bcachefs: BCH_SB_LAYOUT_SIZE_BITS_MAX

Define a constant for the max superblock size, to avoid a too-large
shift.

Reported-by: syzbot+a8b0fb419355c91dda7f@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -903,6 +903,8 @@ unsigned bcachefs_metadata_required_upgrade_below = bcachefs_metadata_version_re
 #define BCH_SB_SECTOR			8
 #define BCH_SB_MEMBERS_MAX		64 /* XXX kill */
 
+#define BCH_SB_LAYOUT_SIZE_BITS_MAX	16 /* 32 MB */
+
 struct bch_sb_layout {
 	__uuid_t		magic;	/* bcachefs superblock UUID */
 	__u8			layout_type;

fs/bcachefs/super-io.c

@@ -649,7 +649,7 @@ static int read_one_super(struct bch_sb_handle *sb, u64 offset, struct printbuf
 
 	bytes = vstruct_bytes(sb->sb);
 
-	if (bytes > 512 << sb->sb->layout.sb_max_size_bits) {
+	if (bytes > 512ULL << min(BCH_SB_LAYOUT_SIZE_BITS_MAX, sb->sb->layout.sb_max_size_bits)) {
 		prt_printf(err, "Invalid superblock: too big (got %zu bytes, layout max %lu)",
 		       bytes, 512UL << sb->sb->layout.sb_max_size_bits);
 		return -BCH_ERR_invalid_sb_too_big;