Re: [PATCH 2/4] mount_service: mount via the shared fsmount helper

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

On Wed, Sep 30, 2026 at 03:44:34PM +0200, Bernd Schubert via B4 Relay wrote:
> From: Bernd Schubert <bernd@bsbernd.com>
> 
> util/mount_service.c issued FSCONFIG_CMD_CREATE, fsmount() and
> move_mount() itself, with the same FSMOUNT_CLOEXEC and
> MOVE_MOUNT_*_EMPTY_PATH flags as lib/mount_fsmount.c. Both mount the
> same filesystem, so a flag changed in one copy silently diverges.
> 
> mount_service.c also called the libc names fsconfig(), fsmount() and
> move_mount() directly. HAVE_NEW_MOUNT_API only checks that the headers
> declare these functions, not that libc exports them. On a system where
> NEED_NEW_MOUNT_API_SYSCALL_WRAPPERS is also set (glibc older than
> 2.36), lib/mount_fsmount.c works around the missing symbols through
> its fuse_-prefixed static inline wrappers, which mount_service.c had
> no access to; calling the bare libc names left it with a declaration
> to compile against but no symbol to link against.
> 
> Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
> ---
>  util/mount_service.c | 49 ++++++++++++-------------------------------------
>  1 file changed, 12 insertions(+), 37 deletions(-)
> 
> diff --git a/util/mount_service.c b/util/mount_service.c
> index 0b3266309a8a..dcdc26df1db4 100644
> --- a/util/mount_service.c
> +++ b/util/mount_service.c
> @@ -28,11 +28,6 @@
>  #include <sys/ioctl.h>
>  #include <linux/fs.h>
>  
> -#ifdef HAVE_NEW_MOUNT_API
> -#include <sys/mount.h>
> -#include <linux/mount.h>
> -#endif
> -
>  #include "mount_util.h"
>  #include "util.h"
>  #include "fuse_i.h"
> @@ -1439,63 +1434,45 @@ static int mount_service_fsopen_mount(struct mount_service *mo,
>  			return FUSE_MOUNT_FALLBACK_NEEDED;
>  
>  		error = -ret;
> -		goto fail_fsconfig;
> +		goto fail_mount;
>  	}
>  
>  	snprintf(tmp, sizeof(tmp), "%i", mo->fusedevfd);
>  	ret = apply_fsconfig_opt_fd(mo->fsopenfd, tmp);
>  	if (ret < 0) {
>  		error = -ret;
> -		goto fail_fsconfig;
> +		goto fail_mount;
>  	}
>  
>  	snprintf(tmp, sizeof(tmp), "%o", stbuf->st_mode & S_IFMT);
>  	ret = apply_fsconfig_opt_string(mo->fsopenfd, "rootmode", tmp);
>  	if (ret < 0) {
>  		error = -ret;
> -		goto fail_fsconfig;
> +		goto fail_mount;
>  	}
>  
>  	snprintf(tmp, sizeof(tmp), "%u", getuid());
>  	ret = apply_fsconfig_opt_string(mo->fsopenfd, "user_id", tmp);
>  	if (ret < 0) {
>  		error = -ret;
> -		goto fail_fsconfig;
> +		goto fail_mount;
>  	}
>  
>  	snprintf(tmp, sizeof(tmp), "%u", getgid());
>  	ret = apply_fsconfig_opt_string(mo->fsopenfd, "group_id", tmp);
>  	if (ret < 0) {
>  		error = -ret;
> -		goto fail_fsconfig;
> -	}
> -
> -	ret = fsconfig(mo->fsopenfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> -	if (ret) {
> -		error = errno;
> -		fprintf(stderr, "%s: creating filesystem: %s\n",
> -			mo->msgtag, strerror(error));
> -		goto fail_fsconfig;
> -	}
> -
> -	mfd = fsmount(mo->fsopenfd, FSMOUNT_CLOEXEC, attr_flags);
> -	if (mfd < 0) {
> -		error = errno;
> -		fprintf(stderr, "%s: fsmount: %s\n",
> -			mo->msgtag, strerror(error));
> -		goto fail_fsconfig;
> -	}
> -
> -	ret = move_mount(mfd, "", mo->mountfd, "",
> -			 MOVE_MOUNT_F_EMPTY_PATH | MOVE_MOUNT_T_EMPTY_PATH);
> -	close(mfd);
> -	if (ret) {
> -		error = errno;
> -		fprintf(stderr, "%s: move_mount: %s\n",
> -			mo->msgtag, strerror(error));
>  		goto fail_mount;
>  	}
>  
> +	ret = fuse_fsmount_create_and_move(mo->fsopenfd, attr_flags,
> +					   mo->mountfd, NULL, &mfd);
> +	if (ret < 0) {
> +		error = -ret;
> +		goto fail_mount;
> +	}
> +	close(mfd);

Slick :)
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> +
>  	/*
>  	 * The mount succeeded, so we send a positive reply even if the mtab
>  	 * update fails.
> @@ -1515,8 +1492,6 @@ static int mount_service_fsopen_mount(struct mount_service *mo,
>  	mo->mounted = true;
>  	return mount_service_send_reply(mo, 0);
>  
> -fail_fsconfig:
> -	log_fsconfig_kmsg(mo->fsopenfd);
>  fail_mount:
>  	return mount_service_send_reply(mo, error);
>  }
> 
> -- 
> 2.53.0
> 
> 
>