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From:   Mauricio Faria de Oliveira <mfo@canonical.com>
To:     Jan Kara <jack@suse.cz>
Cc:     linux-ext4@vger.kernel.org,
        dann frazier <dann.frazier@canonical.com>,
        Mauricio Faria de Oliveira <mauricio.foliveira@gmail.com>,
        Jan Kara <jack@suse.com>
Subject: [RFC PATCH v2 1/5] jbd2: test case for ext4 data=journal/mmap() journal corruption
Date:   Sun,  9 Aug 2020 22:02:04 -0300
Message-Id: <20200810010210.3305322-2-mfo@canonical.com>
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This checks during journal commit, right after calculating the
checksum of a buffer head, whether its contents match the 'BUG'
string (the cookie string in the test case userspace part.)

If so, it sleeps 5 seconds for such contents to change (i.e.,
so that the actual checksum changes from what was calculated.)

And if it changed, set a flag to panic after committing to disk.

Then, on filesystem remount/journal recovery there is an invalid
checksum error, and recovery fails:

  $ sudo mount -o data=journal,journal_checksum $DEV $MNT
  [ 100.832223] EXT4-fs: Warning: mounting with data=journal disables
  delayed allocation, dioread_nolock, and O_DIRECT support!
  [ 100.837488] JBD2: Invalid checksum recovering data block 8706 in log
  [ 100.842010] JBD2: recovery failed
  [ 100.843045] EXT4-fs (loop0): error loading journal
  mount: /ext4: can't read superblock on /dev/loop0.
---
 fs/jbd2/commit.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 6d2da8ad0e6f..51f713089e35 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -26,6 +26,11 @@
 #include <linux/bitops.h>
 #include <trace/events/jbd2.h>
 
+#include <linux/printk.h>
+#include <linux/delay.h>
+
+static journal_t *force_panic;
+
 /*
  * IO end handler for temporary buffer_heads handling writes to the journal.
  */
@@ -331,14 +336,35 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag,
 	__u32 csum32;
 	__be32 seq;
 
+	// For the testcase
+	__u32 csum32_later;
+	__u8 *bh_data;
+
 	if (!jbd2_journal_has_csum_v2or3(j))
 		return;
 
 	seq = cpu_to_be32(sequence);
 	addr = kmap_atomic(page);
 	csum32 = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&seq, sizeof(seq));
+	csum32_later = csum32; // Copy csum32 to check again later
 	csum32 = jbd2_chksum(j, csum32, addr + offset_in_page(bh->b_data),
 			     bh->b_size);
+
+	// Check for testcase cookie 'BUG' in the buffer_head data.
+	bh_data = addr + offset_in_page(bh->b_data);
+	if (bh_data[0] == 'B' &&
+	    bh_data[1] == 'U' &&
+	    bh_data[2] == 'G') {
+		pr_info("TESTCASE: Cookie found. Waiting 5 seconds for changes.\n");
+		msleep(5000);
+		pr_info("TESTCASE: Cookie eaten. Resumed.\n");
+	}
+
+	// Check the checksum again for changes/panic after commit.
+	csum32_later = jbd2_chksum(j, csum32_later, addr + offset_in_page(bh->b_data), bh->b_size);
+	if (csum32 != csum32_later)
+		force_panic = j;
+
 	kunmap_atomic(addr);
 
 	if (jbd2_has_feature_csum3(j))
@@ -885,6 +911,9 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 		blkdev_issue_flush(journal->j_dev, GFP_NOFS);
 	}
 
+	if (force_panic == journal)
+		panic("TESTCASE: checksum changed; commit record done; panic!\n");
+
 	if (err)
 		jbd2_journal_abort(journal, err);
 
-- 
2.17.1