Re: [RFC PATCH] cgroup namespaces: add a 'nsroot=' mountinfo field

["Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org>]

Quoting Tycho Andersen (tycho.andersen-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org):
> Hi Serge,
> 
> On Mon, Mar 21, 2016 at 06:41:33PM -0500, Serge E. Hallyn wrote:
> > One practical problem I've found with cgroup namespaces is that there
> > is no way to disambiguate between a cgroupfs mount which was done in
> > a cgroup namespace, and a bind mount of a cgroupfs directory.  So
> > whether I do
> > 
> > unshare --cgroup -- bash -c "mount -t cgroup -o freezer f /mnt; cat /proc/self/mountinfo"
> > 
> > or whether I just
> > 
> > mount --bind /sys/fs/cgroup/freezer/$(awk -F: '/freezer/ { print $3 }' /proc/self/cgroup) /mnt
> > 
> > 'mount root' field (field 3) in /proc/self/mountinfo will show the
> > same thing, the result of awk -F: '/freezer/ { print $3 }' /proc/self/cgroup.
> > 
> > This patch adds a 'nsroot=' field to cgroup mountinfo entries, so that
> > userspace can distinguish a mount made in a cgroup namespace from a bind
> > mount from a cgroup subdirectory.
> 
> With this patch, mountinfo shows nsroot= in the mount options, but the
> actual mount() call for cgroups doesn't allow nsroot. Would it be
> possible to allow passing nsroot= to mount, as long is it does in fact
> match the current nsroot?

Yeah, that should be possible.  I'll try to send a patch for that later
this week.  That's not to say Tejun will be ok with the behavior, but
it seems to make sense to me.

> The motivation for this is that CRIU just copies the mount options and
> uses them on restore, so with this patch we have to add a special case
> to trim off nsroot= before we restore.
> 
> Tycho
> 
> > Signed-off-by: Serge Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
> > ---
> >  fs/kernfs/mount.c      |  2 +-
> >  include/linux/kernfs.h |  3 ++-
> >  kernel/cgroup.c        | 29 ++++++++++++++++++++++++++++-
> >  3 files changed, 31 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> > index b67dbcc..58f59fd 100644
> > --- a/fs/kernfs/mount.c
> > +++ b/fs/kernfs/mount.c
> > @@ -36,7 +36,7 @@ static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
> >  	struct kernfs_syscall_ops *scops = root->syscall_ops;
> >  
> >  	if (scops && scops->show_options)
> > -		return scops->show_options(sf, root);
> > +		return scops->show_options(sf, dentry, root);
> >  	return 0;
> >  }
> >  
> > diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> > index c06c442..3124b91 100644
> > --- a/include/linux/kernfs.h
> > +++ b/include/linux/kernfs.h
> > @@ -145,7 +145,8 @@ struct kernfs_node {
> >   */
> >  struct kernfs_syscall_ops {
> >  	int (*remount_fs)(struct kernfs_root *root, int *flags, char *data);
> > -	int (*show_options)(struct seq_file *sf, struct kernfs_root *root);
> > +	int (*show_options)(struct seq_file *sf, struct dentry *dentry,
> > +			    struct kernfs_root *root);
> >  
> >  	int (*mkdir)(struct kernfs_node *parent, const char *name,
> >  		     umode_t mode);
> > diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> > index 671dc05..806d1e7 100644
> > --- a/kernel/cgroup.c
> > +++ b/kernel/cgroup.c
> > @@ -1593,7 +1593,32 @@ static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
> >  	return 0;
> >  }
> >  
> > -static int cgroup_show_options(struct seq_file *seq,
> > +static void cgroup_show_nsroot(struct seq_file *seq, struct dentry *dentry,
> > +			       struct kernfs_root *kf_root)
> > +{
> > +	struct kernfs_node *d_kn = dentry->d_fsdata;
> > +	char *nsroot;
> > +	int len, ret;
> > +
> > +	if (!kf_root)
> > +		return;
> > +	len = kernfs_path_from_node(d_kn, kf_root->kn, NULL, 0);
> > +	if (len <= 0)
> > +		return;
> > +	nsroot = kzalloc(len + 1, GFP_ATOMIC);
> > +	if (!nsroot)
> > +		return;
> > +	ret = kernfs_path_from_node(d_kn, kf_root->kn, nsroot, len + 1);
> > +	if (ret <= 0 || ret > len)
> > +		goto out;
> > +
> > +	seq_show_option(seq, "nsroot", nsroot);
> > +
> > +out:
> > +	kfree(nsroot);
> > +}
> > +
> > +static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry,
> >  			       struct kernfs_root *kf_root)
> >  {
> >  	struct cgroup_root *root = cgroup_root_from_kf(kf_root);
> > @@ -1619,6 +1644,8 @@ static int cgroup_show_options(struct seq_file *seq,
> >  		seq_puts(seq, ",clone_children");
> >  	if (strlen(root->name))
> >  		seq_show_option(seq, "name", root->name);
> > +	cgroup_show_nsroot(seq, dentry, kf_root);
> > +
> >  	return 0;
> >  }
> >  
> > -- 
> > 2.7.3
> >