ntfs: restore $MFT mirror contents check

check_mft_mirror() still computes the number of bytes to validate in each
mirrored MFT record, but the actual comparison against $MFTMirr was dropped
when the superblock code was updated.

As a result, mount misses a stale or inconsistent $MFTMirr as long as both
records pass the structural baad-record checks. Restore the comparison and
log an error when the primary $MFT record differs from its mirror copy.

Returning false lets the existing mount error handling mark the volume as
having NTFS errors and, with on_errors=remount-ro, continue read-only. The
default on_errors=continue mount policy still allows the mount to proceed.

Fixes: 6251f0b ("ntfs: update super block operations")
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: charsyam

fs/ntfs/super.c

@@ -993,6 +993,13 @@ static bool check_mft_mirror(struct ntfs_volume *vol)
 			    ntfs_is_baad_recordp((__le32 *)kmirr))
 				bytes = vol->mft_record_size;
 		}
+		/* Compare the two records. */
+		if (memcmp(kmft, kmirr, bytes)) {
+			ntfs_error(sb,
+				   "$MFT and $MFTMirr record %i do not match.  Run chkdsk.",
+				   i);
+			goto mm_unmap_out;
+		}
 		kmft += vol->mft_record_size;
 		kmirr += vol->mft_record_size;
 	} while (++i < vol->mftmirr_size);