[PATCH AUTOSEL 4.9 7/7] ext4: avoid resizing to a partial cluster size

[Sasha Levin]

From: "Kiselev, Oleg" <okiselev@xxxxxxxxxx>

[ Upstream commit 69cb8e9d8cd97cdf5e293b26d70a9dee3e35e6bd ]

This patch avoids an attempt to resize the filesystem to an
unaligned cluster boundary.  An online resize to a size that is not
integral to cluster size results in the last iteration attempting to
grow the fs by a negative amount, which trips a BUG_ON and leaves the fs
with a corrupted in-memory superblock.

Signed-off-by: Oleg Kiselev <okiselev@xxxxxxxxxx>
Link: https://lore.kernel.org/r/0E92A0AB-4F16-4F1A-94B7-702CC6504FDE@xxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/ext4/resize.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index c367129dcdc1..c16bcd3f2e6d 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -1940,6 +1940,16 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
 	}
 	brelse(bh);
 
+	/*
+	 * For bigalloc, trim the requested size to the nearest cluster
+	 * boundary to avoid creating an unusable filesystem. We do this
+	 * silently, instead of returning an error, to avoid breaking
+	 * callers that blindly resize the filesystem to the full size of
+	 * the underlying block device.
+	 */
+	if (ext4_has_feature_bigalloc(sb))
+		n_blocks_count &= ~((1 << EXT4_CLUSTER_BITS(sb)) - 1);
+
 retry:
 	o_blocks_count = ext4_blocks_count(es);
 
-- 
2.35.1