linux-ext4 - [PATCH 1/2] ext4: fix online resize's handling of a too-small final block group

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Date:   Mon,  3 Sep 2018 22:25:45 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     Torsten Hilbrich <torsten.hilbrich@...unet.com>,
        Theodore Ts'o <tytso@....edu>, stable@...r.kernel.org
Subject: [PATCH 1/2] ext4: fix online resize's handling of a too-small final block group

Avoid growing the file system to an extent so that the last block
group is too small to hold all of the metadata that must be stored in
the block group.

This problem can be triggered with the following reproducer:

umount /mnt
mke2fs -F -m0 -b 4096 -t ext4 -O resize_inode,^has_journal \
	-E resize=1073741824 /tmp/foo.img 128M
mount /tmp/foo.img /mnt
truncate --size 1708M /tmp/foo.img
resize2fs /dev/loop0 295400
umount /mnt
e2fsck -fy /tmp/foo.img

Reported-by: Torsten Hilbrich <torsten.hilbrich@...unet.com>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Cc: stable@...r.kernel.org
---
 fs/ext4/resize.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index e5fb38451a73..33655a6eff4d 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -1986,6 +1986,26 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 		}
 	}
 
+	/*
+	 * Make sure the last group has enough space so that it's
+	 * guaranteed to have enough space for all metadata blocks
+	 * that it might need to hold.  (We might not need to store
+	 * the inode table blocks in the last block group, but there
+	 * will be cases where this might be needed.)
+	 */
+	if ((ext4_group_first_block_no(sb, n_group) +
+	     ext4_group_overhead_blocks(sb, n_group) + 2 +
+	     sbi->s_itb_per_group + sbi->s_cluster_ratio) >= n_blocks_count) {
+		n_blocks_count = ext4_group_first_block_no(sb, n_group);
+		n_group--;
+		n_blocks_count_retry = 0;
+		if (resize_inode) {
+			iput(resize_inode);
+			resize_inode = NULL;
+		}
+		goto retry;
+	}
+
 	/* extend the last group */
 	if (n_group == o_group)
 		add = n_blocks_count - o_blocks_count;
-- 
2.18.0.rc0