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

[Viacheslav Dubeyko]

On Tue, 2026-06-23 at 09:37 +0000, Aditya Srivastava wrote:
> From: Aditya Prakash Srivastava <aditya.ansh182@xxxxxxxxx>
> 
> 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, which was highlighted by the maintainer.
> 
> Port this check to HFS:
> 1. Implement hfs_bmap_test_bit() in fs/hfs/btree.c (which safely
> traverses
>    the b-tree map nodes to inspect the desired bit, identical to
> HFS+).
> 2. Invoke hfs_bmap_test_bit() for node 0 at the end of
> hfs_btree_open(),
>    warning and forcing read-only if corruption is detected.
> 
> 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/btree.c | 52
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/hfs/btree.h |  1 +
>  2 files changed, 53 insertions(+)
> 
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 2eb37a2f64e8..4293399d377e 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -155,6 +155,14 @@ struct hfs_btree *hfs_btree_open(struct
> super_block *sb, u32 id, btree_keycmp ke
>  	kunmap_local(head);
>  	folio_unlock(folio);
>  	folio_put(folio);
> +
> +	if (!hfs_bmap_test_bit(tree, 0)) {
> +		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;
> +	}
> +
>  	return tree;
>  
>  fail_folio:
> @@ -356,6 +364,50 @@ struct hfs_bnode *hfs_bmap_alloc(struct
> hfs_btree *tree)
>  	}
>  }
>  
> +bool hfs_bmap_test_bit(struct hfs_btree *tree, u32 nidx)
> +{
> +	struct hfs_bnode *node;
> +	struct page *page;
> +	u16 off, len;
> +	u8 *data, byte, m;
> +	bool res = false;
> +
> +	node = hfs_bnode_find(tree, 0);
> +	if (IS_ERR(node))
> +		return false;
> +
> +	len = hfs_brec_lenoff(node, 2, &off);
> +	while (nidx >= len * 8) {
> +		u32 i;
> +
> +		nidx -= len * 8;
> +		i = node->next;
> +		if (!i) {
> +			hfs_bnode_put(node);
> +			return false;
> +		}
> +		hfs_bnode_put(node);
> +		node = hfs_bnode_find(tree, i);
> +		if (IS_ERR(node))
> +			return false;
> +		if (node->type != HFS_NODE_MAP) {
> +			hfs_bnode_put(node);
> +			return false;
> +		}
> +		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];
> +	res = (byte & m) != 0;
> +	kunmap_local(data);
> +	hfs_bnode_put(node);
> +	return res;
> +}
> +
>  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..efe88f66c856 100644
> --- a/fs/hfs/btree.h
> +++ b/fs/hfs/btree.h
> @@ -93,6 +93,7 @@ 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);
> +extern bool hfs_bmap_test_bit(struct hfs_btree *tree, u32 nidx);
>  
>  /* bnode.c */
>  extern void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32
> off, u32 len);

I am still in personal trip and I am on the move until the end of this
week. I'll be able to review the patch around this Sunday or Monday.
Please, ping me if I am keeping silence too long.

Meanwhile, have you tried to run xfstests for current state of HFS
code? Please, be ready that we still have multiple test-case failures
for the case of HFS file system. The point of my request is to check
that number of failed test-cases is not growing by applying this patch.

Thanks,
Slava.