[PATCH 5.4 175/177] splice: only read in as much information as there is pipe buffer space

[Greg Kroah-Hartman]

From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

commit 3253d9d093376d62b4a56e609f15d2ec5085ac73 upstream.

Andreas GrÃnbacher reports that on the two filesystems that support
iomap directio, it's possible for splice() to return -EAGAIN (instead of
a short splice) if the pipe being written to has less space available in
its pipe buffers than the length supplied by the calling process.

Months ago we fixed splice_direct_to_actor to clamp the length of the
read request to the size of the splice pipe.  Do the same to do_splice.

Fixes: 17614445576b6 ("splice: don't read more than available pipe space")
Reported-by: syzbot+3c01db6025f26530cf8d@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: Andreas GrÃnbacher <andreas.gruenbacher@xxxxxxxxx>
Reviewed-by: Andreas GrÃnbacher <andreas.gruenbacher@xxxxxxxxx>
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/splice.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/fs/splice.c
+++ b/fs/splice.c
@@ -945,12 +945,13 @@ ssize_t splice_direct_to_actor(struct fi
 	WARN_ON_ONCE(pipe->nrbufs != 0);
 
 	while (len) {
+		unsigned int pipe_pages;
 		size_t read_len;
 		loff_t pos = sd->pos, prev_pos = pos;
 
 		/* Don't try to read more the pipe has space for. */
-		read_len = min_t(size_t, len,
-				 (pipe->buffers - pipe->nrbufs) << PAGE_SHIFT);
+		pipe_pages = pipe->buffers - pipe->nrbufs;
+		read_len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
 		ret = do_splice_to(in, &pos, pipe, read_len, flags);
 		if (unlikely(ret <= 0))
 			goto out_release;
@@ -1180,8 +1181,15 @@ static long do_splice(struct file *in, l
 
 		pipe_lock(opipe);
 		ret = wait_for_space(opipe, flags);
-		if (!ret)
+		if (!ret) {
+			unsigned int pipe_pages;
+
+			/* Don't try to read more the pipe has space for. */
+			pipe_pages = opipe->buffers - opipe->nrbufs;
+			len = min(len, (size_t)pipe_pages << PAGE_SHIFT);
+
 			ret = do_splice_to(in, &offset, opipe, len, flags);
+		}
 		pipe_unlock(opipe);
 		if (ret > 0)
 			wakeup_pipe_readers(opipe);