linux-kernel - Re: [ 04/21] target/pscsi: Fix page increment

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Date:	Tue, 19 Mar 2013 01:18:23 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Asias He <asias@...hat.com>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [ 04/21] target/pscsi: Fix page increment

On Tue, 2013-03-19 at 08:56 +0800, Asias He wrote:
> On Mon, Mar 18, 2013 at 11:30:47PM +0000, Ben Hutchings wrote:
> > On Mon, 2013-03-18 at 14:00 -0700, Nicholas A. Bellinger wrote:
> > > On Mon, 2013-03-18 at 13:35 +0800, Asias He wrote:
> > > > On Sat, Mar 16, 2013 at 02:10:22AM +0000, Ben Hutchings wrote:
> > > > > On Tue, 2013-03-12 at 15:44 -0700, Greg Kroah-Hartman wrote:
> > > > > > 3.0-stable review patch.  If anyone has any objections, please let me know.
> > > > > > 
> > > > > > ------------------
> > > > > > 
> > > > > > From: Asias He <asias@...hat.com>
> > > > > > 
> > > > > > commit 472b72f2db7831d7dbe22ffdff4adee3bd49b05d upstream.
> > > > > > 
> > > > > > The page++ is wrong. It makes bio_add_pc_page() pointing to a wrong page
> > > > > > address if the 'while (len > 0 && data_len > 0) { ... }' loop is
> > > > > > executed more than one once.
> > > > > > 
> > > > > > Signed-off-by: Asias He <asias@...hat.com>
> > > > > > Signed-off-by: Nicholas Bellinger <nab@...ux-iscsi.org>
> > > > > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > > > > 
> > > > > > ---
> > > > > >  drivers/target/target_core_pscsi.c |    1 -
> > > > > >  1 file changed, 1 deletion(-)
> > > > > > 
> > > > > > --- a/drivers/target/target_core_pscsi.c
> > > > > > +++ b/drivers/target/target_core_pscsi.c
> > > > > > @@ -1210,7 +1210,6 @@ static int __pscsi_map_task_SG(
> > > > > >  				bio = NULL;
> > > > > >  			}
> > > > > >  
> > > > > > -			page++;
> > > > > >  			len -= bytes;
> > > > > >  			data_len -= bytes;
> > > > > >  			off = 0;
> > > > > 
> > > > > So in case a fragment crosses a page boundary, we wrap around to the
> > > > > beginning of the same page?  That doesn't look right.
> > > > 
> > > > If the fragment crosses a page boundary, what is the correct page
> > > > for it?
> > > > 
> > > > Nicholas, can we assume sg->length + sg->offset should be less than PAGE_SIZE here?
> > > > 
> > > 
> > > sg->length + sg->offset can be less than or equal to PAGE_SIZE here.
> > >
> > > For everything other than tcm_loop + tcm_vhost using externally
> > > allocated SGLs, we can expect fragments to never cross the page
> > > boundary.
> > > 
> > > For tcm_loop + tcm_vhost, there are a few special cases with control CDB
> > > paylaods (usually going through scsi-generic) where we can have a non
> > > zero sg->offset, but at least in the cases I've seen this is still not
> > > using SGL elements that exceed PAGE_SIZE.
> > >
> > > So, I think this logic is OK for SGLs that cross page boundries, given
> > > that it's done outside of the inner loop where *page is set during each
> > > for_each_sg().
> > 
> > The page is set using sg_page() in the outer loop and was then
> > incremented in the inner loop.
> > 
> > 	for_each_sg(sgl, sg, sgl_nents, i) {
> > 		page = sg_page(sg);
> > 		off = sg->offset;
> > 		len = sg->length;
> > 
> > 		while (len > 0 && data_len > 0) {
> > 			bytes = min_t(unsigned int, len, PAGE_SIZE - off);
> > 			bytes = min(bytes, data_len);
> > 			...
> > 			/* page++; */
> > 			len -= bytes;
> > 			data_len -= bytes;
> > 			off = 0;
> > 		}
> > 	}
> > 
> > The inner loop is apparently meant to iterate over pages of a segment,
> > but is now just wrapping around a single page.
> 
> Yes, but we never loop more than once in the inner loop.

If you're sure of that, then:

> So, how about
> 1) fail on  sg->offset + sg->length > PAGE_SIZE (we can not find a
> proper page address in this case)
> 2) remove the inner while loop, run the code was in the loop only once.

this is fine, but then I don't understand why you sent a no-op change to
stable in the first place.

Ben.

-- 
Ben Hutchings
When you say `I wrote a program that crashed Windows', people just stare ...
and say `Hey, I got those with the system, *for free*'. - Linus Torvalds

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