Re: [PATCH 1/2] hfs: detect node 0 btree map corruption at mount time

[Viacheslav Dubeyko]

On Fri, 2026-08-14 at 10:31 +0800, Tao Yu wrote:
> HFS+ already validates that the btree map keeps node 0 marked as in-
> use
> when the tree is opened. HFS lacks the same check, so a corrupted
> volume
> can proceed past mount with a broken node bitmap and only fail later
> in
> write paths.
> 
> Port the node 0 bitmap validation to HFS. If the header node is not
> marked as allocated in the on-disk map, warn that the btree bitmap is
> corrupted and force the filesystem read-only so users can repair it
> with
> fsck.hfs.
> 
> This keeps the HFS mount-time behavior aligned with HFS+ and catches
> the
> corruption closer to where it becomes observable.
> 
> Signed-off-by: Tao Yu <tao1.yu@xxxxxxxxx>
> ---
>  fs/hfs/btree.c | 60
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
> 
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 2eb37a2f64e86..14114318ec724 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -15,12 +15,59 @@
>  
>  #include "btree.h"
>  
> +static bool hfs_bmap_test_bit(struct hfs_bnode *node, u32
> node_bit_idx)
> +{
> +	u16 rec_idx, off, len;
> +	u32 byte_offset;
> +	u8 byte, mask;
> +
> +	if (node->this == 0) {
> +		if (node->type != HFS_NODE_HEADER) {
> +			pr_err("hfs: invalid btree header node\n");
> +			return false;
> +		}
> +		rec_idx = 2;
> +	} else {
> +		if (node->type != HFS_NODE_MAP) {
> +			pr_err("hfs: invalid btree map node\n");
> +			return false;
> +		}
> +		rec_idx = 0;
> +	}
> +
> +	len = hfs_brec_lenoff(node, rec_idx, &off);
> +	if (!len)
> +		return false;
> +
> +	byte_offset = node_bit_idx / BITS_PER_BYTE;
> +	if (byte_offset >= len)
> +		return false;
> +
> +	byte = hfs_bnode_read_u8(node, off + byte_offset);
> +	mask = 1 << (7 - (node_bit_idx % BITS_PER_BYTE));
> +
> +	return byte & mask;
> +}
> +
> +static const char *hfs_btree_name(u32 cnid)
> +{
> +	switch (cnid) {
> +	case HFS_EXT_CNID:
> +		return "Extents Overflow File";
> +	case HFS_CAT_CNID:
> +		return "Catalog File";
> +	default:
> +		return "Unknown B-tree";
> +	}
> +}
> +
>  /* 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)
>  {
>  	struct hfs_btree *tree;
>  	struct hfs_btree_header_rec *head;
>  	struct address_space *mapping;
> +	struct hfs_bnode *node;
>  	struct folio *folio;
>  	struct buffer_head *bh;
>  	unsigned int size;
> @@ -155,6 +202,19 @@ 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);
> +	if (IS_ERR(node))
> +		goto free_inode;
> +
> +	if (!hfs_bmap_test_bit(node, 0)) {
> +		pr_warn("(%s): %s (cnid 0x%x) map record invalid or
> bitmap corruption detected, forcing read-only.\n",
> +			sb->s_id, hfs_btree_name(id), id);
> +		pr_warn("Run fsck.hfs to repair.\n");
> +		sb->s_flags |= SB_RDONLY;
> +	}
> +
> +	hfs_bnode_put(node);
>  	return tree;
>  
>  fail_folio:

Sorry, but this patch [1] already has implemented this functionality.
And this patch is in HFS/HFS+ tree (for-next branch).

Thanks,
Slava.

[1]
https://lore.kernel.org/r/20260716074150.1660-1-aditya.ansh182@xxxxxxxxx