splice: always fsnotify_access(in), fsnotify_modify(out) on success

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/jbyihkyk5dtaohdwjyivambb2gffyjs3dodpofafnkkunxq7bu@jngkdxx65pux/t/#u
Link: https://bugs.debian.org/1039488
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Acked-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Message-Id: <604ec704d933e0e0121d9e107ce914512e045fad.1688393619.git.nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 15 insertions, 17 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 004eb1c4ce31..016b0a83df44 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1267,10 +1267,8 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
 		if ((in->f_flags | out->f_flags) & O_NONBLOCK)
 			flags |= SPLICE_F_NONBLOCK;
 
-		return splice_pipe_to_pipe(ipipe, opipe, len, flags);
-	}
-
-	if (ipipe) {
+		ret = splice_pipe_to_pipe(ipipe, opipe, len, flags);
+	} else if (ipipe) {
 		if (off_in)
 			return -ESPIPE;
 		if (off_out) {
@@ -1295,18 +1293,11 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
 		ret = do_splice_from(ipipe, out, &offset, len, flags);
 		file_end_write(out);
 
-		if (ret > 0)
-			fsnotify_modify(out);
-
 		if (!off_out)
 			out->f_pos = offset;
 		else
 			*off_out = offset;
-
-		return ret;
-	}
-
-	if (opipe) {
+	} else if (opipe) {
 		if (off_out)
 			return -ESPIPE;
 		if (off_in) {
@@ -1322,18 +1313,25 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
 
 		ret = splice_file_to_pipe(in, opipe, &offset, len, flags);
 
-		if (ret > 0)
-			fsnotify_access(in);
-
 		if (!off_in)
 			in->f_pos = offset;
 		else
 			*off_in = offset;
+	} else {
+		ret = -EINVAL;
+	}
 
-		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);
 	}
 
-	return -EINVAL;
+	return ret;
 }
 
 static long __do_splice(struct file *in, loff_t __user *off_in,