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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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.

James