Re: [PATCH v2] hfs: port HFS+ b-tree bitmap corruption check

[Viacheslav Dubeyko <slava@dubeyko.com>]

On Fri, 2026-07-03 at 05:16 +0000, Aditya Srivastava wrote:
> 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.
> 
> 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 the relevant map record indices.
> 2. Port hfs_bmap_get_map_page() and the necessary offset and length
>    validation helpers.
> 3. Implement hfs_bmap_test_bit() 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@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>
> ---
> Changes in v2:
> - Port struct hfs_bmap_ctx and hfs_bmap_get_map_page() helpers to
>   fs/hfs/btree.c, keeping HFS and HFS+ logic unified and easy to
>   library-ize.
> - Re-implement hfs_bmap_test_bit() to take struct hfs_bnode and
>   node_bit_idx, matching HFS+ perfectly.
> - In hfs_btree_open(), use hfs_bnode_find() to retrieve the header
>   node and put it safely after the bitwise verification check.
> 
>  fs/hfs/btree.c | 96
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/hfs/btree.h |  2 ++
>  2 files changed, 98 insertions(+)
> 
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 2eb37a2f64e8..36dc8735d01a 100644
> --- a/fs/hfs/btree.
> +++ b/fs/hfs/btree.c
> @@ -23,6 +23,7 @@ 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 +156,20 @@ 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, 0);

Probably, we need to have a specially named constant for this node ID.

> +	if (IS_ERR(node))
> +		goto free_inode;
> +
> +	if (!hfs_bmap_test_bit(node, 0)) {

Ditto.

> +		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;
> +	}
> +
> +	hfs_bnode_put(node);
> +
>  	return tree;
>  
>  fail_folio:
> @@ -356,6 +371,87 @@ struct hfs_bnode *hfs_bmap_alloc(struct
> hfs_btree *tree)
>  	}
>  }
>  
> +struct hfs_bmap_ctx {
> +	unsigned int page_idx;
> +	unsigned int off;
> +	u16 len;
> +};

I prefer to have comments for this structure likewise HFS+ has.

> +
> +#define HFS_BTREE_HDR_MAP_REC_INDEX	2
> +#define HFS_BTREE_MAP_NODE_REC_INDEX	0

I believe that this should be declared in some proper header.

> +
> +static inline bool is_bnode_offset_valid(struct hfs_bnode *node, u32
> off)

Probably, it makes sense to have this function in some proper header.

> +{
> +	return off < node->tree->node_size;

Are you sure that pointers on node and tree will be always valid?

> +}
> +
> +static inline u32 check_and_correct_requested_length(struct
> hfs_bnode *node, u32 off, u32 len)
> +{
> +	if (off >= node->tree->node_size)

Ditto. Are you sure that pointers on node and tree will be always
valid?

> +		return 0;
> +	if ((u64)off + len > node->tree->node_size)
> +		return node->tree->node_size - off;
> +	return len;
> +}
> +
> +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 == 0) {

It will be good to have named constant for this node ID.

Thanks,
Slava.

> +		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];
> +}
> +
> +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;
> +}
> +
>  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..86036e18095f 100644
> --- a/fs/hfs/btree.h
> +++ b/fs/hfs/btree.h
> @@ -93,6 +93,8 @@ 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);
> +struct hfs_bmap_ctx;
> +extern bool hfs_bmap_test_bit(struct hfs_bnode *node, u32
> node_bit_idx);
>  
>  /* bnode.c */
>  extern void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32
> off, u32 len);