[PATCH v6 1/2] hfsplus: refactor b-tree map page access and add node-type validation

Shardul Bankar
2026-03-15T17:20:59Z

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() and
hfs_bmap_free() to duplicate boilerplate code to validate offsets, correct
lengths, and map the underlying pages via kmap_local_page(). There is
also currently no strict node-type validation before reading these
records, leaving the allocator vulnerable if a corrupted image points a
map linkage to an Index or Leaf node.

Introduce a unified bit-level API to encapsulate the map record access:
1. A new `struct hfs_bmap_ctx` to cleanly pass state and safely handle
page math across all architectures.
2. `hfs_bmap_get_map_page()`: Automatically validates node types
(HFS_NODE_HEADER vs HFS_NODE_MAP), infers the correct record index,
handles page-boundary math, and returns the unmapped `struct page *`
directly to the caller to avoid asymmetric mappings.
3. `hfs_bmap_clear_bit()`: A clean wrapper that internally handles page
mapping/unmapping for single-bit operations.

Refactor hfs_bmap_alloc() and hfs_bmap_free() to utilize this new API.
This deduplicates the allocator logic, hardens the map traversal against
fuzzed images, and provides the exact abstractions needed for upcoming
mount-time validation checks.

Signed-off-by: Shardul Bankar <shardul.b@mpiricsoftware.com>
---
fs/hfsplus/btree.c | 169 ++++++++++++++++++++++++++-----------
include/linux/hfs_common.h | 2 +
2 files changed, 124 insertions(+), 47 deletions(-)

diff --git a/fs/hfsplus/btree.c b/fs/hfsplus/btree.c
index 1220a2f22737..1c6a27e397fb 100644
--- a/fs/hfsplus/btree.c
+++ b/fs/hfsplus/btree.c
@@ -129,6 +129,95 @@ u32 hfsplus_calc_btree_clump_size(u32 block_size, u32 node_size,
return clump_size;
}

+/* 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 == 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;
+ }
+
+ 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 = 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_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, -EALREADY 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)
{
@@ -374,11 +463,9 @@ 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;

@@ -390,30 +477,26 @@ 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;

- if (!is_bnode_offset_valid(node, off)) {
+ page = hfs_bmap_get_map_page(node, &ctx, 0);
+ if (IS_ERR(page)) {
+ res = PTR_ERR(page);
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;
+ 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);
@@ -423,13 +506,14 @@ 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;
@@ -443,22 +527,22 @@ 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);
BUG_ON(!node->this);
@@ -495,24 +579,15 @@ 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)) {
- pr_crit("trying to free free bnode "
- "%u(%d)\n",
- node->this, node->type);
- kunmap_local(data);
- hfs_bnode_put(node);
- return;
+
+ res = hfs_bmap_clear_bit(node, nidx);
+ if (res == -EINVAL) {
+ pr_crit("trying to free free bnode %u(%d)\n",
+ node->this, node->type);
+ } else if (!res) {
+ 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);
}
diff --git a/include/linux/hfs_common.h b/include/linux/hfs_common.h
index dadb5e0aa8a3..be24c687858e 100644
--- a/include/linux/hfs_common.h
+++ b/include/linux/hfs_common.h
@@ -510,6 +510,8 @@ 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

/* btree key type */
--
2.34.1