Re: [PATCH 2/2] libfuse: always send the subtype to the kernel when using fsconfig()

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

On Tue, May 05, 2026 at 09:41:59AM +0200, Bernd Schubert wrote:
> 
> 
> On 5/5/26 07:23, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Although it's possible to supply a filesystem subtype when calling
> > fsopen(), it does not pass the stuff after the dot to the mount option
> > parsing mechanism like the legacy mount() call does.  Therefore, it's
> > necessary to pass the subtype explicitly via fsconfig() or else the
> > subtype will be missing from the /proc/mounts output.
> > 
> > For example, fuse4fs with the old mount API produces this:
> > 
> > /dev/sda /mnt fuse.ext4 rw,nosuid,nodev,relatime,user_id=0,group_id=0 0 0
> > 
> > But with the new mount API, we only get:
> > 
> > /dev/sda /mnt fuse rw,nosuid,nodev,relatime,user_id=0,group_id=0 0 0
> > 
> > ...which breaks fstests, which requires the fsname to match the string
> > that it passes to mount -t.  Update the comment in mount_service.c to
> > match this requirement.
> > 
> > Fixes: 14cb7b93bb9688 ("Add support for the new linux mount API")
> > Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> > ---
> >  lib/mount_fsmount.c  |   13 +++++++++++++
> >  util/mount_service.c |    3 ++-
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> > 
> > 
> > diff --git a/lib/mount_fsmount.c b/lib/mount_fsmount.c
> > index 615271b09758c3..5e13b34ccfc610 100644
> > --- a/lib/mount_fsmount.c
> > +++ b/lib/mount_fsmount.c
> > @@ -359,6 +359,19 @@ int fuse_kern_fsmount(const char *mnt, unsigned long flags, int blkdev,
> >  		goto out_free;
> >  	}
> >  
> > +	/* Configure subtype */
> > +	if (subtype) {
> > +		res = fsconfig(fsfd, FSCONFIG_SET_STRING, "subtype",
> > +			       subtype, 0);
> > +		if (res) {
> > +			err = -errno;
> > +			log_fsconfig_kmsg(fsfd);
> > +			fprintf(stderr, "fuse: fsconfig subtype failed: %s\n",
> > +				strerror(-err));
> > +			goto out_free;
> > +		}
> > +	}
> > +
> >  	/* Configure source */
> >  	res = fsconfig(fsfd, FSCONFIG_SET_STRING, "source", source, 0);
> >  	if (res == -1) {
> 
> Oh yes, that had slipped through. Going to apply this.

Thanks!

> > diff --git a/util/mount_service.c b/util/mount_service.c
> > index bc5940bc900dad..3fbabfdd69300d 100644
> > --- a/util/mount_service.c
> > +++ b/util/mount_service.c
> > @@ -821,7 +821,8 @@ static void try_fsopen(struct mount_service *mo)
> >  	/*
> >  	 * As of Linux 7.0 you can pass subtypes to fsopen, but the manpage for
> >  	 * fsopen only says that you can pass any value of the second column of
> > -	 * /proc/filesystems into fsopen.
> > +	 * /proc/filesystems into fsopen.  You must still call fsconfig() for
> > +	 * the subtype separately, however.
> >  	 */
> >  	mo->fsopenfd = fsopen(fsname(mo), FSOPEN_CLOEXEC);
> >  }
> > 
> 
> Can we integrate that into you series separately, as it is not merged yet?

I'll separate this hunk into a separate patch.

--D

> 
> Thanks,
> Bernd
>