Re: [PATCH 05/19] VFS: add common error checks to lookup_one_qstr()

[Christian Brauner <brauner@kernel.org>]

On Thu, Feb 06, 2025 at 04:42:42PM +1100, NeilBrown wrote:
> Callers of lookup_one_qstr() often check if the result is negative or
> positive.
> These changes can easily be moved into lookup_one_qstr() by checking the
> lookup flags:
> LOOKUP_CREATE means it is NOT an error if the name doesn't exist.
> LOOKUP_EXCL means it IS an error if the name DOES exist.
> 
> This patch adds these checks, then removes error checks from callers,
> and ensures that appropriate flags are passed.
> 
> This subtly changes the meaning of LOOKUP_EXCL.  Previously it could
> only accompany LOOKUP_CREATE.  Now it can accompany LOOKUP_RENAME_TARGET
> as well.  A couple of small changes are needed to accommodate this.  The
> NFS is functionally a no-op but ensures nfs_is_exclusive_create() does
> exactly what the name says.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---

This would be a worthwhile cleanup patch to lookup_one_qstr_excl()
before you've modified it to be lookup_one_qstr(). So this should also
go separately imho.

>  fs/namei.c            | 61 ++++++++++++++-----------------------------
>  fs/nfs/dir.c          |  3 ++-
>  fs/smb/server/vfs.c   | 26 +++++++-----------
>  include/linux/namei.h |  2 +-
>  4 files changed, 33 insertions(+), 59 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 1901120bcbb8..69610047f6c6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1668,6 +1668,8 @@ static struct dentry *lookup_dcache(const struct qstr *name,
>   * Parent directory has inode locked: exclusive or shared.
>   * If @flags contains any LOOKUP_INTENT_FLAGS then d_lookup_done()
>   * must be called after the intended operation is performed - or aborted.
> + * Will return -ENOENT if name isn't found and LOOKUP_CREATE wasn't passed.
> + * Will return -EEXIST if name is found and LOOKUP_EXCL was passed.
>   */
>  struct dentry *lookup_one_qstr(const struct qstr *name,
>  			       struct dentry *base,
> @@ -1678,7 +1680,7 @@ struct dentry *lookup_one_qstr(const struct qstr *name,
>  	struct inode *dir = base->d_inode;
>  
>  	if (dentry)
> -		return dentry;
> +		goto found;
>  
>  	/* Don't create child dentry for a dead directory. */
>  	if (unlikely(IS_DEADDIR(dir)))
> @@ -1689,7 +1691,7 @@ struct dentry *lookup_one_qstr(const struct qstr *name,
>  		return ERR_PTR(-ENOMEM);
>  	if (!d_in_lookup(dentry))
>  		/* Raced with another thread which did the lookup */
> -		return dentry;
> +		goto found;
>  
>  	old = dir->i_op->lookup(dir, dentry, flags);
>  	if (unlikely(old)) {
> @@ -1700,6 +1702,15 @@ struct dentry *lookup_one_qstr(const struct qstr *name,
>  	if ((flags & LOOKUP_INTENT_FLAGS) == 0)
>  		/* ->lookup must have given final answer */
>  		d_lookup_done(dentry);
> +found:
> +	if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
> +		dput(dentry);
> +		return ERR_PTR(-ENOENT);
> +	}
> +	if (d_is_positive(dentry) && (flags & LOOKUP_EXCL)) {
> +		dput(dentry);
> +		return ERR_PTR(-EEXIST);
> +	}
>  	return dentry;
>  }
>  EXPORT_SYMBOL(lookup_one_qstr);
> @@ -2745,10 +2756,6 @@ 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(&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);
> @@ -4085,27 +4092,13 @@ static struct dentry *filename_create(int dfd, struct filename *name,
>  	 * '/', and a directory wasn't requested.
