[PATCH v4 1/2] hfsplus: refactor b-tree map page access and add node-type validation

[Shardul Bankar]

In HFS+ b-trees, the node allocation bitmap is stored across multiple
records. The first chunk resides in the b-tree Header Node at record
index 2, while all subsequent chunks are stored in dedicated Map Nodes
at record index 0.

This structural quirk forces callers like hfs_bmap_alloc() to duplicate
boilerplate code to validate offsets, correct lengths, and map the
underlying pages via kmap_local_page() for both the initial header node
and the subsequent map nodes in the chain.

Introduce a generic helper, hfs_bmap_get_map_page(), to encapsulate
the map record access. This helper:
1. Automatically validates the node->type against HFS_NODE_HEADER and
   HFS_NODE_MAP to prevent misinterpreting corrupted nodes.
2. Infers the correct record index (2 or 0) based on the node type.
3. Handles the offset calculation, length validation, and page mapping.

Refactor hfs_bmap_alloc() to utilize this helper, stripping out the
redundant setup blocks. As part of this cleanup, the double pointer
iterator (struct page **pagep) is replaced with a simpler unsigned int
index (page_idx) for cleaner page boundary crossing.

This deduplicates the allocator logic, hardens the map traversal against
fuzzed/corrupted images, and provides a generic map-access abstraction
that will be utilized by upcoming mount-time validation checks.

Signed-off-by: Shardul Bankar <shardul.b@xxxxxxxxxxxxxxxxxx>
---
 fs/hfsplus/btree.c         | 78 +++++++++++++++++++++++++++-----------
 include/linux/hfs_common.h |  3 ++
 2 files changed, 59 insertions(+), 22 deletions(-)

diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
index 1220a2f22737..22efd6517ef4 100644
--- a/fs/hfsplus/btree.c
+++ b/fs/hfsplus/btree.c
@@ -129,6 +129,47 @@ u32 hfsplus_calc_btree_clump_size(u32 block_size, u32 node_size,
 	return clump_size;
 }
 
+/*
+ * Maps the page containing the b-tree map record and calculates offsets.
+ * Automatically handles the difference between header and map nodes.
+ * Returns the mapped data pointer, or an ERR_PTR on failure.
+ * Note: The caller is responsible for calling kunmap_local(data).
+ */
+static u8 *hfs_bmap_get_map_page(struct hfs_bnode *node, u16 *off, u16 *len,
+				unsigned int *page_idx)
+{
+	u16 rec_idx, off16;
+
+	if (node->this == HFSPLUS_TREE_HEAD) {
+		if (node->type != HFS_NODE_HEADER) {
+			pr_err("hfsplus: invalid btree header node\n");
+			return ERR_PTR(-EIO);
+		}
+		rec_idx = HFSPLUS_BTREE_HDR_MAP_REC_INDEX;
+	} else {
+		if (node->type != HFS_NODE_MAP) {
+			pr_err("hfsplus: invalid btree map node\n");
+			return ERR_PTR(-EIO);
+		}
+		rec_idx = HFSPLUS_BTREE_MAP_NODE_REC_INDEX;
+	}
+
+	*len = hfs_brec_lenoff(node, rec_idx, &off16);
+	if (!*len)
+		return ERR_PTR(-ENOENT);
+
+	if (!is_bnode_offset_valid(node, off16))
+		return ERR_PTR(-EIO);
+
+	*len = check_and_correct_requested_length(node, off16, *len);
+
+	off16 += node->page_offset;
+	*page_idx = off16 >> PAGE_SHIFT;
+	*off = off16 & ~PAGE_MASK;
+
+	return kmap_local_page(node->page[*page_idx]);
+}
+
 /* Get a reference to a B*Tree and do some initial checks */
 struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id)
 {
@@ -374,10 +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;
+	unsigned int page_idx;
 	u32 nidx, idx;
-	unsigned off;
-	u16 off16;
+	u16 off;
 	u16 len;
 	u8 *data, byte, m;
 	int i, res;
@@ -390,30 +430,24 @@ 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;
-
-	if (!is_bnode_offset_valid(node, off)) {
+	data = hfs_bmap_get_map_page(node, &off, &len, &page_idx);
+	if (IS_ERR(data)) {
+		res = PTR_ERR(data);
 		hfs_bnode_put(node);
-		return ERR_PTR(-EIO);
+		return ERR_PTR(res);
 	}
-	len = check_and_correct_requested_length(node, off, len);
 
-	off += node->page_offset;
-	pagep = node->page + (off >> PAGE_SHIFT);
-	data = kmap_local_page(*pagep);
-	off &= ~PAGE_MASK;
 	idx = 0;
 
 	for (;;) {
 		while (len) {
 			byte = data[off];
 			if (byte != 0xff) {
-				for (m = 0x80, i = 0; i < 8; m >>= 1, i++) {
+				for (m = HFSPLUS_BTREE_NODE0_BIT, i = 0; i < 8; m >>= 1, i++) {
 					if (!(byte & m)) {
 						idx += i;
 						data[off] |= m;
-						set_page_dirty(*pagep);
+						set_page_dirty(node->page[page_idx]);
 						kunmap_local(data);
 						tree->free_nodes--;
 						mark_inode_dirty(tree->inode);
@@ -425,7 +459,7 @@ struct hfs_bnode *hfs_bmap_alloc(struct hfs_btree *tree)
 			}
 			if (++off >= PAGE_SIZE) {
 				kunmap_local(data);
-				data = kmap_local_page(*++pagep);
+				data = kmap_local_page(node->page[++page_idx]);
 				off = 0;
 			}
 			idx += 8;
@@ -443,12 +477,12 @@ 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;
+		data = hfs_bmap_get_map_page(node, &off, &len, &page_idx);
+		if (IS_ERR(data)) {
+			res = PTR_ERR(data);
+			hfs_bnode_put(node);
+			return ERR_PTR(res);
+		}
 	}
 }
 
diff --git a/include/linux/hfs_common.h b/include/linux/hfs_common.h
index dadb5e0aa8a3..8238f55dd1d3 100644
--- a/include/linux/hfs_common.h
+++ b/include/linux/hfs_common.h
@@ -510,7 +510,10 @@ struct hfs_btree_header_rec {
 #define HFSPLUS_NODE_MXSZ			32768
 #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 */
 #define HFSPLUS_BTREE_HDR_USER_BYTES		128
+#define HFSPLUS_BTREE_NODE0_BIT			(1 << 7)
 
 /* btree key type */
 #define HFSPLUS_KEY_CASEFOLDING		0xCF	/* case-insensitive */
-- 
2.34.1