[PATCH v2 0/2] Avoid link dependency of dlm on sctp module

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

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 this call
only 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.

"Kernel users" are identified by sockets that don't have a file
descriptor attached to it, as only kernel users are allowed to do that
and, if original socket has a file descriptor, one should just use the
original option.

I kept __user marker on sctp_getsockopt_peeloff_kernel() prototype and
its helpers just to avoid issues with static checkers.

I tried minimizing the code duplication while adding the new option, but
it's just better to add a whole new function to handle it. As the
sockopt arguments are different, the only thing in common between both
call handlers is the call to sctp_do_peeloff(). All rest would have to
have checks around it.

RFC->v1, from Neil:
 - For identifying kernel users, switched from segment_eq() to fd check
 - Reused original option to avoid code duplication
v1->v2, from David:
 - Back to new sockopt implementation, to avoid changing sockopt arg
   format.


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         | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 9 deletions(-)

-- 
2.4.1