linux-ext4 - [PATCH -V2 2/2] ext4: avoid resizing to a partial cluster size

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Message-ID: <0E92A0AB-4F16-4F1A-94B7-702CC6504FDE@amazon.com>
Date:   Wed, 20 Jul 2022 04:27:48 +0000
From:   "Kiselev, Oleg" <okiselev@...zon.com>
To:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
CC:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>
Subject: [PATCH -V2 2/2] ext4: avoid resizing to a partial cluster size

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@...zon.com>
---
v2:
  - Moved the code higher up in the call stack, changed the
    implementation to trim the request size
---
 fs/ext4/resize.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index a69113b4ce4e..b69fda478e9d 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -2007,6 +2007,16 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t )
 	}
 	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);

--
2.34.3