[PATCH 03/10] netfs: Fix enomem handling in buffered reads

[David Howells]

If netfs_read_to_pagecache() gets an error from either ->prepare_read() or
from netfs_prepare_read_iterator(), it needs to decrement ->nr_outstanding,
cancel the subrequest and break out of the issuing loop.  Currently, it
only does this for two of the cases, but there are two more that aren't
handled.

Fix this by moving the handling to a common place and jumping to it from
all four places.  This is in preference to inserting a wrapper around
netfs_prepare_read_iterator() as proposed by Dmitry Antipov[1].

Link: https://lore.kernel.org/r/20241202093943.227786-1-dmantipov@yandex.ru/ [1]

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Reported-by: syzbot+404b4b745080b6210c6c at syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=404b4b745080b6210c6c
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Dmitry Antipov <dmantipov at yandex.ru>
cc: Jeff Layton <jlayton at kernel.org>
cc: netfs at lists.linux.dev
cc: linux-fsdevel at vger.kernel.org
---
 fs/netfs/buffered_read.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 7ac34550c403..4dc9b8286355 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -275,22 +275,14 @@ static void netfs_read_to_pagecache(struct netfs_io_request *rreq)
 			netfs_stat(&netfs_n_rh_download);
 			if (rreq->netfs_ops->prepare_read) {
 				ret = rreq->netfs_ops->prepare_read(subreq);
-				if (ret < 0) {
-					atomic_dec(&rreq->nr_outstanding);
-					netfs_put_subrequest(subreq, false,
-							     netfs_sreq_trace_put_cancel);
-					break;
-				}
+				if (ret < 0)
+					goto prep_failed;
 				trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
 			}
 
 			slice = netfs_prepare_read_iterator(subreq);
-			if (slice < 0) {
-				atomic_dec(&rreq->nr_outstanding);
-				netfs_put_subrequest(subreq, false, netfs_sreq_trace_put_cancel);
-				ret = slice;
-				break;
-			}
+			if (slice < 0)
+				goto prep_iter_failed;
 
 			rreq->netfs_ops->issue_read(subreq);
 			goto done;
@@ -302,6 +294,8 @@ static void netfs_read_to_pagecache(struct netfs_io_request *rreq)
 			trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
 			netfs_stat(&netfs_n_rh_zero);
 			slice = netfs_prepare_read_iterator(subreq);
+			if (slice < 0)
+				goto prep_iter_failed;
 			__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 			netfs_read_subreq_terminated(subreq, 0, false);
 			goto done;
@@ -310,6 +304,8 @@ static void netfs_read_to_pagecache(struct netfs_io_request *rreq)
 		if (source == NETFS_READ_FROM_CACHE) {
 			trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
 			slice = netfs_prepare_read_iterator(subreq);
+			if (slice < 0)
+				goto prep_iter_failed;
 			netfs_read_cache_to_pagecache(rreq, subreq);
 			goto done;
 		}
@@ -318,6 +314,14 @@ static void netfs_read_to_pagecache(struct netfs_io_request *rreq)
 		WARN_ON_ONCE(1);
 		break;
 
+	prep_iter_failed:
+		ret = slice;
+	prep_failed:
+		subreq->error = ret;
+		atomic_dec(&rreq->nr_outstanding);
+		netfs_put_subrequest(subreq, false, netfs_sreq_trace_put_cancel);
+		break;
+
 	done:
 		size -= slice;
 		start += slice;