Re: [PATCH] scsi: properly count the number of pages in scsi_req_map_sg()

[Bryan Holty]

On Tuesday 21 March 2006 14:48, Bryan Holty wrote:
> On Tuesday 21 March 2006 13:17, Mike Christie wrote:
> > Bryan Holty wrote:
> > > On Tuesday 21 March 2006 10:19, Dan Aloni wrote:
> > >>On Tue, Mar 21, 2006 at 09:54:54AM -0600, James Bottomley wrote:
> > >>>This is a good email to discuss on the scsi list:
> > >>>linux-scsi@xxxxxxxxxxxxxxx; whom I've added to the cc list.
> > >>>
> > >>>On Tue, 2006-03-21 at 10:38 +0200, Dan Aloni wrote:
> > >>>>Improper calculation of the number of pages causes bio_alloc() to
> > >>>>be called with nr_iovecs=0, and slab corruption later.
> > >>>>
> > >>>>For example, a simple scatterlist that fails: {(3644,452), (0, 60)},
> > >>>>(offset, size). bufflen=512 => nr_pages=1 => breakage. The proper
> > >>>>page count for this example is 2.
> > >>>
> > >>>Such a scatterlist would likely violate the device's underlying
> > >>>boundaries and is not legal ... there's supposed to be special code
> > >>>checking the queue alignment and copying the bio to an aligned buffer
> > >>> if the limits are violated.  Where are you generating these
> > >>> scatterlists from?
> > >>
> > >>These scatterlists can be generated using the sg driver. Though I am
> > >>actually running a customized version of the sg driver, it seems the
> > >>conversion from a userspace array of sg_iovec_t to scatterlist stays
> > >>the same and also applies to the original driver (see
> > >>st_map_user_pages()).
> > >
> > > Hello,
> > >     I am seeing the same issue when using direct io with sg.  sg will
> > > perform direct io on any date that is aligned with the devices
> > > dma_align. The default for drivers that do not specify is 512.  sg
> > > builds the scatter gather list from the user specified location,
> > > offsetting the first entry in the list if not page aligned.  This is
> > > the case that causes the improper allocation of "nr_iovec" in
> > > scsi_req_map_sgand the later slab corruption.
> > >
> > >     I don't think it is necessary to calculate nr_pages from the entire
> > > list. Only sgl[0] is allowed to have an offset, so we can calculate
> > > from that as follows.
> > > --- a/drivers/scsi/scsi_lib.c	2006-03-03 13:17:22.000000000 -0600
> > > +++ b/drivers/scsi/scsi_lib.c	2006-03-21 11:36:39.389763804 -0600
> > > @@ -368,12 +368,15 @@
> > >  			   int nsegs, unsigned bufflen, gfp_t gfp)
> > >  {
> > >  	struct request_queue *q = rq->q;
> > > -	int nr_pages = (bufflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > > +	int nr_pages = 0;
> > >  	unsigned int data_len = 0, len, bytes, off;
> > >  	struct page *page;
> > >  	struct bio *bio = NULL;
> > >  	int i, err, nr_vecs = 0;
> > > -
> > > +
> > > +	if (nsegs)
> >
> > you can drop that test
> >
> > > + 		nr_pages = (bufflen + sgl[0].offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > > +
> >
> > I think we can do this without looping but I think this is broken. If we
> > had a slight variant of Dan's example but we have a page and some change
> > in the first entry {3644, 4548} and 0,60} in the last one, that would
> > would only calculate two pages but we want three.
> >
> > I think we can have to calculate the first and last entries but the
> > middle ones we can assume have no offset and lengths that are multiples
> > of a page.
>
> You are absolutely correct.
> bufflen is not page masked and when added to the offset of the first entry,
> will generate the correct nr_pages.
>
>     nr_pages = (bufflen + sgl[0].offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
> ==
>     nr_pages = ((bufflen & PAGE_MASK) + (PAGE_SIZE-1) + sgl[0].offset +
> sgl[nsegs-1].length) >> PAGE_SHIFT;
>
> Either will calculate correctly.
>
> --
>  Bryan Holty
> -
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Based on above, I think the most intuitive fix would be the offset addition of 
the first entry to the initialization of nr_pages.  

Without this change, for instance, with 4K io's every sg io that is 
dma_aligned for direct io, but not page aligned will cause slab corruption 
and an oops

I am able to run a number of tests with sg that cause the boundary to be 
crossed, and with this fix there is no slab corruption or data corruption.

Thanks Dan, I had been hunting for this for a couple of days!!

Thoughts??

Signed-off-by: Bryan Holty <lgeek@xxxxxxxxxxxxxxx>

 --- a/drivers/scsi/scsi_lib.c	2006-03-03 13:17:22.000000000 -0600
+++ b/drivers/scsi/scsi_lib.c	2006-03-22 06:09:09.669599539 -0600
@@ -368,7 +368,7 @@
 			   int nsegs, unsigned bufflen, gfp_t gfp)
 {
 	struct request_queue *q = rq->q;
-	int nr_pages = (bufflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	int nr_pages = (bufflen + sgl[0].offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	unsigned int data_len = 0, len, bytes, off;
 	struct page *page;
 	struct bio *bio = NULL;
--
 Bryan Holty
-
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