Quoting Tycho Andersen (tycho.andersen@xxxxxxxxxxxxx):
> 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@xxxxxxxxxx>
> > ---
> > 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
> >
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