linux-ext4 - [PATCH, RFC] ext4: Allow read/only mounts with corrupted block group checksums

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Message-Id: <E1KIteH-0002pP-WE@closure.thunk.org>
Date:	Tue, 15 Jul 2008 19:09:33 -0400
From:	"Theodore Ts'o" <tytso@....edu>
To:	linux-ext4@...r.kernel.org
Subject: [PATCH, RFC] ext4: Allow read/only mounts with corrupted block group checksums


If the block group checksums are corrupted, still allow the mount to
succeed, so e2fsck can have a chance to try to fix things up.  Add
code in the remount r/w path to make sure the block group checksums
are valid before allowing the filesystem to be remounted read/write.

Signed-off-by: "Theodore Ts'o" <tytso@....edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 1cb371d..137b649 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1626,7 +1626,8 @@ static int ext4_check_descriptors(struct super_block *sb)
 			       "Checksum for group %lu failed (%u!=%u)\n",
 			       i, le16_to_cpu(ext4_group_desc_csum(sbi, i,
 			       gdp)), le16_to_cpu(gdp->bg_checksum));
-			return 0;
+			if (!(sb->s_flags & MS_RDONLY))
+				return 0;
 		}
 		if (!flexbg_flag)
 			first_block += EXT4_BLOCKS_PER_GROUP(sb);
@@ -2961,6 +2962,7 @@ static int ext4_remount (struct super_block * sb, int * flags, char * data)
 	ext4_fsblk_t n_blocks_count = 0;
 	unsigned long old_sb_flags;
 	struct ext4_mount_options old_opts;
+	ext4_group_t g;
 	int err;
 #ifdef CONFIG_QUOTA
 	int i;
@@ -3039,6 +3041,26 @@ static int ext4_remount (struct super_block * sb, int * flags, char * data)
 			}
 
 			/*
+			 * Make sure the group descriptor checksums
+			 * are sane.  If they aren't, refuse to
+			 * remount r/w.
+			 */
+			for (g = 0; g < sbi->s_groups_count; g++) {
+				struct ext4_group_desc *gdp = 
+					ext4_get_group_desc(sb, g, NULL);
+
+				if (!ext4_group_desc_csum_verify(sbi, g, gdp)) {
+					printk(KERN_ERR
+	       "EXT4-fs: ext4_remount: "
+		"Checksum for group %lu failed (%u!=%u)\n",
+		g, le16_to_cpu(ext4_group_desc_csum(sbi, g, gdp)),
+					       le16_to_cpu(gdp->bg_checksum));
+					err = -EINVAL;
+					goto restore_opts;
+				}
+			}
+
+			/*
 			 * If we have an unprocessed orphan list hanging
 			 * around from a previously readonly bdev mount,
 			 * require a full umount/remount for now.
--
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