[PATCH] hfs: port HFS+ b-tree bitmap corruption check

[Aditya Srivastava <aditya.ansh182@gmail.com>]

From: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>

In HFS+ filesystems, during b-tree open (hfs_btree_open()), the code
verifies that the allocation map bit for the tree header (node 0) is set.
If not, it indicates a corrupted map record/bitmap and mounts the volume
as read-only (SB_RDONLY) to prevent further damage.

HFS filesystems share the same b-tree structure but currently lack this
corruption detection check, which was highlighted by the maintainer.

Port this check to HFS:
1. Implement hfs_bmap_test_bit() in fs/hfs/btree.c (which safely traverses
   the b-tree map nodes to inspect the desired bit, identical to HFS+).
2. Invoke hfs_bmap_test_bit() for node 0 at the end of hfs_btree_open(),
   warning and forcing read-only if corruption is detected.

Suggested-by: Viacheslav Dubeyko <slava@dubeyko.com>
Link: https://lore.kernel.org/all/6a36101b.be22b350.2a3e9.0001.GAE@google.com/T/#r446d0fed2a2900bd805534bbcb799d86619ae2ea
Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@gmail.com>
---
 fs/hfs/btree.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++
 fs/hfs/btree.h |  1 +
 2 files changed, 53 insertions(+)

diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
index 2eb37a2f64e8..4293399d377e 100644
--- a/fs/hfs/btree.c
+++ b/fs/hfs/btree.c
@@ -155,6 +155,14 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
 	kunmap_local(head);
 	folio_unlock(folio);
 	folio_put(folio);
+
+	if (!hfs_bmap_test_bit(tree, 0)) {
+		pr_warn("(%s): %s (cnid 0x%x) map record invalid or bitmap corruption detected, forcing read-only.\n",
+			sb->s_id, id == HFS_EXT_CNID ? "extents" : "catalog", id);
+		pr_warn("Run fsck.hfs to repair.\n");
+		sb->s_flags |= SB_RDONLY;
+	}
+
 	return tree;
 
 fail_folio:
@@ -356,6 +364,50 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 	}
 }
 
+bool hfs_bmap_test_bit(struct hfs_btree *tree, u32 nidx)
+{
+	struct hfs_bnode *node;
+	struct page *page;
+	u16 off, len;
+	u8 *data, byte, m;
+	bool res = false;
+
+	node = hfs_bnode_find(tree, 0);
+	if (IS_ERR(node))
+		return false;
+
+	len = hfs_brec_lenoff(node, 2, &off);
+	while (nidx >= len * 8) {
+		u32 i;
+
+		nidx -= len * 8;
+		i = node->next;
+		if (!i) {
+			hfs_bnode_put(node);
+			return false;
+		}
+		hfs_bnode_put(node);
+		node = hfs_bnode_find(tree, i);
+		if (IS_ERR(node))
+			return false;
+		if (node->type != HFS_NODE_MAP) {
+			hfs_bnode_put(node);
+			return false;
+		}
+		len = hfs_brec_lenoff(node, 0, &off);
+	}
+	off += node->page_offset + nidx / 8;
+	page = node->page[off >> PAGE_SHIFT];
+	data = kmap_local_page(page);
+	off &= ~PAGE_MASK;
+	m = 1 << (~nidx & 7);
+	byte = data[off];
+	res = (byte & m) != 0;
+	kunmap_local(data);
+	hfs_bnode_put(node);
+	return res;
+}
+
 void hfs_bmap_free(struct hfs_bnode *node)
 {
 	struct hfs_btree *tree;
diff --git a/fs/hfs/btree.h b/fs/hfs/btree.h
index 99be858b2446..efe88f66c856 100644
--- a/fs/hfs/btree.h
+++ b/fs/hfs/btree.h
@@ -93,6 +93,7 @@ extern void hfs_btree_write(struct hfs_btree *tree);
 extern int hfs_bmap_reserve(struct hfs_btree *tree, u32 rsvd_nodes);
 extern struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree);
 extern void hfs_bmap_free(struct hfs_bnode *node);
+extern bool hfs_bmap_test_bit(struct hfs_btree *tree, u32 nidx);
 
 /* bnode.c */
 extern void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32 off, u32 len);
-- 
2.47.3