hfs: port HFS+ b-tree bitmap corruption check

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.

Port this check to HFS, aligning its implementation with HFS+ to
maintain consistent b-tree logic across both filesystems:
1. Define struct hfs_bmap_ctx, and define HFS_TREE_HEAD and the relevant
   map record indices in include/linux/hfs_common.h.
2. Port the necessary offset and length validation helpers to
   fs/hfs/btree.h as static inline functions with robust null pointer
   checks.
3. Implement static hfs_bmap_get_map_page() in fs/hfs/btree.c.
4. Implement static hfs_bmap_test_bit() in fs/hfs/btree.c to inspect
   the B-tree bitmap using the get_map_page helper.
5. Implement static hfs_bmap_clear_bit() in fs/hfs/btree.c.
6. Rewrite hfs_bmap_alloc() in fs/hfs/btree.c to use the unified
   hfs_bmap_get_map_page() helper, eliminating redundant page and offset
   calculation code.
7. Refactor hfs_bmap_free() in fs/hfs/btree.c to use the new
   hfs_bmap_clear_bit() helper.
8. In hfs_btree_open(), retrieve the header node via hfs_bnode_find(),
   test its allocation bit with hfs_bmap_test_bit(), and release it
   using hfs_bnode_put().

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>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
Link: https://lore.kernel.org/r/20260716074150.1660-1-aditya.ansh182@gmail.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>

fs/hfs/bnode.c

@@ -15,48 +15,6 @@
 
 #include "btree.h"
 
-static inline
-bool is_bnode_offset_valid(struct hfs_bnode *node, u32 off)
-{
-	bool is_valid = off < node->tree->node_size;
-
-	if (!is_valid) {
-		pr_err("requested invalid offset: "
-		       "NODE: id %u, type %#x, height %u, "
-		       "node_size %u, offset %u\n",
-		       node->this, node->type, node->height,
-		       node->tree->node_size, off);
-	}
-
-	return is_valid;
-}
-
-static inline
-u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
-{
-	unsigned int node_size;
-
-	if (!is_bnode_offset_valid(node, off))
-		return 0;
-
-	node_size = node->tree->node_size;
-
-	if ((u64)off + len > node_size) {
-		u32 new_len = node_size - off;
-
-		pr_err("requested length has been corrected: "
-		       "NODE: id %u, type %#x, height %u, "
-		       "node_size %u, offset %u, "
-		       "requested_len %u, corrected_len %u\n",
-		       node->this, node->type, node->height,
-		       node->tree->node_size, off, len, new_len);
-
-		return new_len;
-	}
-
-	return len;
-}
-
 void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32 off, u32 len)
 {
 	struct page *page;

fs/hfs/btree.c

@@ -15,6 +15,121 @@
 
 #include "btree.h"
 
