[PATCH 2/2] ext4: replace BUG_ON with graceful retry in ext4_write_inline_data_end

[Aditya Srivastava <aditya.ansh182@gmail.com>]

From: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>

During a buffered write, `ext4_write_inline_data_end()` acquires the
xattr lock after preparing the write. If a concurrent page fault
(`ext4_page_mkwrite()`) converts the inline data to an extent after
the write_end handlers check the state but before
`ext4_write_inline_data_end()` acquires the xattr write lock, the
subsequent check will trigger a kernel panic via
`BUG_ON(!ext4_has_inline_data(inode))`.

Replace the `BUG_ON` check with a graceful error-handling retry path.
If the inline data is cleared after locking the xattr, we safely
release all resources (releasing `iloc.bh`, unlocking/putting the
folio, stopping the active journal transaction handle) and return 0
(VFS retry) to let the generic write path retry the operation safely.

Reported-by: syzbot+0c89d865531d053abb2d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0c89d865531d053abb2d
Fixes: 3fdcfb668fd7 ("ext4: add journalled write support for inline data")
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
---
 fs/ext4/inline.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 8045e4ff270c..cfd591dc1d9c 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -812,7 +812,19 @@ int ext4_write_inline_data_end(struct inode *inode, loff_t pos, unsigned len,
 			goto out;
 		}
 		ext4_write_lock_xattr(inode, &no_expand);
-		BUG_ON(!ext4_has_inline_data(inode));
+		/*
+		 * We could have raced with ext4_page_mkwrite() converting
+		 * the inode and clearing the inline data flag, so we just
+		 * release resources and retry the whole write.
+		 */
+		if (unlikely(!ext4_has_inline_data(inode))) {
+			ext4_write_unlock_xattr(inode, &no_expand);
+			brelse(iloc.bh);
+			folio_unlock(folio);
+			folio_put(folio);
+			ext4_journal_stop(handle);
+			return 0;
+		}
 
 		/*
 		 * ei->i_inline_off may have changed since
-- 
2.47.3