>  	 */
>  	if (last.name[last.len] && !want_dir)
> -		create_flags = 0;
> +		create_flags &= ~LOOKUP_CREATE;
>  	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
>  	dentry = lookup_one_qstr(&last, path->dentry,
>  				 reval_flag | create_flags);
>  	if (IS_ERR(dentry))
>  		goto unlock;
>  
> -	error = -EEXIST;
> -	if (d_is_positive(dentry))
> -		goto fail;
> -
> -	/*
> -	 * Special case - lookup gave negative, but... we had foo/bar/
> -	 * From the vfs_mknod() POV we just have a negative dentry -
> -	 * all is fine. Let's be bastards - you had / on the end, you've
> -	 * been asking for (non-existent) directory. -ENOENT for you.
> -	 */
> -	if (unlikely(!create_flags)) {
> -		error = -ENOENT;
> -		goto fail;
> -	}
>  	if (unlikely(err2)) {
>  		error = err2;
>  		goto fail;
> @@ -4522,10 +4515,6 @@ int do_rmdir(int dfd, struct filename *name)
>  	error = PTR_ERR(dentry);
>  	if (IS_ERR(dentry))
>  		goto exit3;
> -	if (!dentry->d_inode) {
> -		error = -ENOENT;
> -		goto exit4;
> -	}
>  	error = security_path_rmdir(&path, dentry);
>  	if (error)
>  		goto exit4;
> @@ -4656,7 +4645,7 @@ int do_unlinkat(int dfd, struct filename *name)
>  	if (!IS_ERR(dentry)) {
>  
>  		/* Why not before? Because we want correct error value */
> -		if (last.name[last.len] || d_is_negative(dentry))
> +		if (last.name[last.len])
>  			goto slashes;
>  		inode = dentry->d_inode;
>  		ihold(inode);
> @@ -4690,9 +4679,7 @@ int do_unlinkat(int dfd, struct filename *name)
>  	return error;
>  
>  slashes:
> -	if (d_is_negative(dentry))
> -		error = -ENOENT;
> -	else if (d_is_dir(dentry))
> +	if (d_is_dir(dentry))
>  		error = -EISDIR;
>  	else
>  		error = -ENOTDIR;
> @@ -5192,7 +5179,8 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
>  	struct qstr old_last, new_last;
>  	int old_type, new_type;
>  	struct inode *delegated_inode = NULL;
> -	unsigned int lookup_flags = 0, target_flags = LOOKUP_RENAME_TARGET;
> +	unsigned int lookup_flags = 0, target_flags =
> +		LOOKUP_RENAME_TARGET | LOOKUP_CREATE;
>  	bool should_retry = false;
>  	int error = -EINVAL;
>  
> @@ -5205,6 +5193,8 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
>  
>  	if (flags & RENAME_EXCHANGE)
>  		target_flags = 0;
> +	if (flags & RENAME_NOREPLACE)
> +		target_flags |= LOOKUP_EXCL;
>  
>  retry:
>  	error = filename_parentat(olddfd, from, lookup_flags, &old_path,
> @@ -5246,23 +5236,12 @@ int do_renameat2(int olddfd, struct filename *from, int newdfd,
>  	error = PTR_ERR(old_dentry);
>  	if (IS_ERR(old_dentry))
>  		goto exit3;
> -	/* source must exist */
> -	error = -ENOENT;
> -	if (d_is_negative(old_dentry))
> -		goto exit4;
>  	new_dentry = lookup_one_qstr(&new_last, new_path.dentry,
>  				     lookup_flags | target_flags);
>  	error = PTR_ERR(new_dentry);
>  	if (IS_ERR(new_dentry))
>  		goto exit4;
> -	error = -EEXIST;
> -	if ((flags & RENAME_NOREPLACE) && d_is_positive(new_dentry))
> -		goto exit5;
>  	if (flags & RENAME_EXCHANGE) {
> -		error = -ENOENT;
> -		if (d_is_negative(new_dentry))
> -			goto exit5;
> -
>  		if (!d_is_dir(new_dentry)) {
>  			error = -ENOTDIR;
>  			if (new_last.name[new_last.len])
