linux-ext4 - [PATCH] jbd2: fix the inconsistency between checksum and data in memory for journal sb

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Message-Id: <20251028064728.91827-1-yebin@huaweicloud.com>
Date: Tue, 28 Oct 2025 14:47:28 +0800
From: Ye Bin <yebin@...weicloud.com>
To: tytso@....edu,
	adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org
Cc: jack@...e.cz
Subject: [PATCH] jbd2: fix the inconsistency between checksum and data in memory for journal sb

From: Ye Bin <yebin10@...wei.com>

Copying the file system while it is mounted as read-only results in
a mount failure:
[~]# mkfs.ext4 -F /dev/sdc
[~]# mount /dev/sdc -o ro /mnt/test
[~]# dd if=/dev/sdc of=/dev/sda bs=1M
[~]# mount /dev/sda /mnt/test1
[ 1094.849826] JBD2: journal checksum error
[ 1094.850927] EXT4-fs (sda): Could not load journal inode
mount: mount /dev/sda on /mnt/test1 failed: Bad message

Above issue may happen as follows:
ext4_fill_super
 set_journal_csum_feature_set(sb)
  if (ext4_has_metadata_csum(sb))
   incompat = JBD2_FEATURE_INCOMPAT_CSUM_V3;
  if (test_opt(sb, JOURNAL_CHECKSUM)
   jbd2_journal_set_features(sbi->s_journal, compat, 0, incompat);
    lock_buffer(journal->j_sb_buffer);
    sb->s_feature_incompat  |= cpu_to_be32(incompat);
    //The data in the journal sb was modified, but the checksum was not
      updated, so the data remaining in memory has a mismatch between the
      data and the checksum.
    unlock_buffer(journal->j_sb_buffer);

In this case, the journal sb copied over is in a state where the checksum
and data are inconsistent, so mounting fails.
To solve the above issue, update the checksum in memory after modifying
the journal sb.

Fixes: 4fd5ea43bc11 ("jbd2: checksum journal superblock")
Signed-off-by: Ye Bin <yebin10@...wei.com>
---
 fs/jbd2/journal.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index d480b94117cd..5b6e8c1a5e6a 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2349,6 +2349,8 @@ int jbd2_journal_set_features(journal_t *journal, unsigned long compat,
 	sb->s_feature_compat    |= cpu_to_be32(compat);
 	sb->s_feature_ro_compat |= cpu_to_be32(ro);
 	sb->s_feature_incompat  |= cpu_to_be32(incompat);
+	if (jbd2_journal_has_csum_v2or3(journal))
+		sb->s_checksum = jbd2_superblock_csum(sb);
 	unlock_buffer(journal->j_sb_buffer);
 	jbd2_journal_init_transaction_limits(journal);
 
@@ -2378,9 +2380,13 @@ void jbd2_journal_clear_features(journal_t *journal, unsigned long compat,
 
 	sb = journal->j_superblock;
 
+	lock_buffer(journal->j_sb_buffer);
 	sb->s_feature_compat    &= ~cpu_to_be32(compat);
 	sb->s_feature_ro_compat &= ~cpu_to_be32(ro);
 	sb->s_feature_incompat  &= ~cpu_to_be32(incompat);
+	if (jbd2_journal_has_csum_v2or3(journal))
+		sb->s_checksum = jbd2_superblock_csum(sb);
+	unlock_buffer(journal->j_sb_buffer);
 	jbd2_journal_init_transaction_limits(journal);
 }
 EXPORT_SYMBOL(jbd2_journal_clear_features);
-- 
2.34.1