Date:	Sun, 17 Apr 2016 23:11:27 -0500
From:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
To:	serge.hallyn@...ntu.com
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	containers@...ts.linux-foundation.org, hannes@...xchg.org,
	ebiederm@...ssion.com, gregkh@...uxfoundation.org, tj@...nel.org,
	cgroups@...r.kernel.org, akpm@...ux-foundation.org,
	serge@...lyn.com
Subject: Re: [PATCH 2/2] mountinfo: implement show_path for kernfs and cgroup

On Sun, Apr 17, 2016 at 03:04:32PM -0500, serge.hallyn@...ntu.com wrote:
> From: Serge Hallyn <serge.hallyn@...ntu.com>
> 
> When showing a cgroupfs entry in mountinfo, show the
> path of the mount root dentry relative to the reader's
> cgroup namespace root.
> 
> Signed-off-by: Serge Hallyn <serge.hallyn@...ntu.com>
> ---
>  fs/kernfs/mount.c      | 14 ++++++++++++++
>  include/linux/kernfs.h |  2 ++
>  kernel/cgroup.c        | 35 +++++++++++++++++++++++++++++++++++
>  3 files changed, 51 insertions(+)
> 
> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> index f73541f..3b78724 100644
> --- a/fs/kernfs/mount.c
> +++ b/fs/kernfs/mount.c
> @@ -15,6 +15,7 @@
>  #include <linux/slab.h>
>  #include <linux/pagemap.h>
>  #include <linux/namei.h>
> +#include <linux/seq_file.h>
>  
>  #include "kernfs-internal.h"
>  
> @@ -40,6 +41,18 @@ static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
>  	return 0;
>  }
>  
> +static int kernfs_sop_show_path(struct seq_file *sf, struct dentry *dentry)
> +{
> +	struct kernfs_node *node = dentry->d_fsdata;
> +	struct kernfs_root *root = kernfs_root(node);
> +	struct kernfs_syscall_ops *scops = root->syscall_ops;
> +
> +	if (scops && scops->show_path)
> +		return scops->show_path(sf, node, root);
> +
> +	return seq_dentry(sf, dentry, " \t\n\\");
> +}
> +
>  const struct super_operations kernfs_sops = {
>  	.statfs		= simple_statfs,
>  	.drop_inode	= generic_delete_inode,
> @@ -47,6 +60,7 @@ const struct super_operations kernfs_sops = {
>  
>  	.remount_fs	= kernfs_sop_remount_fs,
>  	.show_options	= kernfs_sop_show_options,
> +	.show_path	= kernfs_sop_show_path,
>  };
>  
>  /**
> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> index c06c442..30f089e 100644
> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -152,6 +152,8 @@ struct kernfs_syscall_ops {
>  	int (*rmdir)(struct kernfs_node *kn);
>  	int (*rename)(struct kernfs_node *kn, struct kernfs_node *new_parent,
>  		      const char *new_name);
> +	int (*show_path)(struct seq_file *sf, struct kernfs_node *kn,
> +			 struct kernfs_root *root);
>  };
>  
>  struct kernfs_root {
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 671dc05..9a0d7b3 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1593,6 +1593,40 @@ static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
>  	return 0;
>  }
>  
> +static int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
> +			    struct kernfs_root *kf_root)
> +{
> +	int len = 0, ret = 0;
> +	char *buf = NULL;
> +	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
> +	struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
> +	struct cgroup *ns_cgroup;
> +
> +	mutex_lock(&cgroup_mutex);

Hm, I can't grab the cgroup mutex here because I already have the
namespace_sem.  But that's required by cset_cgroup_from_root().  Can
I just call that under rcu_read_lock() instead?  (Not without
changing the lockdep_assert_help()).  Is there another way to get the
info needed here?

> +	spin_lock_bh(&css_set_lock);
> +	ns_cgroup = cset_cgroup_from_root(ns->root_cset, kf_cgroot);
> +	len = kernfs_path_from_node(kf_node, ns_cgroup->kn, NULL, 0);
> +	if (len > 0)
> +		buf = kmalloc(len + 1, GFP_ATOMIC);
> +	if (buf)
> +		ret = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, len + 1);
> +
> +	spin_unlock_bh(&css_set_lock);
> +	mutex_unlock(&cgroup_mutex);
> +
> +	if (len <= 0)
> +		return len;
> +	if (!buf)
> +		return -ENOMEM;
> +	if (ret == len) {
> +		seq_escape(sf, buf, " \t\n\\");
> +		ret = 0;
> +	} else if (ret >= 0)
> +		ret = -EINVAL;
> +	kfree(buf);
> +	return ret;
> +}
> +
>  static int cgroup_show_options(struct seq_file *seq,
>  			       struct kernfs_root *kf_root)
>  {
> @@ -5430,6 +5464,7 @@ static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
>  	.mkdir			= cgroup_mkdir,
>  	.rmdir			= cgroup_rmdir,
>  	.rename			= cgroup_rename,
> +	.show_path		= cgroup_show_path,
>  };
>  
>  static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
> -- 
> 2.7.4
> 
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