linux-kernel - Re: [RFC PATCH 0/5] net: socket bind to file descriptor introduced

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Message-ID: <20120904190007.GB29369@fieldses.org>
Date:	Tue, 4 Sep 2012 15:00:07 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"hpa@...or.com" <hpa@...or.com>,
	"thierry.reding@...onic-design.de" <thierry.reding@...onic-design.de>,
	"bfields@...hat.com" <bfields@...hat.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	Pavel Emelianov <xemul@...allels.com>,
	"neilb@...e.de" <neilb@...e.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"paul.gortmaker@...driver.com" <paul.gortmaker@...driver.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"gorcunov@...nvz.org" <gorcunov@...nvz.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [RFC PATCH 0/5] net: socket bind to file descriptor introduced

On Mon, Aug 20, 2012 at 02:18:13PM +0400, Stanislav Kinsbursky wrote:
> 16.08.2012 07:03, Eric W. Biederman пишет:
> >Stanislav Kinsbursky <skinsbursky@...allels.com> writes:
> >
> >>This patch set introduces new socket operation and new system call:
> >>sys_fbind(), which allows to bind socket to opened file.
> >>File to bind to can be created by sys_mknod(S_IFSOCK) and opened by
> >>open(O_PATH).
> >>
> >>This system call is especially required for UNIX sockets, which has name
> >>lenght limitation.
> >>
> >>The following series implements...
> >
> >Hmm.  I just realized this patchset is even sillier than I thought.
> >
> >Stanislav is the problem you are ultimately trying to solve nfs clients
> >in a container connecting to the wrong user space rpciod?
> >
> 
> Hi, Eric.
> The problem you mentioned was the reason why I started to think about this.
> But currently I believe, that limitations in unix sockets connect or
> bind should be removed, because it will be useful it least for CRIU
> project.
> 
> >Aka net/sunrpc/xprtsock.c:xs_setup_local only taking an absolute path
> >and then creating a delayed work item to actually open the unix domain
> >socket?
> >
> >The straight correct and straight forward thing to do appears to be:
> >- Capture the root from current->fs in xs_setup_local.
> >- In xs_local_finish_connect change current->fs.root to the captured
> >   version of root before kernel_connect, and restore current->fs.root
> >   after kernel_connect.
> >
> >It might not be a bad idea to implement open on unix domain sockets in
> >a filesystem as create(AF_LOCAL)+connect() which would allow you to
> >replace __sock_create + kernel_connect with a simple file_open_root.
> >
> 
> I like the idea of introducing new family (AF_LOCAL_AT for example)
> and new sockaddr for connecting or binding from specified root. The
> only thing I'm worrying is passing file descriptor to unix bind or
> connect routine. Because this approach doesn't provide easy way to
> use such family and sockaddr in kernel (like in NFS example).
> 
> >But I think the simple scheme of:
> >struct path old_root;
> >old_root = current->fs.root;
> >kernel_connect(...);
> >current->fs.root = old_root;
> >
> >Is more than sufficient and will remove the need for anything
> >except a purely local change to get nfs clients to connect from
> >containers.
> >
> 
> That was my first idea.

So is this what you're planning on doing now?

> And probably it would be worth to change all
> fs_struct to support sockets with relative path.
> What do you think about it?

I didn't understand the question.  Are you suggesting that changes to
fs_struct would be required to make this work?  I don't see why.

--b.
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