linux-ext4 - [PATCH] ext4: Fix warning in ext4_dio_write_end

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Message-Id: <20231122181440.12043-1-jack@suse.cz>
Date: Wed, 22 Nov 2023 19:14:40 +0100
From: Jan Kara <jack@...e.cz>
To: Ted Tso <tytso@....edu>
Cc: <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>,
	syzbot+47479b71cdfc78f56d30@...kaller.appspotmail.com
Subject: [PATCH] ext4: Fix warning in ext4_dio_write_end_io()

The syzbot has reported that it can hit the warning in
ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
reproducer creates a race between DIO IO completion and truncate
expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
inode state where i_disksize is already updated but i_size is not
updated yet. Since we are careful when setting up DIO write and consider
it extending (and thus performing the IO synchronously with i_rwsem held
exclusively) whenever it goes past either of i_size or i_disksize, we
can use the same test during IO completion without risking entering
ext4_handle_inode_extension() without i_rwsem held. This way we make it
obvious both i_size and i_disksize are large enough when we report DIO
completion without relying on unreliable WARN_ON.

Reported-by: syzbot+47479b71cdfc78f56d30@...kaller.appspotmail.com
Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/ext4/file.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0166bb9ca160..ba497aabdd1e 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -386,10 +386,11 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
 	 * blocks. But the code in ext4_iomap_alloc() is careful to use
 	 * zeroed/unwritten extents if this is possible; thus we won't leave
 	 * uninitialized blocks in a file even if we didn't succeed in writing
-	 * as much as we intended.
+	 * as much as we intended. Also we can race with truncate or write
+	 * expanding the file so we have to be a bit careful here.
 	 */
-	WARN_ON_ONCE(i_size_read(inode) < READ_ONCE(EXT4_I(inode)->i_disksize));
-	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize))
+	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
+	    pos + size <= i_size_read(inode))
 		return size;
 	return ext4_handle_inode_extension(inode, pos, size);
 }
-- 
2.35.3