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Date:   Wed, 20 Oct 2021 16:34:18 +0000
From:   Sargun Dhillon <sargun@sargun.me>
To:     Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: Retrieving the network namespace of a socket
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On Wed, Oct 20, 2021 at 05:03:56PM +0300, Sergey Ryazanov wrote:
> Hello Sargun,
> 
> On Wed, Oct 20, 2021 at 12:57 PM Sargun Dhillon <sargun@sargun.me> wrote:
> > I'm working on a problem where I need to determine which network namespace a
> > given socket is in. I can currently bruteforce this by using INET_DIAG, and
> > enumerating namespaces and working backwards.
> 
> Namespace is not a per-socket, but a per-process attribute. So each
> socket of a process belongs to the same namespace.
> 
> Could you elaborate what kind of problem you are trying to solve?
> Maybe there is a more simple solution. for it.
> 
> -- 
> Sergey

That's not entirely true. See the folowing code:

int main() {
	int fd1, fd2;
	fd1 = socket(AF_INET, SOCK_STREAM, 0);
	assert(fd1 >= 0);
	assert(unshare(CLONE_NEWNET) == 0);
	fd2 = socket(AF_INET, SOCK_STREAM, 0);
	assert(fd2 >= 0);
}

fd1 and fd2 have different sock_net.

The context for this is:
https://linuxplumbersconf.org/event/11/contributions/932/

We need to figure out, for a given socket, if it has reachability to a given IP.