+/* Context for iterating b-tree map pages
+ * @page_idx: The index of the page within the b-node's page array
+ * @off: The byte offset within the mapped page
+ * @len: The remaining length of the map record
+ */
+struct hfs_bmap_ctx {
+	unsigned int page_idx;
+	unsigned int off;
+	u16 len;
+};
+
+/*
+ * Finds the specific page containing the requested byte offset within the map
+ * record. Automatically handles the difference between header and map nodes.
+ * Returns the struct page pointer, or an ERR_PTR on failure.
+ * Note: The caller is responsible for mapping/unmapping the returned page.
+ */
+static struct page *hfs_bmap_get_map_page(struct hfs_bnode *node,
+					  struct hfs_bmap_ctx *ctx,
+					  u32 byte_offset)
+{
+	u16 rec_idx, off16;
+	unsigned int page_off;
+
+	if (node->this == HFS_TREE_HEAD) {
+		if (node->type != HFS_NODE_HEADER) {
+			pr_err("hfs: invalid btree header node\n");
+			return ERR_PTR(-EIO);
+		}
+		rec_idx = HFS_BTREE_HDR_MAP_REC_INDEX;
+	} else {
+		if (node->type != HFS_NODE_MAP) {
+			pr_err("hfs: invalid btree map node\n");
+			return ERR_PTR(-EIO);
+		}
+		rec_idx = HFS_BTREE_MAP_NODE_REC_INDEX;
+	}
+
+	ctx->len = hfs_brec_lenoff(node, rec_idx, &off16);
+	if (!ctx->len)
+		return ERR_PTR(-ENOENT);
+
+	if (!is_bnode_offset_valid(node, off16))
+		return ERR_PTR(-EIO);
+
+	ctx->len = check_and_correct_requested_length(node, off16, ctx->len);
+
+	if (byte_offset >= ctx->len)
+		return ERR_PTR(-EINVAL);
+
+	page_off = (u32)off16 + node->page_offset + byte_offset;
+	ctx->page_idx = page_off >> PAGE_SHIFT;
+	ctx->off = page_off & ~PAGE_MASK;
+
+	return node->page[ctx->page_idx];
+}
+
+/**
+ * hfs_bmap_test_bit - test a bit in the b-tree map
+ * @node: the b-tree node containing the map record
+ * @node_bit_idx: the relative bit index within the node's map record
+ *
+ * Returns true if set, false if clear or on failure.
+ */
+static bool hfs_bmap_test_bit(struct hfs_bnode *node, u32 node_bit_idx)
+{
+	struct hfs_bmap_ctx ctx;
+	struct page *page;
+	u8 *bmap, byte, mask;
+
+	page = hfs_bmap_get_map_page(node, &ctx, node_bit_idx / BITS_PER_BYTE);
+	if (IS_ERR(page))
+		return false;
+
+	bmap = kmap_local_page(page);
+	byte = bmap[ctx.off];
+	kunmap_local(bmap);
+
+	mask = 1 << (7 - (node_bit_idx % BITS_PER_BYTE));
+	return (byte & mask) != 0;
+}
+
+/**
+ * hfs_bmap_clear_bit - clear a bit in the b-tree map
+ * @node: the b-tree node containing the map record
+ * @node_bit_idx: the relative bit index within the node's map record
+ *
+ * Returns 0 on success, -EINVAL if already clear, or negative error code.
+ */
+static int hfs_bmap_clear_bit(struct hfs_bnode *node, u32 node_bit_idx)
+{
+	struct hfs_bmap_ctx ctx;
+	struct page *page;
+	u8 *bmap, mask;
+
+	page = hfs_bmap_get_map_page(node, &ctx, node_bit_idx / BITS_PER_BYTE);
+	if (IS_ERR(page))
+		return PTR_ERR(page);
+
+	bmap = kmap_local_page(page);
+
+	mask = 1 << (7 - (node_bit_idx % BITS_PER_BYTE));
+
+	if (!(bmap[ctx.off] & mask)) {
+		kunmap_local(bmap);
+		return -EINVAL;
+	}
+
+	bmap[ctx.off] &= ~mask;
+	set_page_dirty(page);
+	kunmap_local(bmap);
+
+	return 0;
+}
+
 /* Get a reference to a B*Tree and do some initial checks */
 struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp keycmp)
 {
@@ -23,6 +138,7 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
 	struct address_space *mapping;
 	struct folio *folio;
 	struct buffer_head *bh;
+	struct hfs_bnode *node;
 	unsigned int size;
 	u16 dblock;
 	sector_t start_block;
@@ -155,6 +271,20 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
 	kunmap_local(head);
 	folio_unlock(folio);
 	folio_put(folio);
+
+	node = hfs_bnode_find(tree, HFS_TREE_HEAD);
+	if (IS_ERR(node))
+		goto free_inode;
+
+	if (!hfs_bmap_test_bit(node, HFS_TREE_HEAD)) {
+		pr_warn("(%s): %s (cnid 0x%x) bitmap corrupted, forcing rdonly\n",
+			sb->s_id, id == HFS_EXT_CNID ? "extents" : "catalog", id);
+		pr_warn("Run fsck.hfs to repair.\n");
+		sb->s_flags |= SB_RDONLY;
+	}
+
+	hfs_bnode_put(node);
+
 	return tree;
 
