Date:	Wed, 17 Jun 2015 09:16:58 -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 Wed, Jun 17, 2015 at 09:40:32AM -0300, Marcelo Ricardo Leitner wrote:
> On 17-06-2015 09:20, Neil Horman wrote:
> >On Wed, Jun 17, 2015 at 08:38:10AM -0300, Marcelo Ricardo Leitner wrote:
> >>On 17-06-2015 07:21, Neil Horman wrote:
> >>>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.
> >>
> >>It was like that before commit 2f2d76cc3e93 ("dlm: Do not allocate a fd for
> >>peeloff"), but it caused serious issues due to the fd allocation, so that's
> >>what I'm willing to avoid now.
> >>
> >>References:
> >>http://article.gmane.org/gmane.linux.network.drbd/22529
> >>https://bugzilla.redhat.com/show_bug.cgi?id=1075629 (this one is closed,
> >>sorry)
> >>
> >>   Marcelo
> >>
> >Ah, I see.  You're using the new socket option as a differentiator to just skip
> >the creation of an FD.
> 
> Exactly.
> 
> >I get your reasoning, but I'm still not in love with the idea of duplicating
> >code paths to avoid that action.  Can we use some data inside the socket
> >structure to do this differentiation?  Specifically here I'm thinking of
> >sock->file.  IIRC that will be non-null for any sockets created in user space,
> 
> I had thought about using some socket flags like netlink does but couldn't
> get around with that. Hadn't thought about sock->file though, nice idea.
> 
> >but will always be NULL for dlm created sockets (since we use sock_create
> >directly to create them.  If that is a sufficient differentiator, then we can
> >just optionally allocate the new socket fd for the peeled off socket, iff the
> >parent sock->file pointer is non-null.
> >
> >Thoughts?
> >Neil
> 
> We can re-use the current code path, by either checking it via sock->file or
> via get_fs(). That will require us to change the option arg format so we
> keep it nice and clean but as it would be kernel-side only, it should be ok
> right? It currently is:
> 
> typedef struct {
>         sctp_assoc_t associd;
>         int sd;
> } sctp_peeloff_arg_t;
> 
> And we would have to fit a pointer in there, something like:
> typedef union {
> 	struct {
> 	        sctp_assoc_t associd;
> 	        int sd;
> 	};
> 	void *sock;
> } sctp_peeloff_arg_t;
> 
> Sounds good?
> 
Yes, sounds reasonable.

Thanks!
Neil

>   Marcelo
> 
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