Or use the for_each_sg() loop helper, if possible.

Drivers are usually very simple, the iommu code does more sg tricks and
thus is more complex to audit.

> I would suggest that other sg_last() uses be audited for the same bug.

Agree.

-- 
Jens Axboe