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Date:   Mon, 3 Jul 2023 16:42:13 +0200
From:   Ahelenia =?utf-8?Q?Ziemia=C5=84ska?= 
        <nabijaczleweli@nabijaczleweli.xyz>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>,
        Amir Goldstein <amir73il@gmail.com>,
        Chung-Chiang Cheng <cccheng@synology.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v5 1/3] splice: always fsnotify_access(in),
 fsnotify_modify(out) on success
Message-ID: <604ec704d933e0e0121d9e107ce914512e045fad.1688393619.git.nabijaczleweli@nabijaczleweli.xyz>
References: <cover.1688393619.git.nabijaczleweli@nabijaczleweli.xyz>
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The current behaviour caused an asymmetry where some write APIs
(write, sendfile) would notify the written-to/read-from objects,
but splice wouldn't.

This affected userspace which uses inotify, most notably coreutils
tail -f, to monitor pipes.
If the pipe buffer had been filled by a splice-family function:
  * tail wouldn't know and thus wouldn't service the pipe, and
  * all writes to the pipe would block because it's full,
thus service was denied.
(For the particular case of tail -f this could be worked around
 with ---disable-inotify.)

Fixes: 983652c69199 ("splice: report related fsnotify events")
Link: https://lore.kernel.org/linux-fsdevel/jbyihkyk5dtaohdwjyivambb2gffyjs=
3dodpofafnkkunxq7bu@jngkdxx65pux/t/#u
Link: https://bugs.debian.org/1039488
Signed-off-by: Ahelenia Ziemia=C5=84ska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Jan Kara <jack@suse.cz>
---
 fs/splice.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 3e06611d19ae..6ae6da52eba9 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1154,10 +1154,8 @@ long do_splice(struct file *in, loff_t *off_in, stru=
ct file *out,
 		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
 			flags |=3D SPLICE_F_NONBLOCK;
=20
-		return splice_pipe_to_pipe(ipipe, opipe, len, flags);
-	}
-
-	if (ipipe) {
+		ret =3D splice_pipe_to_pipe(ipipe, opipe, len, flags);
+	} else if (ipipe) {
 		if (off_in)
 			return -ESPIPE;
 		if (off_out) {
@@ -1182,18 +1180,11 @@ long do_splice(struct file *in, loff_t *off_in, str=
uct file *out,
 		ret =3D do_splice_from(ipipe, out, &offset, len, flags);
 		file_end_write(out);
=20
-		if (ret > 0)
-			fsnotify_modify(out);
-
 		if (!off_out)
 			out->f_pos =3D offset;
 		else
 			*off_out =3D offset;
-
-		return ret;
-	}
-
-	if (opipe) {
+	} else if (opipe) {
 		if (off_out)
 			return -ESPIPE;
 		if (off_in) {
@@ -1209,18 +1200,24 @@ long do_splice(struct file *in, loff_t *off_in, str=
uct file *out,
=20
 		ret =3D splice_file_to_pipe(in, opipe, &offset, len, flags);
=20
-		if (ret > 0)
-			fsnotify_access(in);
-
 		if (!off_in)
 			in->f_pos =3D offset;
 		else
 			*off_in =3D offset;
+	} else
+		return -EINVAL;
=20
-		return ret;
+	if (ret > 0) {
+		/*
+		 * Generate modify out before access in:
+		 * do_splice_from() may've already sent modify out,
+		 * and this ensures the events get merged.
+		 */
+		fsnotify_modify(out);
+		fsnotify_access(in);
 	}
=20
-	return -EINVAL;
+	return ret;
 }
=20
 static long __do_splice(struct file *in, loff_t __user *off_in,
--=20
2.39.2


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