Re: [PATCH 1/4] mount_fsmount: split superblock creation and move_mount into a helper

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Sep 30, 2026 at 03:44:33PM +0200, Bernd Schubert via B4 Relay wrote:
> From: Bernd Schubert <bernd@bsbernd.com>
> 
> This is a preparation patch. fuse_kern_fsmount() ran fsconfig
> CREATE, fsmount() and move_mount() inline. A later commit will make
> mount_service.c call the same three calls after it has run FSOPEN,
> SOURCE and MNTOPTS itself, so this moves them into
> fuse_fsmount_create_and_move(). The fsopen fd stays owned by the
> caller, because mount_service.c will close mo->fsopenfd itself
> later.
> 
> fuse_kern_fsmount() now converts MS_* flags before calling the
> helper, so unsupported flags are caught before the superblock is
> created, not after.
> 
> Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

This is a simple hoist, so
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  lib/mount_fsmount.c | 86 +++++++++++++++++++++++++++++++----------------------
>  lib/mount_i_linux.h | 18 +++++++++++
>  2 files changed, 69 insertions(+), 35 deletions(-)
> 
> diff --git a/lib/mount_fsmount.c b/lib/mount_fsmount.c
> index b2d0453e5c93..0b1fd4300d2d 100644
> --- a/lib/mount_fsmount.c
> +++ b/lib/mount_fsmount.c
> @@ -489,6 +489,53 @@ void fuse_kern_umount_mountfd(int mountfd)
>  	}
>  }
>  
> +int fuse_fsmount_create_and_move(int fsfd, unsigned int mount_attrs,
> +				 int dest_mnt_fd, const char *dest_path,
> +				 int *mountfd_out)
> +{
> +	int mountfd;
> +	int res;
> +
> +	res = fuse_fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +	if (res == -1) {
> +		int save_errno = errno;
> +
> +		log_fsconfig_kmsg(fsfd);
> +		fuse_log(FUSE_LOG_ERR, "fuse: fsconfig CREATE failed: %s\n",
> +			 strerror(save_errno));
> +		return -save_errno;
> +	}
> +
> +	mountfd = fuse_fsmount(fsfd, FSMOUNT_CLOEXEC, mount_attrs);
> +	if (mountfd == -1) {
> +		int save_errno = errno;
> +
> +		log_fsconfig_kmsg(fsfd);
> +		fuse_log(FUSE_LOG_ERR, "fuse: fsmount failed: %s\n",
> +			 strerror(save_errno));
> +		return -save_errno;
> +	}
> +
> +	if (dest_mnt_fd >= 0)
> +		res = fuse_move_mount(mountfd, "", dest_mnt_fd, "",
> +				      MOVE_MOUNT_F_EMPTY_PATH |
> +					      MOVE_MOUNT_T_EMPTY_PATH);
> +	else
> +		res = fuse_move_mount(mountfd, "", AT_FDCWD, dest_path,
> +				      MOVE_MOUNT_F_EMPTY_PATH);
> +	if (res == -1) {
> +		int save_errno = errno;
> +
> +		fuse_log(FUSE_LOG_ERR, "fuse: move_mount failed: %s\n",
> +			 strerror(save_errno));
> +		close(mountfd);
> +		return -save_errno;
> +	}
> +
> +	*mountfd_out = mountfd;
> +	return 0;
> +}
> +
>  int fuse_kern_fsmount(const char *mnt, int dest_mnt_fd, unsigned long flags,
>  		      int blkdev, const char *fsname, const char *subtype,
>  		      const char *source_dev, const char *kernel_opts,
> @@ -584,16 +631,6 @@ int fuse_kern_fsmount(const char *mnt, int dest_mnt_fd, unsigned long flags,
>  		goto out_free;
>  	}
>  
> -	/* Create the filesystem instance */
> -	res = fuse_fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> -	if (res == -1) {
