Re: [PATCH v2] hfs: replace BUG_ONs with error handling

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Tue, 2025-11-25 at 22:13 +0100, Jori Koolstra wrote:
> In a06ec283e125 next_id, folder_count, and file_count in the super block
> info were expanded to 64 bits, and BUG_ONs were added to detect
> overflow. This triggered an error reported by syzbot: if the MDB is
> corrupted, the BUG_ON is triggered. This patch replaces this mechanism
> with proper error handling and resolves the syzbot reported bug.
> 
> Singed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
> Reported-by: syzbot+17cc9bb6d8d69b4139f0@syzkaller.appspotmail.com
> Closes: https://syzbot.org/bug?extid=17cc9bb6d8d69b4139f0  
> Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
> ---
>  fs/hfs/dir.c    | 12 ++++++------
>  fs/hfs/hfs.h    |  3 +++
>  fs/hfs/hfs_fs.h |  2 +-
>  fs/hfs/inode.c  | 40 ++++++++++++++++++++++++++++++++--------
>  fs/hfs/mdb.c    | 15 ++++++++++++---
>  5 files changed, 54 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/hfs/dir.c b/fs/hfs/dir.c
> index 86a6b317b474..03881a91f869 100644
> --- a/fs/hfs/dir.c
> +++ b/fs/hfs/dir.c
> @@ -196,8 +196,8 @@ static int hfs_create(struct mnt_idmap *idmap, struct inode *dir,
>  	int res;
>  
>  	inode = hfs_new_inode(dir, &dentry->d_name, mode);
> -	if (!inode)
> -		return -ENOMEM;
> +	if (IS_ERR(inode))
> +		return PTR_ERR(inode);
>  
>  	res = hfs_cat_create(inode->i_ino, dir, &dentry->d_name, inode);
>  	if (res) {
> @@ -226,8 +226,8 @@ static struct dentry *hfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
>  	int res;
>  
>  	inode = hfs_new_inode(dir, &dentry->d_name, S_IFDIR | mode);
> -	if (!inode)
> -		return ERR_PTR(-ENOMEM);
> +	if (IS_ERR(inode))
> +		return ERR_CAST(inode);
>  
>  	res = hfs_cat_create(inode->i_ino, dir, &dentry->d_name, inode);
>  	if (res) {
> @@ -264,9 +264,9 @@ static int hfs_remove(struct inode *dir, struct dentry *dentry)
>  		return res;
>  	clear_nlink(inode);
>  	inode_set_ctime_current(inode);
> -	hfs_delete_inode(inode);
> +	res = hfs_delete_inode(inode);
>  	mark_inode_dirty(inode);
> -	return 0;
> +	return res;

This modification doesn't look good, frankly speaking. The hfs_delete_inode()
will return error code pretty at the beginning of execution. So, it doesn't make
sense to call mark_inode_dirty() then. However, we already did a lot of activity
before hfs_delete_inode() call:

static int hfs_remove(struct inode *dir, struct dentry *dentry)
{
	struct inode *inode = d_inode(dentry);
	int res;

	if (S_ISDIR(inode->i_mode) && inode->i_size != 2)
		return -ENOTEMPTY;
	res = hfs_cat_delete(inode->i_ino, dir, &dentry->d_name);
	if (res)
		return res;
	clear_nlink(inode);
	inode_set_ctime_current(inode);
	hfs_delete_inode(inode);
	mark_inode_dirty(inode);
	return 0;
}

So, not full executing of hfs_delete_inode() makes situation really bad.
Because, we deleted record from Catalog File but rejected of execution of
hfs_delete_inode() functionality.

I am thinking that, maybe, better course of action is to check HFS_SB(sb)-
>folder_count and HFS_SB(sb)->file_count at the beginning of hfs_remove():

static int hfs_remove(struct inode *dir, struct dentry *dentry)
{
	struct inode *inode = d_inode(dentry);
	int res;

	if (S_ISDIR(inode->i_mode) && inode->i_size != 2)
		return -ENOTEMPTY;

<<-- Check it here and return error

	res = hfs_cat_delete(inode->i_ino, dir, &dentry->d_name);
	if (res)
		return res;
	clear_nlink(inode);
	inode_set_ctime_current(inode);
	hfs_delete_inode(inode);
	mark_inode_dirty(inode);
	return 0;
}

In such case, we reject to make the removal, to return error and no activity
will happened. Let's move the check from hfs_delete_inode() to hfs_remove(). We
can ignore hfs_create() [1] and hfs_mkdir() [2] because these methods simply
processing erroneous situation.

>  }
>  
>  /*
> diff --git a/fs/hfs/hfs.h b/fs/hfs/hfs.h
> index 6f194d0768b6..4b4797ef4e50 100644
> --- a/fs/hfs/hfs.h
> +++ b/fs/hfs/hfs.h
> @@ -287,3 +287,6 @@ struct hfs_readdir_data {
>  };
>  
>  #endif
> +
> +
> +#define EFSCORRUPTED	EUCLEAN		/* Filesystem is corrupted */

I don't think that rename existing error code is good idea. Especially, because
we will not need the newly introduce error code's name. Please, see my comments
below.

