splice: don't read more than available pipe space

[ Upstream commit 17614445576b6af24e9cf36607c6448164719c96 ]

In commit 4721a601099, we tried to fix a problem wherein directio reads
into a splice pipe will bounce EFAULT/EAGAIN all the way out to
userspace by simulating a zero-byte short read.  This happens because
some directio read implementations (xfs) will call
bio_iov_iter_get_pages to grab pipe buffer pages and issue asynchronous
reads, but as soon as we run out of pipe buffers that _get_pages call
returns EFAULT, which the splice code translates to EAGAIN and bounces
out to userspace.

In that commit, the iomap code catches the EFAULT and simulates a
zero-byte read, but that causes assertion errors on regular splice reads
because xfs doesn't allow short directio reads.

The brokenness is compounded by splice_direct_to_actor immediately
bailing on do_splice_to returning <= 0 without ever calling ->actor
(which empties out the pipe), so if userspace calls back we'll EFAULT
again on the full pipe, and nothing ever gets copied.

Therefore, teach splice_direct_to_actor to clamp its requests to the
amount of free space in the pipe and remove the simulated short read
from the iomap directio code.

Fixes: 4721a601099 ("iomap: dio data corruption and spurious errors when pipes fill")
Reported-by: Murphy Zhou <jencce.kernel@gmail.com>
Ranted-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/fs/splice.c b/fs/splice.c
index 485e409..8af8c31 100644 (file)
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -946,11 +946,16 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
        sd->flags &= ~SPLICE_F_NONBLOCK;
        more = sd->flags & SPLICE_F_MORE;
 
+       WARN_ON_ONCE(pipe->nrbufs != 0);
+
        while (len) {
                size_t read_len;
                loff_t pos = sd->pos, prev_pos = pos;
 
-               ret = do_splice_to(in, &pos, pipe, len, flags);
+               /* Don't try to read more the pipe has space for. */
+               read_len = min_t(size_t, len,
+                                (pipe->buffers - pipe->nrbufs) << PAGE_SHIFT);
+               ret = do_splice_to(in, &pos, pipe, read_len, flags);
                if (unlikely(ret <= 0))
                        goto out_release;