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 27c7a5c4e91b..8cbe63f4089a 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -1531,7 +1531,8 @@ static int nfs_is_exclusive_create(struct inode *dir, unsigned int flags)
>  {
>  	if (NFS_PROTO(dir)->version == 2)
>  		return 0;
> -	return flags & LOOKUP_EXCL;
> +	return (flags & (LOOKUP_CREATE | LOOKUP_EXCL)) ==
> +		(LOOKUP_CREATE | LOOKUP_EXCL);
>  }
>  
>  /*
> diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
> index 89b3823f6405..bf8ac43c39b0 100644
> --- a/fs/smb/server/vfs.c
> +++ b/fs/smb/server/vfs.c
> @@ -113,11 +113,6 @@ static int ksmbd_vfs_path_lookup_locked(struct ksmbd_share_config *share_conf,
>  	if (IS_ERR(d))
>  		goto err_out;
>  
> -	if (d_is_negative(d)) {
> -		dput(d);
> -		goto err_out;
> -	}
> -
>  	path->dentry = d;
>  	path->mnt = mntget(parent_path->mnt);
>  
> @@ -677,6 +672,7 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
>  	struct ksmbd_file *parent_fp;
>  	int new_type;
>  	int err, lookup_flags = LOOKUP_NO_SYMLINKS;
> +	int target_lookup_flags = LOOKUP_RENAME_TARGET;
>  
>  	if (ksmbd_override_fsids(work))
>  		return -ENOMEM;
> @@ -687,6 +683,14 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
>  		goto revert_fsids;
>  	}
>  
> +	/*
> +	 * explicitly handle file overwrite case, for compatibility with
> +	 * filesystems that may not support rename flags (e.g: fuse)
> +	 */
> +	if (flags & RENAME_NOREPLACE)
> +		target_lookup_flags |= LOOKUP_EXCL;
> +	flags &= ~(RENAME_NOREPLACE);
> +
>  retry:
>  	err = vfs_path_parent_lookup(to, lookup_flags | LOOKUP_BENEATH,
>  				     &new_path, &new_last, &new_type,
> @@ -727,7 +731,7 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
>  	}
>  
>  	new_dentry = lookup_one_qstr(&new_last, new_path.dentry,
> -				     lookup_flags | LOOKUP_RENAME_TARGET);
> +				     lookup_flags | target_lookup_flags);
>  	if (IS_ERR(new_dentry)) {
>  		err = PTR_ERR(new_dentry);
>  		goto out3;
> @@ -738,16 +742,6 @@ int ksmbd_vfs_rename(struct ksmbd_work *work, const struct path *old_path,
>  		goto out4;
>  	}
>  
> -	/*
> -	 * explicitly handle file overwrite case, for compatibility with
> -	 * filesystems that may not support rename flags (e.g: fuse)
> -	 */
> -	if ((flags & RENAME_NOREPLACE) && d_is_positive(new_dentry)) {
> -		err = -EEXIST;
> -		goto out4;
> -	}
> -	flags &= ~(RENAME_NOREPLACE);
> -
>  	if (old_child == trap) {
>  		err = -EINVAL;
>  		goto out4;
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 06bb3ea65beb..839a64d07f8c 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -31,7 +31,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
>  /* These tell filesystem methods that we are dealing with the final component... */
>  #define LOOKUP_OPEN		0x0100	/* ... in open */
>  #define LOOKUP_CREATE		0x0200	/* ... in object creation */
> -#define LOOKUP_EXCL		0x0400	/* ... in exclusive creation */
> +#define LOOKUP_EXCL		0x0400	/* ... in target must not exist */
>  #define LOOKUP_RENAME_TARGET	0x0800	/* ... in destination of rename() */
>  
>  #define LOOKUP_INTENT_FLAGS	(LOOKUP_OPEN | LOOKUP_CREATE | LOOKUP_EXCL |	\
> -- 
> 2.47.1
>