> -		err = -errno;
> -		log_fsconfig_kmsg(fsfd);
> -		fuse_log(FUSE_LOG_ERR, "fuse: fsconfig CREATE failed: %s\n",
> -			 strerror(errno));
> -		goto out_free;
> -	}
> -
>  	/* Convert MS_* flags to MOUNT_ATTR_* for fsmount() */
>  	flags = ms_flags_to_mount_attrs(flags, &mount_attrs);
>  	if (flags != 0) {
> @@ -602,33 +639,14 @@ int fuse_kern_fsmount(const char *mnt, int dest_mnt_fd, unsigned long flags,
>  		goto out_free;
>  	}
>  
> -	/* Create mount object with mount attributes */
> -	mountfd = fuse_fsmount(fsfd, FSMOUNT_CLOEXEC, mount_attrs);
> -	if (mountfd == -1) {
> -		err = -errno;
> -		log_fsconfig_kmsg(fsfd);
> -		fuse_log(FUSE_LOG_ERR, "fuse: fsmount failed: %s\n",
> -			 strerror(errno));
> +	err = fuse_fsmount_create_and_move(fsfd, mount_attrs, dest_mnt_fd, mnt,
> +					   &mountfd);
> +	if (err)
>  		goto out_free;
> -	}
>  
>  	close(fsfd);
>  	fsfd = -1;
>  
> -	if (dest_mnt_fd >= 0)
> -		res = fuse_move_mount(mountfd, "", dest_mnt_fd, "",
> -				 MOVE_MOUNT_F_EMPTY_PATH |
> -					 MOVE_MOUNT_T_EMPTY_PATH);
> -	else
> -		res = fuse_move_mount(mountfd, "", AT_FDCWD, mnt,
> -				 MOVE_MOUNT_F_EMPTY_PATH);
> -	if (res == -1) {
> -		err = -errno;
> -		fuse_log(FUSE_LOG_ERR, "fuse: move_mount failed: %s\n",
> -			 strerror(errno));
> -		goto out_close_mntfd;
> -	}
> -
>  	res = fuse_mnt_add_mount_helper(mnt, source, type, mtab_opts);
>  	if (res == -1) {
>  		err = -EIO;
> @@ -645,9 +663,7 @@ int fuse_kern_fsmount(const char *mnt, int dest_mnt_fd, unsigned long flags,
>  
>  out_umount:
>  	fuse_kern_umount_mountfd(mountfd);
> -out_close_mntfd:
> -	if (mountfd != -1)
> -		close(mountfd);
> +	close(mountfd);
>  out_free:
>  	free(source);
>  	free(type);
> diff --git a/lib/mount_i_linux.h b/lib/mount_i_linux.h
> index 8cf7ff4532c6..35bb54c82236 100644
> --- a/lib/mount_i_linux.h
> +++ b/lib/mount_i_linux.h
> @@ -86,6 +86,24 @@ int fuse_kern_fsmount(const char *mnt, int mnt_fd, unsigned long flags,
>  int fuse_kern_fsmount_mo(const char *mnt, const struct mount_opts *mo,
>  			 const char *mtab_opts, int *mountfd_out);
>  
> +/**
> + * Create the superblock and attach the resulting mount to a destination.
> + *
> + * The fsopen fd stays owned by the caller - a caller that keeps it in
> + * long-lived state must not be left with a descriptor closed behind its back.
> + *
> + * @fsfd fsopen fd, fully configured
> + * @mount_attrs MOUNT_ATTR_* flags, from ms_flags_to_mount_attrs()
> + * @dest_mnt_fd pre-resolved destination fd, or -1 to use @dest_path
> + * @dest_path destination path, used only when @dest_mnt_fd is -1
> + * @mountfd_out on success receives the fsmount() fd, to be closed by the
> + *              caller and usable for fuse_kern_umount_mountfd()
> + * @return 0 on success, negative error code on failure
> + */
> +int fuse_fsmount_create_and_move(int fsfd, unsigned int mount_attrs,
> +				 int dest_mnt_fd, const char *dest_path,
> +				 int *mountfd_out);
> +
>  /**
>   * Unmount (lazily) the mount @mountfd was created for, without a path lookup.
>   *
> 
> -- 
> 2.53.0
> 
> 
>