linux-kernel - Re: [PATCH 2/2] mountinfo: implement show

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Date:	Mon, 18 Apr 2016 20:12:39 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serge.hallyn@...ntu.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	containers@...ts.linux-foundation.org, hannes@...xchg.org,
	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

"Serge E. Hallyn" <serge.hallyn@...ntu.com> writes:

>> 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?

Do we need the current cgroup namespace information at all?

Could we not get the relevant cgroup namespace from the mount of
cgroupfs?

In general the better path is not to have the contents of files depend on
who is reading the file.

Eric