> diff --git a/fs/hfs/hfs_fs.h b/fs/hfs/hfs_fs.h
> index fff149af89da..21dfdde71b14 100644
> --- a/fs/hfs/hfs_fs.h
> +++ b/fs/hfs/hfs_fs.h
> @@ -182,7 +182,7 @@ extern void hfs_inode_read_fork(struct inode *inode, struct hfs_extent *ext,
>  			__be32 log_size, __be32 phys_size, u32 clump_size);
>  extern struct inode *hfs_iget(struct super_block *, struct hfs_cat_key *, hfs_cat_rec *);
>  extern void hfs_evict_inode(struct inode *);
> -extern void hfs_delete_inode(struct inode *);
> +extern int hfs_delete_inode(struct inode *);
>  
>  /* attr.c */
>  extern const struct xattr_handler * const hfs_xattr_handlers[];
> diff --git a/fs/hfs/inode.c b/fs/hfs/inode.c
> index 9cd449913dc8..ce27d49c41e4 100644
> --- a/fs/hfs/inode.c
> +++ b/fs/hfs/inode.c
> @@ -186,16 +186,22 @@ struct inode *hfs_new_inode(struct inode *dir, const struct qstr *name, umode_t
>  	s64 next_id;
>  	s64 file_count;
>  	s64 folder_count;
> +	int err = -ENOMEM;
>  
>  	if (!inode)
> -		return NULL;
> +		goto out_err;
> +
> +	err = -EFSCORRUPTED;

In 99% of cases, this logic will be called for file system internal logic when
mount was successful. So, file system volume is not corrupted. Even if we
suspect that volume is corrupted, then potential reason could be failed read (-
EIO). It needs to run FSCK tool to be sure that volume is really corrupted.

>  
>  	mutex_init(&HFS_I(inode)->extents_lock);
>  	INIT_LIST_HEAD(&HFS_I(inode)->open_dir_list);
>  	spin_lock_init(&HFS_I(inode)->open_dir_lock);
>  	hfs_cat_build_key(sb, (btree_key *)&HFS_I(inode)->cat_key, dir->i_ino, name);
>  	next_id = atomic64_inc_return(&HFS_SB(sb)->next_id);
> -	BUG_ON(next_id > U32_MAX);
> +	if (next_id > U32_MAX) {
> +		pr_err("next CNID exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

File system volume is not corrupted here. Because, it is only error of file
system logic. And we will not store this not correct number to the volume,
anyway. At minimum, we should protect the logic from doing this. And it doesn't
need to recommend to run FSCK tool here.

Probably, it makes sense to decrement erroneous back.

Potentially, if we have such situation, maybe, it makes sense to consider to
make file system READ-ONLY. But I am not fully sure.

> +		goto out_discard;
> +	}
>  	inode->i_ino = (u32)next_id;
>  	inode->i_mode = mode;
>  	inode->i_uid = current_fsuid();
> @@ -209,7 +215,10 @@ struct inode *hfs_new_inode(struct inode *dir, const struct qstr *name, umode_t
>  	if (S_ISDIR(mode)) {
>  		inode->i_size = 2;
>  		folder_count = atomic64_inc_return(&HFS_SB(sb)->folder_count);
> -		BUG_ON(folder_count > U32_MAX);
> +		if (folder_count > U32_MAX) {
> +			pr_err("folder count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto. File system volume is not corrupted here.

> +			goto out_discard;
> +		}
>  		if (dir->i_ino == HFS_ROOT_CNID)
>  			HFS_SB(sb)->root_dirs++;
>  		inode->i_op = &hfs_dir_inode_operations;
> @@ -219,7 +228,10 @@ struct inode *hfs_new_inode(struct inode *dir, const struct qstr *name, umode_t
>  	} else if (S_ISREG(mode)) {
>  		HFS_I(inode)->clump_blocks = HFS_SB(sb)->clumpablks;
>  		file_count = atomic64_inc_return(&HFS_SB(sb)->file_count);
> -		BUG_ON(file_count > U32_MAX);
> +		if (file_count > U32_MAX) {
> +			pr_err("file count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto. File system volume is not corrupted here.

> +			goto out_discard;
> +		}
>  		if (dir->i_ino == HFS_ROOT_CNID)
>  			HFS_SB(sb)->root_files++;
>  		inode->i_op = &hfs_file_inode_operations;
> @@ -243,24 +255,35 @@ struct inode *hfs_new_inode(struct inode *dir, const struct qstr *name, umode_t
>  	hfs_mark_mdb_dirty(sb);
>  
>  	return inode;
> +
> +	out_discard:
> +		iput(inode);	
> +	out_err:
> +		return ERR_PTR(err); 
>  }
>  
> -void hfs_delete_inode(struct inode *inode)
> +int hfs_delete_inode(struct inode *inode)
>  {
>  	struct super_block *sb = inode->i_sb;
>  
>  	hfs_dbg("ino %lu\n", inode->i_ino);
>  	if (S_ISDIR(inode->i_mode)) {
> -		BUG_ON(atomic64_read(&HFS_SB(sb)->folder_count) > U32_MAX);
> +		if (atomic64_read(&HFS_SB(sb)->folder_count) > U32_MAX) {
> +			pr_err("folder count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto. File system volume is not corrupted here.