 fail_folio:
@@ -285,11 +415,9 @@ int hfs_bmap_reserve(struct hfs_btree *tree, u32 rsvd_nodes)
 struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 {
 	struct hfs_bnode *node, *next_node;
-	struct page **pagep;
+	struct hfs_bmap_ctx ctx;
+	struct page *page;
 	u32 nidx, idx;
-	unsigned off;
-	u16 off16;
-	u16 len;
 	u8 *data, byte, m;
 	int i, res;
 
@@ -301,24 +429,26 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 	node = hfs_bnode_find(tree, nidx);
 	if (IS_ERR(node))
 		return node;
-	len = hfs_brec_lenoff(node, 2, &off16);
-	off = off16;
 
-	off += node->page_offset;
-	pagep = node->page + (off >> PAGE_SHIFT);
-	data = kmap_local_page(*pagep);
-	off &= ~PAGE_MASK;
+	page = hfs_bmap_get_map_page(node, &ctx, 0);
+	if (IS_ERR(page)) {
+		res = PTR_ERR(page);
+		hfs_bnode_put(node);
+		return ERR_PTR(res);
+	}
+
+	data = kmap_local_page(page);
 	idx = 0;
 
 	for (;;) {
-		while (len) {
-			byte = data[off];
+		while (ctx.len) {
+			byte = data[ctx.off];
 			if (byte != 0xff) {
 				for (m = 0x80, i = 0; i < 8; m >>= 1, i++) {
 					if (!(byte & m)) {
 						idx += i;
-						data[off] |= m;
-						set_page_dirty(*pagep);
+						data[ctx.off] |= m;
+						set_page_dirty(page);
 						kunmap_local(data);
 						tree->free_nodes--;
 						mark_inode_dirty(tree->inode);
@@ -327,13 +457,14 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 					}
 				}
 			}
-			if (++off >= PAGE_SIZE) {
+			if (++ctx.off >= PAGE_SIZE) {
 				kunmap_local(data);
-				data = kmap_local_page(*++pagep);
-				off = 0;
+				page = node->page[++ctx.page_idx];
+				data = kmap_local_page(page);
+				ctx.off = 0;
 			}
 			idx += 8;
-			len--;
+			ctx.len--;
 		}
 		kunmap_local(data);
 		nidx = node->next;
@@ -347,22 +478,22 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 			return next_node;
 		node = next_node;
 
-		len = hfs_brec_lenoff(node, 0, &off16);
-		off = off16;
-		off += node->page_offset;
-		pagep = node->page + (off >> PAGE_SHIFT);
-		data = kmap_local_page(*pagep);
-		off &= ~PAGE_MASK;
+		page = hfs_bmap_get_map_page(node, &ctx, 0);
+		if (IS_ERR(page)) {
+			res = PTR_ERR(page);
+			hfs_bnode_put(node);
+			return ERR_PTR(res);
+		}
+		data = kmap_local_page(page);
 	}
 }
 
 void hfs_bmap_free(struct hfs_bnode *node)
 {
 	struct hfs_btree *tree;
-	struct page *page;
 	u16 off, len;
 	u32 nidx;
-	u8 *data, byte, m;
+	int res;
 
