linux-kernel - Re: [PATCH] vfs: add FS_USERNS_DELEGATABLE flag and set it for NFS

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Message-ID: <20260203-genehm-senden-f0375c2ca2b6@brauner>
Date: Tue, 3 Feb 2026 17:41:27 +0100
From: Christian Brauner <brauner@...nel.org>
To: Jeff Layton <jlayton@...nel.org>, 
	Alexander Mikhalitsyn <alexander@...alicyn.com>
Cc: Trond Myklebust <trondmy@...nel.org>, Anna Schumaker <anna@...nel.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	"Seth Forshee (DigitalOcean)" <sforshee@...nel.org>, linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: add FS_USERNS_DELEGATABLE flag and set it for NFS

On Tue, Feb 03, 2026 at 11:21:25AM -0500, Jeff Layton wrote:
> On Tue, 2026-02-03 at 17:11 +0100, Alexander Mikhalitsyn wrote:
> > Am Do., 29. Jan. 2026 um 22:48 Uhr schrieb Jeff Layton <jlayton@...nel.org>:
> > > 
> > > Commit e1c5ae59c0f2 ("fs: don't allow non-init s_user_ns for filesystems
> > > without FS_USERNS_MOUNT") prevents the mount of any filesystem inside a
> > > container that doesn't have FS_USERNS_MOUNT set.
> > > 
> > 
> > Hi Jeff,
> > 
> > > This broke NFS mounts in our containerized environment. We have a daemon
> > > somewhat like systemd-mountfsd running in the init_ns. A process does a
> > > fsopen() inside the container and passes it to the daemon via unix
> > > socket.
> > > 
> > > The daemon then vets that the request is for an allowed NFS server and
> > > performs the mount. This now fails because the fc->user_ns is set to the
> > > value in the container and NFS doesn't set FS_USERNS_MOUNT.  We don't
> > > want to add FS_USERNS_MOUNT to NFS since that would allow the container
> > > to mount any NFS server (even malicious ones).
> > > 
> > > Add a new FS_USERNS_DELEGATABLE flag, and enable it on NFS.
> > 
> > Great idea, very similar to what we have with BPFFS/BPF Tokens.
> > 
> > Taking into account this patch, shouldn't we drop FS_USERNS_MOUNT and
> > replace it with
> > FS_USERNS_DELEGATABLE for bpffs too?
> > 
> > I mean something like:
> > 
> > ======================
> > $ git diff
> > diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> > index 9f866a010dad..d8dfdc846bd0 100644
> > --- a/kernel/bpf/inode.c
> > +++ b/kernel/bpf/inode.c
> > @@ -1009,10 +1009,6 @@ static int bpf_fill_super(struct super_block
> > *sb, struct fs_context *fc)
> >         struct inode *inode;
> >         int ret;
> > 
> > -       /* Mounting an instance of BPF FS requires privileges */
> > -       if (fc->user_ns != &init_user_ns && !capable(CAP_SYS_ADMIN))
> > -               return -EPERM;
> > -
> >         ret = simple_fill_super(sb, BPF_FS_MAGIC, bpf_rfiles);
> >         if (ret)
> >                 return ret;
> > @@ -1085,7 +1081,7 @@ static struct file_system_type bpf_fs_type = {
> >         .init_fs_context = bpf_init_fs_context,
> >         .parameters     = bpf_fs_parameters,
> >         .kill_sb        = bpf_kill_super,
> > -       .fs_flags       = FS_USERNS_MOUNT,
> > +       .fs_flags       = FS_USERNS_DELEGATABLE,
> >  };
> > 
> >  static int __init bpf_init(void)
> > ======================
> > 
> > Because it feels like we were basically implementing this FS_USERNS_DELEGATABLE
> > flag implicitly for BPFFS before. I can submit a patch for BPFFS later
> > after testing.

Can you send that to the list, please?
Thanks!