[lxc-devel] [PATCH] aufs: Support unprivileged clone, mount

[Serge Hallyn]

Quoting KATOH Yasufumi (karma at jazz.email.ne.jp):
> >>> On Thu, 30 Apr 2015 14:02:55 +0000
>     in message   "Re: [lxc-devel] [PATCH] aufs: Support unprivileged clone, mount"
>                   Serge Hallyn-san wrote:
> 
> > Quoting KATOH Yasufumi (karma at jazz.email.ne.jp):
> > > Current aufs supports FS_USERNS_MOUNT by using allow_userns module
> > > parameter. It allows root in userns to mount aufs.
> > > 
> > > This patch allows an unprivileged container to use aufs.
> > > 
> > > Signed-off-by: KATOH Yasufumi <karma at jazz.email.ne.jp>
> > > ---
> > >  src/lxc/bdev.c | 65 +++++++++++++++++++++++++++++++---------------------------
> > >  1 file changed, 35 insertions(+), 30 deletions(-)
> > > 
> > > diff --git a/src/lxc/bdev.c b/src/lxc/bdev.c
> > > index cf7c043..eb516c8 100644
> > > --- a/src/lxc/bdev.c
> > > +++ b/src/lxc/bdev.c
> > > @@ -2552,12 +2552,12 @@ static int aufs_detect(const char *path)
> > >  //
> > >  static int aufs_mount(struct bdev *bdev)
> > >  {
> > > -	char *options, *dup, *lower, *upper, *rundir;
> > > +	char *options, *dup, *lower, *upper;
> > >  	int len;
> > >  	unsigned long mntflags;
> > >  	char *mntdata;
> > > -	char *runpath;
> > >  	int ret;
> > > +	const char *xinopath = "/dev/shm/aufs.xino";
> 
> > So who will create this directory?  What are its permissions?
> > Does aufs somehow autocreate that in such a way that anyone
> > can create files (and immediately delete) files there?
> 
> Thanks for your reply.
> 
> '/dev/shm' may not always exists, or may not always be 1777, so this
> code may cause a problem on such a system.

/dev/shm yes, but what about /dev/shm/aufs.xino itself.

Wait - is /dev/shm/aufs.xino a directory under which files will be
created (and immediately deleted), or /dev/shm/aufs.xino itself going
to be created and immediately unlinked?  If the latter then what I'm
asking doesn't matter.

> But many software expect the existent of /dev/shm for shared memory
> area. So many distributions have /dev/shm. And it is usually mounted
> as tmpfs, and its permission is usually set 1777. For example,
> 
>   - systemd creates /dev/shm, and mount as tmpfs, and set mode=1777 at
>     boot time.
>   - Ubuntu Trusty, Debian wheezy has /dev/shm as the link to /run/shm
>     (mode=1777)
> 
> xino file is created per aufs branch automatically.

Automatically at boot, or at the first mount time?  I.e. whose credentials
are used to created it.

> We can specify the
> same xino to multiple aufs mounts. After a aufs priv container is
> started, I could run a unpriv aufs container at the same time.
> 
> So I think that it is not a problem in many cases.
> 
> Does that answer your question?

no, though I may just be being dense.  But can you please reboot
such a system, start a aufs-backed root-owned container, then
show the result of 'ls -ld /dev/shm/aufs.xino' and
'ls -l /dev/shm/aufs.xino' ?

thanks,
-serge