netdev - Re: [PATCH] netns: add /proc/*/net/id symlink

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Date:	Sat, 21 May 2011 08:39:37 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, equinox@...c24.net
Subject: Re: [PATCH] netns: add /proc/*/net/id symlink

Alexey Dobriyan <adobriyan@...il.com> writes:

> David Lamparter pointed some real scenarios where knowing
> if two processes live in same netns is important,
> like "how do I kill _all_ processes in netns to shutdown it".

Currently today the way I do this is md5sum /proc/<pid>/mounts.

That works because it is usually necessary to have a separate mount
namespace with a separate set of mounts to accommodate sysfs.

> Currently only kernel knows if two netns are the same.
> Userspace maybe can look at different proc files to find a match
> indirectly sysconf-style but result will be ugly no matter what.

Somewhat. 

Right now today without patches if we limit ourselves to the network
namespace there is a pretty valid way to do this.

stat /proc/<pid>/net/dev and compare the inode numbers.

Or any other file in /proc/*/net/.  The inode numbers are the
same if you are in the same network namespace.

> Add /proc/*/net/id symlink which "points" to an integer.
>
> 	$ readlink /proc/net/id
> 	0
>
> 	$ readlink /proc/2941/net/id
> 	1
>
> "id" is not a file because 1 syscall is faster than 3 syscalls.
>
> The only rules and expectations for userspace are:
> [as if they will comply, ha-ha]
>
> * init_net always has id 0
> * two netns do not have same id
> * id is unsigned integer

I don't like this patch because we already have a proc interface
that already solves this in production kernels today.

- stat is a single syscall
- two netns do not have the same id
- id is an ino_t.

Now it probably needs to be better documented that /proc/*/net/*
have the same inode number if the network namespace is the
same, as everyone including myself overlooked this very handy
existing property.

Writing this it occurs to me there is a misfeature in my pending
namespace file descriptor code.  Right now /proc/<pid>/ns/net
has a floating inode number and it would be good if I could make
that a inode number be the same for every file that refers to
the same network namespace. Ugh.

Eric

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