bcachefs: Fix bch_member.btree_bitmap_shift validation

Needs to match the assert later when we resize...

Reported-by: syzbot+e8eff054face85d7ea41@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/sb-members.c

@@ -163,7 +163,7 @@ static int validate_member(struct printbuf *err,
 		return -BCH_ERR_invalid_sb_members;
 	}
 
-	if (m.btree_bitmap_shift >= 64) {
+	if (m.btree_bitmap_shift >= BCH_MI_BTREE_BITMAP_SHIFT_MAX) {
 		prt_printf(err, "device %u: invalid btree_bitmap_shift %u", i, m.btree_bitmap_shift);
 		return -BCH_ERR_invalid_sb_members;
 	}
@@ -450,7 +450,7 @@ static void __bch2_dev_btree_bitmap_mark(struct bch_sb_field_members_v2 *mi, uns
 		m->btree_bitmap_shift += resize;
 	}
 
-	BUG_ON(m->btree_bitmap_shift > 57);
+	BUG_ON(m->btree_bitmap_shift >= BCH_MI_BTREE_BITMAP_SHIFT_MAX);
 	BUG_ON(end > 64ULL << m->btree_bitmap_shift);
 
 	for (unsigned bit = start >> m->btree_bitmap_shift;

fs/bcachefs/sb-members_format.h

@@ -65,6 +65,12 @@ struct bch_member {
 	__le32			last_journal_bucket_offset;
 };
 
+/*
+ * btree_allocated_bitmap can represent sector addresses of a u64: it itself has
+ * 64 elements, so 64 - ilog2(64)
+ */
+#define BCH_MI_BTREE_BITMAP_SHIFT_MAX	58
+
 /*
  * This limit comes from the bucket_gens array - it's a single allocation, and
  * kernel allocation are limited to INT_MAX