Re: [PATCH v5] hfs: port HFS+ b-tree bitmap corruption check
From: Viacheslav Dubeyko
Date: Wed Jul 15 2026 - 13:38:06 EST
On Wed, 2026-07-15 at 03:38 +0000, Aditya Prakash Srivastava wrote:
> 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. Implement static hfs_bmap_get_map_page() in fs/hfs/btree.c, and
> 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_test_bit() in fs/hfs/btree.c to inspect
> the B-tree bitmap using the new get_map_page helper.
> 4. 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@xxxxxxxxxxx>
> Link:
> https://lore.kernel.org/all/6a36101b.be22b350.2a3e9.0001.GAE@xxxxxxxxxx/T/#r446d0fed2a2900bd805534bbcb799d86619ae2ea
> Signed-off-by: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>
> ---
> fs/hfs/bnode.c | 42 -------------------
> fs/hfs/btree.c | 84
> ++++++++++++++++++++++++++++++++++++++
> fs/hfs/btree.h | 40 ++++++++++++++++++
> include/linux/hfs_common.h | 13 ++++--
> 4 files changed, 134 insertions(+), 45 deletions(-)
>
> diff --git a/fs/hfs/bnode.c b/fs/hfs/bnode.c
> index da8e5342c91c..1b331108d9c0 100644
> --- a/fs/hfs/bnode.c
> +++ b/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;
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 2eb37a2f64e8..88810b1c90c3 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -15,6 +15,75 @@
>
> #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;
> +};
> +
> +static struct page *hfs_bmap_get_map_page(struct hfs_bnode *node,
> + struct hfs_bmap_ctx *ctx,
> + u32 byte_offset)
We have really nice comments of functions [3,4] for the case of HFS+. I
believe it makes to keep the comments for the case of HFS too.
Mostly, patch looks good. However, we have one critical issue in the
patch. Current code uses hfs_bmap_get_map_page() only in
hfs_bmap_test_bit(). But, we have the same logic in hfs_bmap_alloc()
[1, 2]. It is not good to have the same logic implemented several times
in different places. Also, hfs_bmap_free() could benefit from
hfs_bmap_clear_bit() function. Could you please improve your patch?
> +{
> + 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];
> +}
> +
> +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;
> +}
> +
> /* 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 +92,7 @@ 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 +225,20 @@ 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:
> diff --git a/fs/hfs/btree.h b/fs/hfs/btree.h
> index 99be858b2446..f8afa33f709a 100644
> --- a/fs/hfs/btree.h
> +++ b/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",
type %#x
> + 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",
type %#x
> + node->this, node->type, node->height,
> + node_size, off, len, new_len);
> +
> + return new_len;
> + }
> +
> + return len;
> +}
> diff --git a/include/linux/hfs_common.h b/include/linux/hfs_common.h
> index 45fb4c9ff9f5..d6a615e74b26 100644
> --- a/include/linux/hfs_common.h
> +++ b/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
Thanks,
Slava.
[1]
https://elixir.bootlin.com/linux/v7.2-rc3/source/fs/hfs/btree.c#L307
[2]
https://elixir.bootlin.com/linux/v7.2-rc3/source/fs/hfs/btree.c#L352
[3]
https://elixir.bootlin.com/linux/v7.2-rc3/source/fs/hfsplus/btree.c#L144
[4]
https://elixir.bootlin.com/linux/v7.2-rc3/source/fs/hfsplus/btree.c#L190