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

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

Quoting Serge E. Hallyn (serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org):
> 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.

Hi Tejun,

no rush on the patch itself, I don't mind if i have to rewrite it from
scratch, but I'd like to get the patch into docker/libcontainer using it,
so can we decide on whether the syntax as shown here is ok?

thanks,
-serge

> 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
>