Please, see my comments above related to hfs_remove() logic.

> +			return -EFSCORRUPTED;
> +		}
>  		atomic64_dec(&HFS_SB(sb)->folder_count);
>  		if (HFS_I(inode)->cat_key.ParID == cpu_to_be32(HFS_ROOT_CNID))
>  			HFS_SB(sb)->root_dirs--;
>  		set_bit(HFS_FLG_MDB_DIRTY, &HFS_SB(sb)->flags);
>  		hfs_mark_mdb_dirty(sb);
> -		return;
> +		return 0;
>  	}
>  
> -	BUG_ON(atomic64_read(&HFS_SB(sb)->file_count) > U32_MAX);
> +	if (atomic64_read(&HFS_SB(sb)->file_count) > U32_MAX) {
> +		pr_err("file count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto. File system volume is not corrupted here.

Please, see my comments above related to hfs_remove() logic.

> +		return -EFSCORRUPTED;
> +	}
>  	atomic64_dec(&HFS_SB(sb)->file_count);
>  	if (HFS_I(inode)->cat_key.ParID == cpu_to_be32(HFS_ROOT_CNID))
>  		HFS_SB(sb)->root_files--;
> @@ -272,6 +295,7 @@ void hfs_delete_inode(struct inode *inode)
>  	}
>  	set_bit(HFS_FLG_MDB_DIRTY, &HFS_SB(sb)->flags);
>  	hfs_mark_mdb_dirty(sb);
> +	return 0;
>  }
>  
>  void hfs_inode_read_fork(struct inode *inode, struct hfs_extent *ext,
> diff --git a/fs/hfs/mdb.c b/fs/hfs/mdb.c
> index 53f3fae60217..45b690ab4ba5 100644
> --- a/fs/hfs/mdb.c
> +++ b/fs/hfs/mdb.c
> @@ -273,15 +273,24 @@ void hfs_mdb_commit(struct super_block *sb)
>  		/* These parameters may have been modified, so write them back */
>  		mdb->drLsMod = hfs_mtime();
>  		mdb->drFreeBks = cpu_to_be16(HFS_SB(sb)->free_ablocks);
> -		BUG_ON(atomic64_read(&HFS_SB(sb)->next_id) > U32_MAX);
> +		if (atomic64_read(&HFS_SB(sb)->next_id) > U32_MAX) {
> +			pr_err("next CNID exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto. File system volume is not corrupted here yet.

Breaking logic of hfs_mdb_commit() looks like really bad idea. Especially,
because we don't return any error message. I am thinking that, probably, we need
to consider of moving the check of next_id, file_count, folder_count from
hfs_mdb_commit() into hfs_sync_fs() [3] with the goal of of converting file
system in READ-ONLY state and returning error code.



> +			return;
> +		}
>  		mdb->drNxtCNID =
>  			cpu_to_be32((u32)atomic64_read(&HFS_SB(sb)->next_id));
>  		mdb->drNmFls = cpu_to_be16(HFS_SB(sb)->root_files);
>  		mdb->drNmRtDirs = cpu_to_be16(HFS_SB(sb)->root_dirs);
> -		BUG_ON(atomic64_read(&HFS_SB(sb)->file_count) > U32_MAX);
> +		if (atomic64_read(&HFS_SB(sb)->file_count) > U32_MAX) {
> +			pr_err("file count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto, please, see my comments above.

> +			return;
> +		}
>  		mdb->drFilCnt =
>  			cpu_to_be32((u32)atomic64_read(&HFS_SB(sb)->file_count));
> -		BUG_ON(atomic64_read(&HFS_SB(sb)->folder_count) > U32_MAX);
> +		if (atomic64_read(&HFS_SB(sb)->folder_count) > U32_MAX) {
> +			pr_err("folder count exceeds limit — filesystem corrupted. It is recommended to run fsck\n");

Ditto, please, see my comments above.

Thanks,
Slava.

> +			return;
> +		}
>  		mdb->drDirCnt =
>  			cpu_to_be32((u32)atomic64_read(&HFS_SB(sb)->folder_count));
>  

[1] https://elixir.bootlin.com/linux/v6.18-rc6/source/fs/hfs/dir.c#L205
[2] https://elixir.bootlin.com/linux/v6.18-rc6/source/fs/hfs/dir.c#L235
[3] 
https://elixir.bootlin.com/linux/v6.18-rc6/source/fs/hfs/super.c#L37