linux-kernel - Re: [PATCH v5 4/4] fuse: Allow user namespace mounts

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Date:	Wed, 22 Oct 2014 14:51:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel@...ts.sourceforge.net,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 4/4] fuse: Allow user namespace mounts

On Wed, Oct 22, 2014 at 2:24 PM, Seth Forshee
<seth.forshee@...onical.com> wrote:
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Serge H. Hallyn <serge.hallyn@...ntu.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> ---
>  fs/fuse/inode.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b88b5a780228..7d0e73e36e7b 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1201,7 +1201,7 @@ static void fuse_kill_sb_anon(struct super_block *sb)
>  static struct file_system_type fuse_fs_type = {
>         .owner          = THIS_MODULE,
>         .name           = "fuse",
> -       .fs_flags       = FS_HAS_SUBTYPE,
> +       .fs_flags       = FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
>         .mount          = fuse_mount,
>         .kill_sb        = fuse_kill_sb_anon,
>  };
> @@ -1233,7 +1233,7 @@ static struct file_system_type fuseblk_fs_type = {
>         .name           = "fuseblk",
>         .mount          = fuse_mount_blk,
>         .kill_sb        = fuse_kill_sb_blk,
> -       .fs_flags       = FS_REQUIRES_DEV | FS_HAS_SUBTYPE,
> +       .fs_flags       = FS_REQUIRES_DEV | FS_HAS_SUBTYPE | FS_USERNS_MOUNT,
>  };
>  MODULE_ALIAS_FS("fuseblk");
>
> --
> 1.9.1
>

This is mostly a sign of my ignorance, but how does this actually end
up working?  I assume that the mounter opens /dev/fuse and then passes
the fd to the mount call.  Which userns is captured?  The opener of
/dev/fuse or the mounter of the fs?

--Andy
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