[PATCH v2 2/3] ext4: reduce max cluster size to match documented 256MB limit

[Baokun Li <libaokun@linux.alibaba.com>]

The mke2fs man page documents:

  Valid cluster-size values are from 2048 to 256M bytes per cluster.

but EXT4_MAX_CLUSTER_LOG_SIZE was set to 30 (1GB), allowing crafted
filesystem images to specify cluster sizes up to 1GB.

On 32-bit systems with bigalloc enabled, the consistency check in
ext4_handle_clustersize():

  s_blocks_per_group == s_clusters_per_group * (clustersize / blocksize)

can overflow when the cluster ratio is large enough. Since
s_blocks_per_group is not range-checked in the bigalloc path, the
wrapped product can pass the consistency check, leading to inconsistent
group geometry and potential out-of-bounds block allocation.

Reduce EXT4_MAX_CLUSTER_LOG_SIZE to 28 to match the documented 256MB
limit. With this cap, the maximum product is:

  (blocksize * 8) * (256M / blocksize) = 2^31

which fits safely in a 32-bit unsigned long for all block sizes.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260608061112.392391-1-libaokun%40linux.alibaba.com
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/ext4/ext4.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 94283a991e5c..11e41a864db8 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -334,7 +334,7 @@ struct ext4_io_submit {
 #define	EXT4_MAX_BLOCK_SIZE		65536
 #define EXT4_MIN_BLOCK_LOG_SIZE		10
 #define EXT4_MAX_BLOCK_LOG_SIZE		16
-#define EXT4_MAX_CLUSTER_LOG_SIZE	30
+#define EXT4_MAX_CLUSTER_LOG_SIZE	28
 #ifdef __KERNEL__
 # define EXT4_BLOCK_SIZE(s)		((s)->s_blocksize)
 #else
-- 
2.43.7