[PATCH v2 03/11] fuse: fuse_dev_splice_read: use nonblocking I/O

["Ahelenia Ziemiańska" <nabijaczleweli@nabijaczleweli.xyz>]

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Otherwise we risk sleeping with the pipe locked for indeterminate
lengths of time ‒ since FUSE is usually installed with the fusermount
helper suid, given
	cat > fusedev.c <<^D
	#define _GNU_SOURCE
	#include <fcntl.h>
	#define FUSE_USE_VERSION 30
	#include <fuse.h>
	static void *fop_init(struct fuse_conn_info *conn, struct fuse_config *cfg)
	{
		for (;;)
			splice(3, 0, 4, 0, 128 * 1024 * 1024, 0);
	}
	static const struct fuse_operations fops = { .init = fop_init };
	int main(int argc, char **argv)
	{
		return fuse_main(argc, argv, &fops, NULL);
	}
	^D
	cc nullsleep.c $(pkg-config fuse3 --cflags --libs) -o nullsleep
	mkfifo fifo
	mkdir dir
	./nullsleep dir 4>fifo &
	read -r _ < fifo &
	sleep 0.1
	echo zupa > fifo
nullsleep used to sleep in splice and the shell used to enter an
uninterruptible sleep in open("fifo");
now the splice returns -EAGAIN and the whole program completes.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
---
 fs/fuse/dev.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 1a8f82f478cb..4e8caf66c01e 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1202,7 +1202,8 @@ __releases(fiq->lock)
  * the 'sent' flag.
  */
 static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
-				struct fuse_copy_state *cs, size_t nbytes)
+				struct fuse_copy_state *cs, size_t nbytes,
+				bool nonblock)
 {
 	ssize_t err;
 	struct fuse_conn *fc = fud->fc;
@@ -1238,7 +1239,7 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 			break;
 		spin_unlock(&fiq->lock);
 
-		if (file->f_flags & O_NONBLOCK)
+		if (nonblock)
 			return -EAGAIN;
 		err = wait_event_interruptible_exclusive(fiq->waitq,
 				!fiq->connected || request_pending(fiq));
@@ -1364,7 +1365,8 @@ static ssize_t fuse_dev_read(struct kiocb *iocb, struct iov_iter *to)
 
 	fuse_copy_init(&cs, 1, to);
 
-	return fuse_dev_do_read(fud, file, &cs, iov_iter_count(to));
+	return fuse_dev_do_read(fud, file, &cs, iov_iter_count(to),
+				file->f_flags & O_NONBLOCK);
 }
 
 static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
@@ -1388,7 +1390,7 @@ static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
 	fuse_copy_init(&cs, 1, NULL);
 	cs.pipebufs = bufs;
 	cs.pipe = pipe;
-	ret = fuse_dev_do_read(fud, in, &cs, len);
+	ret = fuse_dev_do_read(fud, in, &cs, len, true);
 	if (ret < 0)
 		goto out;
 
-- 
2.39.2

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