Re: [PATCH 1/2] VFS: change kern_path_locked() and user_path_locked_at() to never return negative dentry

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2025-02-17 at 11:27 +1100, NeilBrown wrote:
> No callers of kern_path_locked() or user_path_locked_at() want a
> negative dentry.  So change them to return -ENOENT instead.  This
> simplifies callers.
> 
> This results in a subtle change to bcachefs in that an ioctl will now
> return -ENOENT in preference to -EXDEV.  I believe this restores the
> behaviour to what it was prior to
>  Commit bbe6a7c899e7 ("bch2_ioctl_subvolume_destroy(): fix locking")
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> Link: https://lore.kernel.org/r/20250207034040.3402438-2-neilb@suse.de
> Acked-by: Paul Moore <paul@paul-moore.com>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  Documentation/filesystems/porting.rst |  8 ++++
>  drivers/base/devtmpfs.c               | 65 +++++++++++++--------------
>  fs/bcachefs/fs-ioctl.c                |  4 --
>  fs/namei.c                            |  4 ++
>  kernel/audit_watch.c                  | 12 ++---
>  5 files changed, 48 insertions(+), 45 deletions(-)
> 
> diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
> index 1639e78e3146..2ead47e20677 100644
> --- a/Documentation/filesystems/porting.rst
> +++ b/Documentation/filesystems/porting.rst
> @@ -1157,3 +1157,11 @@ in normal case it points into the pathname being looked up.
>  NOTE: if you need something like full path from the root of filesystem,
>  you are still on your own - this assists with simple cases, but it's not
>  magic.
> +
> +---
> +
> +** recommended**
> +
> +kern_path_locked() and user_path_locked() no longer return a negative
> +dentry so this doesn't need to be checked.  If the name cannot be found,
> +ERR_PTR(-ENOENT) is returned.
> diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
> index b848764ef018..c9e34842139f 100644
> --- a/drivers/base/devtmpfs.c
> +++ b/drivers/base/devtmpfs.c
> @@ -245,15 +245,12 @@ static int dev_rmdir(const char *name)
>  	dentry = kern_path_locked(name, &parent);
>  	if (IS_ERR(dentry))
>  		return PTR_ERR(dentry);
> -	if (d_really_is_positive(dentry)) {
> -		if (d_inode(dentry)->i_private == &thread)
> -			err = vfs_rmdir(&nop_mnt_idmap, d_inode(parent.dentry),
> -					dentry);
> -		else
> -			err = -EPERM;
> -	} else {
> -		err = -ENOENT;
> -	}
> +	if (d_inode(dentry)->i_private == &thread)
> +		err = vfs_rmdir(&nop_mnt_idmap, d_inode(parent.dentry),
> +				dentry);
> +	else
> +		err = -EPERM;
> +
>  	dput(dentry);
>  	inode_unlock(d_inode(parent.dentry));
>  	path_put(&parent);
> @@ -310,6 +307,8 @@ static int handle_remove(const char *nodename, struct device *dev)
>  {
>  	struct path parent;
>  	struct dentry *dentry;
> +	struct kstat stat;
> +	struct path p;
>  	int deleted = 0;
>  	int err;
>  
> @@ -317,32 +316,28 @@ static int handle_remove(const char *nodename, struct device *dev)
>  	if (IS_ERR(dentry))
>  		return PTR_ERR(dentry);
>  
> -	if (d_really_is_positive(dentry)) {
> -		struct kstat stat;
> -		struct path p = {.mnt = parent.mnt, .dentry = dentry};
> -		err = vfs_getattr(&p, &stat, STATX_TYPE | STATX_MODE,
> -				  AT_STATX_SYNC_AS_STAT);
> -		if (!err && dev_mynode(dev, d_inode(dentry), &stat)) {
> -			struct iattr newattrs;
> -			/*
> -			 * before unlinking this node, reset permissions
> -			 * of possible references like hardlinks
> -			 */
> -			newattrs.ia_uid = GLOBAL_ROOT_UID;
