From 62913ae96de747091c4dacd06d158e7729c1a76d Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Tue, 7 Mar 2023 23:15:49 -0500
Subject: [PATCH] ext4, jbd2: add an optimized bmap for the journal inode

The generic bmap() function exported by the VFS takes locks and does
checks that are not necessary for the journal inode.  So allow the
file system to set a journal-optimized bmap function in
journal->j_bmap.

Reported-by: syzbot+9543479984ae9e576000@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=e4aaa78795e490421c79f76ec3679006c8ff4cf0
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/ext4/super.c      | 23 +++++++++++++++++++++++
 fs/jbd2/journal.c    |  9 ++++++---
 include/linux/jbd2.h |  8 ++++++++
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 2192b41..46b7345 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5742,6 +5742,28 @@ static struct inode *ext4_get_journal_inode(struct super_block *sb,
 	return journal_inode;
 }
 
+static int ext4_journal_bmap(journal_t *journal, sector_t *block)
+{
+	struct ext4_map_blocks map;
+	int ret;
+
+	if (journal->j_inode == NULL)
+		return 0;
+
+	map.m_lblk = *block;
+	map.m_len = 1;
+	ret = ext4_map_blocks(NULL, journal->j_inode, &map, 0);
+	if (ret <= 0) {
+		ext4_msg(journal->j_inode->i_sb, KERN_CRIT,
+			 "journal bmap failed: block %llu ret %d\n",
+			 *block, ret);
+		jbd2_journal_abort(journal, ret ? ret : -EIO);
+		return ret;
+	}
+	*block = map.m_pblk;
+	return 0;
+}
+
 static journal_t *ext4_get_journal(struct super_block *sb,
 				   unsigned int journal_inum)
 {
@@ -5762,6 +5784,7 @@ static journal_t *ext4_get_journal(struct super_block *sb,
 		return NULL;
 	}
 	journal->j_private = sb;
+	journal->j_bmap = ext4_journal_bmap;
 	ext4_init_journal_params(sb, journal);
 	return journal;
 }
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 2696f43..c84f588 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -970,10 +970,13 @@ int jbd2_journal_bmap(journal_t *journal, unsigned long blocknr,
 {
 	int err = 0;
 	unsigned long long ret;
-	sector_t block = 0;
+	sector_t block = blocknr;
 
-	if (journal->j_inode) {
-		block = blocknr;
+	if (journal->j_bmap) {
+		err = journal->j_bmap(journal, &block);
+		if (err == 0)
+			*retp = block;
+	} else if (journal->j_inode) {
 		ret = bmap(journal->j_inode, &block);
 
 		if (ret || !block) {
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 2170e0c..6ffa34c 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1308,6 +1308,14 @@ struct journal_s
 				    struct buffer_head *bh,
 				    enum passtype pass, int off,
 				    tid_t expected_commit_id);
+
+	/**
+	 * @j_bmap:
+	 *
+	 * Bmap function that should be used instead of the generic
+	 * VFS bmap function.
+	 */
+	int (*j_bmap)(struct journal_s *journal, sector_t *block);
 };
 
 #define jbd2_might_wait_for_commit(j) \
-- 
2.7.4