netdev - Re: [RFC PATCH 0/2] sctp: add new getsockopt option SCTP_SOCKOPT_PEELOFF

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Message-ID: <20150617102119.GA24677@hmsreliant.think-freely.org>
Date:	Wed, 17 Jun 2015 06:21:19 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:	netdev@...r.kernel.org, Vlad Yasevich <vyasevich@...il.com>,
	linux-sctp@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] sctp: add new getsockopt option
 SCTP_SOCKOPT_PEELOFF_KERNEL

On Tue, Jun 16, 2015 at 07:42:31PM -0300, Marcelo Ricardo Leitner wrote:
> Hi,
> 
> I'm trying to remove a direct dependency of dlm module on sctp one.
> Currently dlm code is calling sctp_do_peeloff() directly and only this
> call is causing the load of sctp module together with dlm. For that, we
> have basically 3 options:
> - Doing a module split on dlm
>   - which I'm avoiding because it was already split and was merged (more
>     info on patch2 changelog)
>   - and the sctp code on it is rather small if compared with sctp module
>     itself
> - Using some other infra that gets indirectly activated, like getsockopt()
>   - It was like this before, but the exposed sockopt created a file
>     descriptor for the new socket and that create some serious issues.
>     More info on 2f2d76cc3e93 ("dlm: Do not allocate a fd for peeloff")
> - Doing something like ipv6_stub (which is used by vxlan) or similar
>   - but I don't feel that's a good way out here, it doesn't feel right.
> 
> So I'm approaching this by going with 2nd option again but this time
> also creating a new sockopt that is only accessible for kernel users of
> this protocol, so that we are safe to directly return a struct socket *
> via getsockopt() results. This is the tricky part of it of this series. 
> 
> It smells hacky yes but currently most of sctp calls are wrapped behind
> kernel_*(). Even if we set a flag (like netlink does) saying that this
> is a kernel socket, we still have the issue of getting the function call
> through and returning such non-usual return value.
> 
> I kept __user marker on sctp_getsockopt_peeloff_kernel() prototype and
> its helpers just to avoid issues with static checkers.
> 
> Kernel path not really tested yet.. mainly willing to know what do you
> think, is this feasible? getsockopt option only reachable by kernel
> itself? Couldn't find any other like this.
> 
> Thanks,
> Marcelo
> 
> Marcelo Ricardo Leitner (2):
>   sctp: add new getsockopt option SCTP_SOCKOPT_PEELOFF_KERNEL
>   dlm: avoid using sctp_do_peeloff directly
> 
>  fs/dlm/lowcomms.c         | 17 ++++++++---------
>  include/uapi/linux/sctp.h | 12 ++++++++++++
>  net/sctp/socket.c         | 39 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 59 insertions(+), 9 deletions(-)
> 
> -- 
> 2.4.1
> 
> 

Why not just use the existing PEELOFF socket option with the kernel_getsockopt
interface, and sockfd_lookup to translate the returned value back to a socket
struct?  That seems less redundant and less hack-ish to me.

Neil
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