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Date: Thu, 21 Dec 2023 04:08:56 +0100
From: 
	Ahelenia =?utf-8?Q?Ziemia=C5=84ska?= <nabijaczleweli@nabijaczleweli.xyz>
Cc: Jens Axboe <axboe@kernel.dk>, Christian Brauner <brauner@kernel.org>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, linux-fsdevel@vger.kernel.org, 
	Miklos Szeredi <miklos@szeredi.hu>, linux-kernel@vger.kernel.org
Subject: [PATCH v2 03/11] fuse: fuse_dev_splice_read: use nonblocking I/O
Message-ID: <b6ae6cc54272011a24130723f3344616602ebb55.1703126594.git.nabijaczleweli@nabijaczleweli.xyz>
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Otherwise we risk sleeping with the pipe locked for indeterminate
lengths of time =E2=80=92 since FUSE is usually installed with the fusermou=
nt
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 =3D { .init =3D 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 Ziemia=C5=84ska <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 =3D 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);
=20
-		if (file->f_flags & O_NONBLOCK)
+		if (nonblock)
 			return -EAGAIN;
 		err =3D wait_event_interruptible_exclusive(fiq->waitq,
 				!fiq->connected || request_pending(fiq));
@@ -1364,7 +1365,8 @@ static ssize_t fuse_dev_read(struct kiocb *iocb, stru=
ct iov_iter *to)
=20
 	fuse_copy_init(&cs, 1, to);
=20
-	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);
 }
=20
 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 =3D bufs;
 	cs.pipe =3D pipe;
-	ret =3D fuse_dev_do_read(fud, in, &cs, len);
+	ret =3D fuse_dev_do_read(fud, in, &cs, len, true);
 	if (ret < 0)
 		goto out;
=20
--=20
2.39.2

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