jbd2: fix block tag checksum verification brokenness

Al Viro complained of a ton of bogosity with regards to the jbd2 block
tag header checksum.  This one checksum is 16 bits, so cut off the
upper 16 bits and treat it as a 16-bit value and don't mess around
with be32* conversions.  Fortunately metadata checksumming is still
"experimental" and not in a shipping e2fsprogs, so there should be few
users affected by this.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 0f53946f13c1..e61d7224a729 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -343,20 +343,21 @@ static void jbd2_block_tag_csum_set(journal_t *j, journal_block_tag_t *tag,
 {
         struct page *page = bh->b_page;
         __u8 *addr;
-        __u32 csum;
+        __u32 csum32;
 
         if (!JBD2_HAS_INCOMPAT_FEATURE(j, JBD2_FEATURE_INCOMPAT_CSUM_V2))
                 return;
 
         sequence = cpu_to_be32(sequence);
         addr = kmap_atomic(page);
-        csum = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
-                          sizeof(sequence));
-        csum = jbd2_chksum(j, csum, addr + offset_in_page(bh->b_data),
-                          bh->b_size);
+        csum32 = jbd2_chksum(j, j->j_csum_seed, (__u8 *)&sequence,
+                             sizeof(sequence));
+        csum32 = jbd2_chksum(j, csum32, addr + offset_in_page(bh->b_data),
+                             bh->b_size);
         kunmap_atomic(addr);
 
-        tag->t_checksum = cpu_to_be32(csum);
+        /* We only have space to store the lower 16 bits of the crc32c. */
+        tag->t_checksum = cpu_to_be16(csum32);
 }
 /*
  * jbd2_journal_commit_transaction