Re: [PATCH] bsg: bidi bio map failure fix

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

On Tue, Feb 12 2008, James Bottomley wrote:
> On Tue, 2008-02-12 at 15:40 -0500, Pete Wyckoff wrote:
> > If blk_rq_map_user requires more than one bio, and fails mapping
> > somewhere after the first bio, it will return with rq->bio set to
> > non-NULL, but it will have already unmapped the partial bio.  The
> > "out:" error exit section will see the non-null bio and try to unmap
> > it again, triggering a mapcount bug via bad_page().
> > 
> > Signed-off-by: Pete Wyckoff <pw@osc.edu>
> > ---
> >  block/bsg.c |    4 +++-
> >  1 files changed, 3 insertions(+), 1 deletions(-)
> > 
> > diff --git a/block/bsg.c b/block/bsg.c
> > index 3337125..bba7154 100644
> > --- a/block/bsg.c
> > +++ b/block/bsg.c
> > @@ -295,8 +295,10 @@ bsg_map_hdr(struct bsg_device *bd, struct sg_io_v4 *hdr)
> >  
> >  		dxferp = (void*)(unsigned long)hdr->din_xferp;
> >  		ret =  blk_rq_map_user(q, next_rq, dxferp, hdr->din_xfer_len);
> > -		if (ret)
> > +		if (ret) {
> > +			next_rq->bio = NULL;  /* do not unmap twice */
> 
> Nice ... that's a nasty asymmetry of the blk_rq_map_user API.  The map
> takes a request gets a ref and fills in the bio.  The unmap has to be
> called on the bio leaving you to clear the now removed bio reference
> manually.

It is nasty, how about fixing that instead?

diff --git a/block/blk-map.c b/block/blk-map.c
index 955d75c..bc5ce60 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -143,6 +143,7 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
 	return 0;
 unmap_rq:
 	blk_rq_unmap_user(bio);
+	rq->bio = NULL;
 	return ret;
 }
 EXPORT_SYMBOL(blk_rq_map_user);

-- 
Jens Axboe