 	hfs_dbg("node %u\n", node->this);
 	tree = node->tree;
@@ -396,23 +527,18 @@ void hfs_bmap_free(struct hfs_bnode *node)
 		}
 		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];
-	if (!(byte & m)) {
+
+	res = hfs_bmap_clear_bit(node, nidx);
+	if (res == -EINVAL) {
 		pr_crit("trying to free free bnode %u(%d)\n",
-			node->this, node->type);
-		kunmap_local(data);
-		hfs_bnode_put(node);
-		return;
+			nidx, node->type);
+	} else if (res) {
+		pr_crit("fail to free bnode %u(%d)\n",
+			nidx, node->type);
+	} else {
+		tree->free_nodes++;
+		mark_inode_dirty(tree->inode);
 	}
-	data[off] = byte & ~m;
-	set_page_dirty(page);
-	kunmap_local(data);
+
 	hfs_bnode_put(node);
-	tree->free_nodes++;
-	mark_inode_dirty(tree->inode);
 }

fs/hfs/btree.h

@@ -129,3 +129,43 @@ extern int __hfs_brec_find(struct hfs_bnode *bnode, struct hfs_find_data *fd);
 extern int hfs_brec_find(struct hfs_find_data *fd);
 extern int hfs_brec_read(struct hfs_find_data *fd, void *rec, u32 rec_len);
 extern int hfs_brec_goto(struct hfs_find_data *fd, int cnt);
+
+static inline bool is_bnode_offset_valid(struct hfs_bnode *node, u32 off)
+{
+	bool is_valid;
+
+	if (!node || !node->tree)
+		return false;
+
+	is_valid = off < node->tree->node_size;
+
+	if (!is_valid) {
+		pr_err("invalid offset: id %u, type %#x, h %u, sz %u, off %u\n",
+		       node->this, node->type, node->height,
+		       node->tree->node_size, off);
+	}
+
+	return is_valid;
+}
+
+static inline u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
+{
+	unsigned int node_size;
+
+	if (!is_bnode_offset_valid(node, off))
+		return 0;
+
+	node_size = node->tree->node_size;
+
+	if ((u64)off + len > node_size) {
+		u32 new_len = node_size - off;
+
+		pr_err("corrected len: id %u, type %#x, h %u, sz %u, off %u, len %u->%u\n",
+		       node->this, node->type, node->height,
+		       node_size, off, len, new_len);
+
+		return new_len;
+	}
+
+	return len;
+}

include/linux/hfs_common.h

@@ -510,14 +510,21 @@ struct hfs_btree_header_rec {
 						   b-tree but not in extents
 						   b-tree (hfsplus). */
 
+/* HFS BTree misc info */
+#define HFS_TREE_HEAD				0
+#define HFS_BTREE_HDR_MAP_REC_INDEX		2	/* Map (bitmap) record in Header node */
+#define HFS_BTREE_MAP_NODE_REC_INDEX		0	/* Map record in Map Node */
+
 /* HFS+ BTree misc info */
-#define HFSPLUS_TREE_HEAD			0
+#define HFSPLUS_TREE_HEAD			HFS_TREE_HEAD
 #define HFSPLUS_NODE_MXSZ			32768
 #define HFSPLUS_NODE_MINSZ			512
 #define HFSPLUS_ATTR_TREE_NODE_SIZE		8192
 #define HFSPLUS_BTREE_HDR_NODE_RECS_COUNT	3
-#define HFSPLUS_BTREE_HDR_MAP_REC_INDEX		2	/* Map (bitmap) record in Header node */
-#define HFSPLUS_BTREE_MAP_NODE_REC_INDEX	0	/* Map record in Map Node */
+/* Map (bitmap) record in Header node */
+#define HFSPLUS_BTREE_HDR_MAP_REC_INDEX		HFS_BTREE_HDR_MAP_REC_INDEX
+/* Map record in Map Node */
+#define HFSPLUS_BTREE_MAP_NODE_REC_INDEX	HFS_BTREE_MAP_NODE_REC_INDEX
 #define HFSPLUS_BTREE_HDR_USER_BYTES		128
 #define HFSPLUS_BTREE_MAP_NODE_RECS_COUNT	2
 #define HFSPLUS_BTREE_MAP_NODE_RESERVED_BYTES	2