linux-kernel - Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

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Date:	Tue, 26 Jul 2016 21:17:31 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Andrew Vagin <avagin@...tuozzo.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"criu@...nvz.org" <criu@...nvz.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/5 RFC] Add an interface to discover relationships
 between namespaces

Hello Andrew,

On 26 July 2016 at 20:25, Andrew Vagin <avagin@...tuozzo.com> wrote:
> On Tue, Jul 26, 2016 at 10:03:25AM +0200, Michael Kerrisk (man-pages) wrote:
>> On 07/26/2016 04:54 AM, Andrew Vagin wrote:
>> > On Mon, Jul 25, 2016 at 09:59:43AM -0500, Eric W. Biederman wrote:
>> > > "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>> >
>> > [snip]
>> >
>> > > [snip]
>> > > > > > So, from my point of view, the important piece that was missing from
>> > > > > > your commit message was the note to use readlink("/proc/self/fd/%d")
>> > > > > > on the returned FDs. I think that detail needs to be part of the
>> > > > > > commit message (and also the man page text). I think it even be
>> > > > > > helpful to include the above program as part of the commit message:
>> > > > > > it helps people more quickly grasp the API.
>> > > > >
>> > > > > Please, please make the standard way to compare these things fstat.
>> > > > > That is much less magic than a symlink, and a little more future proof.
>> > > > > Possibly even kcmp.
>> >
>> > I like the idea to use kcmp to compare namespaces. I am going to add this
>> > functionality to kcmp and describe all these in the man page.
>>
>> Hi Andrey,
>>
>> Can you briefly sketch out the proposed API and how it would be used?
>> I'd find it useful to see that even before the implementation.
>
> Sure. If a process wants to compare two namespaces, it needs to get file
> descriptors for them (open /proc/PID/ns/XXX, use new ioctl-s, find a
> process which has them),
> and then it calls kcmp(pid1, pid2, KCMP_NSFD, ns_fd1, ns_fd2)
>
> For example, if we want to compare pid namespaces for 1 and 2 processes:
>

What's the purpose of the following line, and the use of 'pid' in the
kcmp() call?:

> pid = getpid();
> ns_fd1 = open("/proc/1/ns/pid")
> ns_fd2 = open("/proc/2/ns/pid")
>
> if (!kcmp(pid, pid, KCMP_NSFD, ns_fd1, ns_fd2))
>         printf("Both processes live in the same pid namespace\n");


Thanks,

Michael