block: fix sg SG_DXFER_TO_FROM_DEV regression

I overlooked SG_DXFER_TO_FROM_DEV support when I converted sg to use
the block layer mapping API (2.6.28).

Douglas Gilbert explained SG_DXFER_TO_FROM_DEV:

http://www.spinics.net/lists/linux-scsi/msg37135.html

=
The semantics of SG_DXFER_TO_FROM_DEV were:
   - copy user space buffer to kernel (LLD) buffer
   - do SCSI command which is assumed to be of the DATA_IN
     (data from device) variety. This would overwrite
     some or all of the kernel buffer
   - copy kernel (LLD) buffer back to the user space.

The idea was to detect short reads by filling the original
user space buffer with some marker bytes ("0xec" it would
seem in this report). The "resid" value is a better way
of detecting short reads but that was only added this century
and requires co-operation from the LLD.
=

This patch changes the block layer mapping API to support this
semantics. This simply adds another field to struct rq_map_data and
enables __bio_copy_iov() to copy data from user space even with READ
requests.

It's better to add the flags field and kills null_mapped and the new
from_user fields in struct rq_map_data but that approach makes it
difficult to send this patch to stable trees because st and osst
drivers use struct rq_map_data (they were converted to use the block
layer in 2.6.29 and 2.6.30). Well, I should clean up the block layer
mapping API.

zhou sf reported this regiression and tested this patch:

http://www.spinics.net/lists/linux-scsi/msg37128.html
http://www.spinics.net/lists/linux-scsi/msg37168.htmlReported-by: Nzhou sf <sxzzsf@gmail.com>
Tested-by: Nzhou sf <sxzzsf@gmail.com>
Cc: stable@kernel.org
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

drivers/scsi/sg.c

@@ -1656,6 +1656,10 @@ static int sg_start_req(Sg_request *srp, unsigned char *cmd)
 		md->nr_entries = req_schp->k_use_sg;
 		md->offset = 0;
 		md->null_mapped = hp->dxferp ? 0 : 1;
+		if (dxfer_dir == SG_DXFER_TO_FROM_DEV)
+			md->from_user = 1;
+		else
+			md->from_user = 0;
 	}
 
 	if (iov_count) {

fs/bio.c

@@ -705,14 +705,13 @@ static struct bio_map_data *bio_alloc_map_data(int nr_segs, int iov_count,
 }
 
 static int __bio_copy_iov(struct bio *bio, struct bio_vec *iovecs,
-			  struct sg_iovec *iov, int iov_count, int uncopy,
-			  int do_free_page)
+			  struct sg_iovec *iov, int iov_count,
+			  int to_user, int from_user, int do_free_page)
 {
 	int ret = 0, i;
 	struct bio_vec *bvec;
 	int iov_idx = 0;
 	unsigned int iov_off = 0;
-	int read = bio_data_dir(bio) == READ;
 
 	__bio_for_each_segment(bvec, bio, i, 0) {
 		char *bv_addr = page_address(bvec->bv_page);
@@ -727,13 +726,14 @@ static int __bio_copy_iov(struct bio *bio, struct bio_vec *iovecs,
 			iov_addr = iov[iov_idx].iov_base + iov_off;
 
 			if (!ret) {
-				if (!read && !uncopy)
-					ret = copy_from_user(bv_addr, iov_addr,
-							     bytes);
-				if (read && uncopy)
+				if (to_user)
 					ret = copy_to_user(iov_addr, bv_addr,
 							   bytes);
 
+				if (from_user)
+					ret = copy_from_user(bv_addr, iov_addr,
+							     bytes);
+
 				if (ret)
 					ret = -EFAULT;
 			}
@@ -770,7 +770,8 @@ int bio_uncopy_user(struct bio *bio)
 
 	if (!bio_flagged(bio, BIO_NULL_MAPPED))
 		ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs,
-				     bmd->nr_sgvecs, 1, bmd->is_our_pages);
+				     bmd->nr_sgvecs, bio_data_dir(bio) == READ,
+				     0, bmd->is_our_pages);
 	bio_free_map_data(bmd);
 	bio_put(bio);
 	return ret;
@@ -875,8 +876,9 @@ struct bio *bio_copy_user_iov(struct request_queue *q,
 	/*
 	 * success
 	 */
-	if (!write_to_vm && (!map_data || !map_data->null_mapped)) {
-		ret = __bio_copy_iov(bio, bio->bi_io_vec, iov, iov_count, 0, 0);
+	if ((!write_to_vm && (!map_data || !map_data->null_mapped)) ||
+	    (map_data && map_data->from_user)) {
+		ret = __bio_copy_iov(bio, bio->bi_io_vec, iov, iov_count, 0, 1, 0);
 		if (ret)
 			goto cleanup;
 	}

include/linux/blkdev.h

@@ -723,6 +723,7 @@ struct rq_map_data {
 	int nr_entries;
 	unsigned long offset;
 	int null_mapped;
+	int from_user;
 };
 
 struct req_iterator {