bdev: add back PAGE_SIZE block size validation for sb_set_blocksize()

The commit titled "block/bdev: lift block size restrictions to 64k"
lifted the block layer's max supported block size to 64k inside the
helper blk_validate_block_size() now that we support large folios.
However in lifting the block size we also removed the silly use
cases many filesystems have to use sb_set_blocksize() to *verify*
that the block size <= PAGE_SIZE. The call to sb_set_blocksize() was
used to check the block size <= PAGE_SIZE since historically we've
always supported userspace to create for example 64k block size
filesystems even on 4k page size systems, but what we didn't allow
was mounting them. Older filesystems have been using the check with
sb_set_blocksize() for years.

While, we could argue that such checks should be filesystem specific,
there are much more users of sb_set_blocksize() than LBS enabled
filesystem on upstream, so just do the easier thing and bring back
the PAGE_SIZE check for sb_set_blocksize() users and only skip it
for LBS enabled filesystems.

This will ensure that tests such as generic/466 when run in a loop
against say, ext4, won't try to try to actually mount a filesystem with
a block size larger than your filesystem supports given your PAGE_SIZE
and in the worst case crash.

Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20250307020403.3068567-1-mcgrof@kernel.org
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

4 files changed, 6 insertions, 2 deletions

diff --git a/block/bdev.c b/block/bdev.c
index 3bd948e6438d..4844d1e27b6f 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -181,6 +181,8 @@ EXPORT_SYMBOL(set_blocksize);
 
 int sb_set_blocksize(struct super_block *sb, int size)
 {
+	if (!(sb->s_type->fs_flags & FS_LBS) && size > PAGE_SIZE)
+		return 0;
 	if (set_blocksize(sb->s_bdev_file, size))
 		return 0;
 	/* If we get here, we know size is validated */
diff --git a/fs/bcachefs/fs.c b/fs/bcachefs/fs.c
index 90ade8f648d9..e99e378d68ea 100644
--- a/fs/bcachefs/fs.c
+++ b/fs/bcachefs/fs.c
@@ -2396,7 +2396,7 @@ static struct file_system_type bcache_fs_type = {
 	.name			= "bcachefs",
 	.init_fs_context	= bch2_init_fs_context,
 	.kill_sb		= bch2_kill_sb,
-	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
+	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_LBS,
 };
 
 MODULE_ALIAS_FS("bcachefs");
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index d92d7a07ea89..3d8b80165d48 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -2118,7 +2118,8 @@ static struct file_system_type xfs_fs_type = {
 	.init_fs_context	= xfs_init_fs_context,
 	.parameters		= xfs_fs_parameters,
 	.kill_sb		= xfs_kill_sb,
-	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME,
+	.fs_flags		= FS_REQUIRES_DEV | FS_ALLOW_IDMAP | FS_MGTIME |
+				  FS_LBS,
 };
 MODULE_ALIAS_FS("xfs");
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index be3ad155ec9f..f47fbe16b101 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2604,6 +2604,7 @@ struct file_system_type {
 #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
 #define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
 #define FS_MGTIME		64	/* FS uses multigrain timestamps */
+#define FS_LBS			128	/* FS supports LBS */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	int (*init_fs_context)(struct fs_context *);
 	const struct fs_parameter_spec *parameters;