On Sun, Feb 10, 2019 at 10:46:16AM -0200, Marcelo Ricardo Leitner wrote:

On Sat, Feb 09, 2019 at 03:12:17PM -0800, David Miller wrote:

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From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Date: Wed, 6 Feb 2019 18:37:54 -0200

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On Wed, Feb 06, 2019 at 12:14:30PM -0800, Julien Gomes wrote:

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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.

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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.

Need to clarify the "clearing out", I think it was meant differently.
It was more about on how to ensure that the 16-bytes long of the v3
supplied to a v1-only kernel is compatible with the 12-bytes long v1.
The kernel would have to check the trailing 4 bytes after v1-size and
make sure they are all zeroed in order for the old kernel to accept it
as a v1. But, as I said above, there are situations that this will not
be enough.

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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.

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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.

We have issues on read path too. 52ccb8e90c0a ("[SCTP]: Update
SCTP_PEER_ADDR_PARAMS socket option to the latest api draft.")
extended struct sctp_paddrparams and its getsockopt goes with:

sctp_getsockopt_peer_addr_params()
...
        if (len < sizeof(struct sctp_paddrparams))
                return -EINVAL;
        len = sizeof(struct sctp_paddrparams);

By then, we didn't have the /uapi/ folder yet. There may other cases.