Re: [PATCH net] sctp: make sctp_setsockopt_events() less strict about the option length

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

On Sat, Feb 09, 2019 at 03:12:17PM -0800, David Miller wrote:
> From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Date: Wed, 6 Feb 2019 18:37:54 -0200
> 
> > On Wed, Feb 06, 2019 at 12:14:30PM -0800, Julien Gomes wrote:
> >> Make sctp_setsockopt_events() able to accept sctp_event_subscribe
> >> structures longer than the current definitions.
> >> 
> >> This should prevent unjustified setsockopt() failures due to struct
> >> sctp_event_subscribe extensions (as in 4.11 and 4.12) when using
> >> binaries that should be compatible, but were built with later kernel
> >> uapi headers.
> > 
> > Not sure if we support backwards compatibility like this?
> 
> What a complete mess we have here.
> 
> Use new socket option numbers next time, do not change the size and/or
> layout of existing socket options.

What about reusing the same socket option, but defining a new struct?
Say, MYSOCKOPT supports struct mysockopt, struct mysockopt2, struct
mysockopt3...

That way we have a clear definition of the user's intent.

> 
> This whole thread, if you read it, is basically "if we compatability
> this way, that breaks, and if we do compatability this other way oh
> shit this other thing doesn't work."
> 
> I think we really need to specifically check for the difference sizes
> that existed one by one, clear out the part not given by the user, and
> backport this as far back as possible in a way that in the older kernels
> we see if the user is actually trying to use the new features and if so
> error out.

I'm afraid clearing out may not be enough, though seems it's the best
we can do so far. If the struct is allocated but not fully initialized
via a memset, but by setting its fields one by one, the remaining new
fields will be left uninitinialized.

> 
> Which, btw, is terrible behavior.  Newly compiled apps should work on
> older kernels if they don't try to use the new features, and if they

One use case here is: a given distro is using kernel X and app Foo is
built against it. Then upgrades to X+1, Foo is patched to fix an issue
and is rebuilt against X+1. The user upgrades Foo package but for
whatever reason, doesn't upgrade kernel or reboot the system. Here,
Foo doesn't work anymore until the new kernel is also running.

> can the ones that want to try to use the new features should be able
> to fall back when that feature isn't available in a non-ambiguous
> and precisely defined way.
> 
> The fact that the use of the new feature is hidden in the new
> structure elements is really rotten.
> 
> This patch, at best, needs some work and definitely a longer and more
> detailed commit message.