Re: [PATCH 04/18] VFS: add vfs_lookup_open()

> Op 01-06-2026 08:37 CEST schreef NeilBrown <[email protected]>:
> 
>  
> From: NeilBrown <neil-+NVA1uvv1dVBDLzU/[email protected]>
> 
> vfs_lookup_open() is a limited version of lookup_open() which is
> exported for nfsd to use - to replace dentry_create().
> 
> It is limited in that no filename is given (thus no auditing) and no
> LOOKUP_ flags are passed.  A few "intent" LOOKUP flags are deduced from
> the open flags.
> 
> Signed-off-by: NeilBrown <neil-+NVA1uvv1dVBDLzU/[email protected]>
> ---
>  fs/namei.c            | 54 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/namei.h |  3 +++
>  2 files changed, 57 insertions(+)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 18a43c24d7f1..db3fddbccd21 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -4571,6 +4571,60 @@ static struct dentry *lookup_open(const struct path *path, struct file *file,
>  	goto out;
>  }
>  
> +/**
> + * vfs_lookup_open - open and possibly create a regular file
> + * @parent: directory to contain file
> + * @last: final component of file name
> + * @open_flag: O_flags
> + * @mode: initial permissions for file
> + *
> + * Open a file after lookup and/or create.  This provides similar
> + * functionality open_last_lookups() for in-kernel users, particularly
> + * nfsd.
> + * It uses ->atomic_open or ->lookup / ->create / ->open as appropriate.
> + *
> + * Returns: the opened struct file, or an error.
> + */
> +struct file *vfs_lookup_open(struct path *parent, struct qstr *last,
> +			     int open_flag, umode_t mode)
> +{
> +	struct file *file __free(fput) = NULL;
> +	unsigned int lookup_flags = LOOKUP_OPEN;
> +	struct dentry *dentry;
> +	int error = 0;
> +
> +	error = lookup_noperm_common(last, parent->dentry);
> +	if (error)
> +		return ERR_PTR(error);
> +
> +	file = alloc_empty_file(open_flag, current_cred());
> +	if (IS_ERR(file))
> +		return file;
> +
> +	if (open_flag & O_CREAT) {
> +		lookup_flags |= LOOKUP_CREATE;
> +		if (open_flag & O_EXCL)
> +			lookup_flags |= LOOKUP_EXCL;

Is O_EXCL enforced anywhere in this call path? I don't really know nfsd, but
for the standard O_CREAT call chain, this happens in do_open(), and EEXIST is
returned.

Also, maybe warn when nonsensical bits here, like __O_TMPFILE, are set in
open_flag?

> +	}
> +	dentry = lookup_open(parent, file, last, lookup_flags, NULL,
> +			     open_flag, S_IFREG | mode);
> +	if (IS_ERR(dentry))
> +		return ERR_CAST(dentry);
> +
> +	if (d_really_is_negative(dentry)) {
> +		error = -ENOENT;
> +	} else if (!(file->f_mode & FMODE_OPENED)) {
> +		struct path path = {.mnt = parent->mnt, .dentry = dentry };
> +
> +		error = vfs_open(&path, file);

Ah, good, this fixes the bug in dentry_create() where on failing the open
part of atomic_open() no other attempt was done by the vfs to do the open.

> +	}
> +	dput(dentry);
> +
> +	if (error)
> +		return ERR_PTR(error);
> +	return no_free_ptr(file);
> +}
> +
>  static inline bool trailing_slashes(struct nameidata *nd)
>  {
>  	return (bool)nd->last.name[nd->last.len];
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 2ad6dd9987b9..8c048c97a7f7 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -103,6 +103,9 @@ struct dentry *start_creating_dentry(struct dentry *parent,
>  struct dentry *start_removing_dentry(struct dentry *parent,
>  				     struct dentry *child);
>  
> +struct file *vfs_lookup_open(struct path *parent, struct qstr *last,
> +			     int open_flag, umode_t mode);
> +
>  /* end_creating - finish action started with start_creating
>   * @child: dentry returned by start_creating() or vfs_mkdir()
>   *
> -- 
> 2.50.0.107.gf914562f5916.dirty