On Mon, May 21, 2018 at 02:16:56PM +0200, Michael Tuexen wrote: > > On 21. May 2018, at 13:39, Neil Horman <nhorman@xxxxxxxxxxxxx> wrote: > > > > On Sun, May 20, 2018 at 10:54:04PM -0300, Marcelo Ricardo Leitner wrote: > >> On Sun, May 20, 2018 at 08:50:59PM -0400, Neil Horman wrote: > >>> On Sat, May 19, 2018 at 03:44:40PM +0800, Xin Long wrote: > >>>> This feature is actually already supported by sk->sk_reuse which can be > >>>> set by SO_REUSEADDR. But it's not working exactly as RFC6458 demands in > >>>> section 8.1.27, like: > >>>> > >>>> - This option only supports one-to-one style SCTP sockets > >>>> - This socket option must not be used after calling bind() > >>>> or sctp_bindx(). > >>>> > >>>> Besides, SCTP_REUSE_PORT sockopt should be provided for user's programs. > >>>> Otherwise, the programs with SCTP_REUSE_PORT from other systems will not > >>>> work in linux. > >>>> > >>>> This patch reuses sk->sk_reuse and works pretty much as SO_REUSEADDR, > >>>> just with some extra setup limitations that are neeeded when it is being > >>>> enabled. > >>>> > >>>> "It should be noted that the behavior of the socket-level socket option > >>>> to reuse ports and/or addresses for SCTP sockets is unspecified", so it > >>>> leaves SO_REUSEADDR as is for the compatibility. > >>>> > >>>> Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx> > >>>> --- > >>>> include/uapi/linux/sctp.h | 1 + > >>>> net/sctp/socket.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++ > >>>> 2 files changed, 49 insertions(+) > >>>> > >>> A few things: > >>> > >>> 1) I agree with Tom, this feature is a complete duplication of the SK_REUSEPORT > >>> socket option. I understand that this is an implementation of the option in the > >>> RFC, but its definately a duplication of a feature, which makes several things > >>> really messy. > >>> > >>> 2) The overloading of the sk_reuse opeion is a bad idea, for several reasons. > >>> Chief among them is the behavioral interference between this patch and the > >>> SO_REUSEADDR socket level option, that also sets this feature. If you set > >>> sk_reuse via SO_REUSEADDR, you will set the SCTP port reuse feature regardless > >>> of the bind or 1:1/1:m state of the socket. Vice versa, if you set this socket > >>> option via the SCTP_PORT_REUSE option you will inadvertently turn on address > >>> reuse for the socket. We can't do that. > >> > >> Given your comments, going a bit further here, one other big > >> implication is that a port would never be able to be considered to > >> fully meet SCTP standards regarding reuse because a rogue application > >> may always abuse of the socket level opt to gain access to the port. > >> > >> IOW, the patch allows the application to use such restrictions against > >> itself and nothing else, which undermines the patch idea. > >> > > Agreed. > > > >> I lack the knowledge on why the SCTP option was proposed in the RFC. I > >> guess they had a good reason to add the restriction on 1:1/1:m style. > >> Does the usage of the current imply in any risk to SCTP sockets? If > >> yes, that would give some grounds for going forward with the SCTP > >> option. > >> > > I'm also not privy to why the sctp option was proposed, though I expect that the > > lack of standardization of SO_REUSEPORT probably had something to do with it. > > As for the reasoning behind restriction to only 1:1 sockets, if I had to guess, > > I would say it likely because it creates ordering difficulty at the application > > level. > > > > CC-ing Michael Tuxen, who I believe had some input on this RFC. Hopefully he > > can shed some light on this. > Dear all, > > the reason this was added is to have a specified way to allow a system to > behave like a client and server making use of the INIT collision. > > For 1-to-many style sockets you can do this by creating a socket, binding it, > calling listen on it and trying to connect to the peer. > > For 1-to-1 style sockets you need two sockets for it. One listener and one > you use to connect (and close it in case of failure, open a new one...). > > It was not clear if one can achieve this with SO_REUSEPORT and/or SO_REUSEADDR > on all platforms. We left that unspecified. > > I hope this makes the intention clearer. > I think it makes the intention clearer yes, but it unfortunately does nothing in my mind to clarify how the implementation should best handle the potential overlap in functionality. What I see here is that we have two functional paths (the SO_REUSEPORT path and the SCTP_PORT_REUSE path), which may or may not (depending on the OS implementation achieve the same functional goal (allowing multiple sockets to share a port while allowing one socket to listen and the other connect to a remote peer). If both implementations do the same thing on a given platform, we can either just alias one to another and be done, but if they don't then we either have to implement both paths, and ensure that the SO_REUSEPORT path is a no-op/error return for SCTP sockets, or that each path implements a distinct feature set that is cleaarly documented. That said, I think we may be in luck. Looking at the connect and listen paths, it appears to me that: 1) Sockets ignore SO_REUSEPORT in the connect and listen paths (save for any autobinding) so it would appear that the intent of the SCTP rfc can be honored via SO_REUSEPORT on linux. 2) SO_REUSEPORT prevents changing state after a bind has occured, so we can honr that part of the SCTP RFC. The only missing part is the restriction that SCTP_REUSE_PORT has which is unaccounted for is that 1:M sockets aren't allowed to enable port reuse. However, I think the implication from Michaels description above is that port reuse on a 1:M socket is implicit because a single socket can connect and listen in that use case, rather than there being a danger to doing so. As such, I would propose that we implement this socket option by simply setting the sk->sk_reuseport field in the sock structure, and document the fact that linux does not restrict port reuse from 1:M sockets. Thoughts? Neil -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
