Subject: Re: [RFC PATCH 0/2] sctp: add new getsockopt option SCTP_SOCKOPT_PEELOFF_KERNEL

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Wed, 17 Jun 2015 08:38:10 -0300

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