> -			newattrs.ia_gid = GLOBAL_ROOT_GID;
> -			newattrs.ia_mode = stat.mode & ~0777;
> -			newattrs.ia_valid =
> -				ATTR_UID|ATTR_GID|ATTR_MODE;
> -			inode_lock(d_inode(dentry));
> -			notify_change(&nop_mnt_idmap, dentry, &newattrs, NULL);
> -			inode_unlock(d_inode(dentry));
> -			err = vfs_unlink(&nop_mnt_idmap, d_inode(parent.dentry),
> -					 dentry, NULL);
> -			if (!err || err == -ENOENT)
> -				deleted = 1;
> -		}
> -	} else {
> -		err = -ENOENT;
> +	p.mnt = parent.mnt;
> +	p.dentry = dentry;
> +	err = vfs_getattr(&p, &stat, STATX_TYPE | STATX_MODE,
> +			  AT_STATX_SYNC_AS_STAT);
> +	if (!err && dev_mynode(dev, d_inode(dentry), &stat)) {
> +		struct iattr newattrs;
> +		/*
> +		 * before unlinking this node, reset permissions
> +		 * of possible references like hardlinks
> +		 */
> +		newattrs.ia_uid = GLOBAL_ROOT_UID;
> +		newattrs.ia_gid = GLOBAL_ROOT_GID;
> +		newattrs.ia_mode = stat.mode & ~0777;
> +		newattrs.ia_valid =
> +			ATTR_UID|ATTR_GID|ATTR_MODE;
> +		inode_lock(d_inode(dentry));
> +		notify_change(&nop_mnt_idmap, dentry, &newattrs, NULL);
> +		inode_unlock(d_inode(dentry));
> +		err = vfs_unlink(&nop_mnt_idmap, d_inode(parent.dentry),
> +				 dentry, NULL);
> +		if (!err || err == -ENOENT)
> +			deleted = 1;
>  	}
>  	dput(dentry);
>  	inode_unlock(d_inode(parent.dentry));
> diff --git a/fs/bcachefs/fs-ioctl.c b/fs/bcachefs/fs-ioctl.c
> index 15725b4ce393..595b57fabc9a 100644
> --- a/fs/bcachefs/fs-ioctl.c
> +++ b/fs/bcachefs/fs-ioctl.c
> @@ -511,10 +511,6 @@ static long bch2_ioctl_subvolume_destroy(struct bch_fs *c, struct file *filp,
>  		ret = -EXDEV;
>  		goto err;
>  	}
> -	if (!d_is_positive(victim)) {
> -		ret = -ENOENT;
> -		goto err;
> -	}
>  	ret = __bch2_unlink(dir, victim, true);
>  	if (!ret) {
>  		fsnotify_rmdir(dir, victim);
> diff --git a/fs/namei.c b/fs/namei.c
> index 3ab9440c5b93..fb6da3ca0ca5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2741,6 +2741,10 @@ static struct dentry *__kern_path_locked(int dfd, struct filename *name, struct
>  	}
>  	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
>  	d = lookup_one_qstr_excl(&last, path->dentry, 0);
> +	if (!IS_ERR(d) && d_is_negative(d)) {
> +		dput(d);
> +		d = ERR_PTR(-ENOENT);
> +	}
>  	if (IS_ERR(d)) {
>  		inode_unlock(path->dentry->d_inode);
>  		path_put(path);
> diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
> index 7f358740e958..367eaf2c78b7 100644
> --- a/kernel/audit_watch.c
> +++ b/kernel/audit_watch.c
> @@ -350,11 +350,10 @@ static int audit_get_nd(struct audit_watch *watch, struct path *parent)
>  	struct dentry *d = kern_path_locked(watch->path, parent);
>  	if (IS_ERR(d))
>  		return PTR_ERR(d);
> -	if (d_is_positive(d)) {
> -		/* update watch filter fields */
> -		watch->dev = d->d_sb->s_dev;
> -		watch->ino = d_backing_inode(d)->i_ino;
> -	}
> +	/* update watch filter fields */
> +	watch->dev = d->d_sb->s_dev;
> +	watch->ino = d_backing_inode(d)->i_ino;
> +
>  	inode_unlock(d_backing_inode(parent->dentry));
>  	dput(d);
>  	return 0;
> @@ -419,10 +418,11 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
>  	/* caller expects mutex locked */
>  	mutex_lock(&audit_filter_mutex);
>  
> -	if (ret) {
> +	if (ret && ret != -ENOENT) {
>  		audit_put_watch(watch);
>  		return ret;
>  	}
> +	ret = 0;
>  
>  	/* either find an old parent or attach a new one */
>  	parent = audit_find_parent(d_backing_inode(parent_path.dentry));

Reviewed-by: Jeff Layton <jlayton@kernel.org>