block: make blk_rq_map_user take a NULL user-space buffer

This patch changes blk_rq_map_user to accept a NULL user-space buffer
with a READ command if rq_map_data is not NULL. Thus a caller can pass
page frames to lk_rq_map_user to just set up a request and bios with
page frames propely. bio_uncopy_user (called via blk_rq_unmap_user)
doesn't copy data to user space with such request.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

3 files changed, 17 insertions, 8 deletions

diff --git a/block/blk-map.c b/block/blk-map.c
index 572140cda5ff..4849fa36161e 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -42,7 +42,7 @@ static int __blk_rq_unmap_user(struct bio *bio)
 
 static int __blk_rq_map_user(struct request_queue *q, struct request *rq,
                              struct rq_map_data *map_data, void __user *ubuf,
-                             unsigned int len, gfp_t gfp_mask)
+                             unsigned int len, int null_mapped, gfp_t gfp_mask)
 {
         unsigned long uaddr;
         struct bio *bio, *orig_bio;
@@ -63,6 +63,9 @@ static int __blk_rq_map_user(struct request_queue *q, struct request *rq,
         if (IS_ERR(bio))
                 return PTR_ERR(bio);
 
+        if (null_mapped)
+                bio->bi_flags |= (1 << BIO_NULL_MAPPED);
+
         orig_bio = bio;
         blk_queue_bounce(q, &bio);
 
@@ -111,12 +114,17 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
 {
         unsigned long bytes_read = 0;
         struct bio *bio = NULL;
-        int ret;
+        int ret, null_mapped = 0;
 
         if (len > (q->max_hw_sectors << 9))
                 return -EINVAL;
-        if (!len || !ubuf)
+        if (!len)
                 return -EINVAL;
+        if (!ubuf) {
+                if (!map_data || rq_data_dir(rq) != READ)
+                        return -EINVAL;
+                null_mapped = 1;
+        }
 
         while (bytes_read != len) {
                 unsigned long map_len, end, start;
@@ -135,7 +143,7 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
                         map_len -= PAGE_SIZE;
 
                 ret = __blk_rq_map_user(q, rq, map_data, ubuf, map_len,
-                                        gfp_mask);
+                                        null_mapped, gfp_mask);
                 if (ret < 0)
                         goto unmap_rq;
                 if (!bio)
diff --git a/fs/bio.c b/fs/bio.c
index 9d68ddb89b71..355302985e22 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -547,11 +547,11 @@ static int __bio_copy_iov(struct bio *bio, struct bio_vec *iovecs,
 int bio_uncopy_user(struct bio *bio)
 {
         struct bio_map_data *bmd = bio->bi_private;
-        int ret;
-
-        ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs, bmd->nr_sgvecs, 1,
-                             bmd->is_our_pages);
+        int ret = 0;
 
+        if (!bio_flagged(bio, BIO_NULL_MAPPED))
+                ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs,
+                                     bmd->nr_sgvecs, 1, bmd->is_our_pages);
         bio_free_map_data(bmd);
         bio_put(bio);
         return ret;
diff --git a/include/linux/bio.h b/include/linux/bio.h
index bc386cd5e996..7af373f253dc 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -108,6 +108,7 @@ struct bio {
 #define BIO_USER_MAPPED 6       /* contains user pages */
 #define BIO_EOPNOTSUPP  7       /* not supported */
 #define BIO_CPU_AFFINE  8       /* complete bio on same CPU as submitted */
+#define BIO_NULL_MAPPED 9       /* contains invalid user pages */
 #define bio_flagged(bio, flag)  ((bio)->bi_flags & (1 << (flag)))
 
 /*