Date:   Tue, 10 Dec 2019 09:31:11 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux Network Development Mailing List 
        <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Sean Tranchetti <stranche@...eaurora.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux SCTP <linux-sctp@...r.kernel.org>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
Subject: Re: [PATCH v2] net: introduce ip_local_unbindable_ports sysctl

On Tue, 10 Dec 2019 12:46:29 +0100, Maciej Żenczykowski wrote:
> > Okay, that's what I was suspecting.  It'd be great if the real
> > motivation for a patch was spelled out in the commit message :/  
> 
> It is, but the commit message is already extremely long.

Long, yet it doesn't mention the _real_ reason for the patch.

> At some point essays and discussions belong in email and not in the
> commit message.

Ugh just admit you didn't mention the primary use case in the commit
log, and we can move on.

> Here's another use case:
> 
> A network where firewall policy or network behaviour blocks all
> traffic using specific ports.
> 
> I've seen generic firewalls that unconditionally drop all BGP or SMTP
> port traffic, or all traffic on ports 5060/5061 (regardless of
> direction) or on 25/53/80/123/443/853/3128/8000/8080/8088/8888
> (usually due to some ill guided security policies against sip or open
> proxies or xxx). If you happen to use port XXXX as your source port
> your connection just hangs (packets are blackholed).
> 
> Sure you can argue the network is broken, but in the real world you
> often can't fix it... Go try and convince your ISP that they should
> only drop inbound connections to port 8000, but not outgoing
> connections from port 8000 - you'll go crazy before you find someone
> who even understands what you're talking about - and even if you find
> such a person, they'll probably be too busy to change things - and
> even though it might be a 1 letter change (port -> dport) - it still
> might take months of testing and rollout before it's fully deployed.
> 
> I've seen networks where specific ports are automatically classified
> as super high priority (network control) so you don't want anything
> using these ports without very good reason (common for BGP for
> example, or for encap schemes).
> 
> Or a specific port number being reserved by GUE or other udp encap
> schemes and thus unsafe to use for generic traffic (because the
> network or even the kernel itself might for example auto decapsulate
> it [via tc ebpf for example], or parse the interior of the packet for
> flowhashing purposes...).
> 
> [I'll take this opportunity to point out that due to poor flow hashing
> behaviour GRE is basically unusable at scale (not to mention poorly
> extensible), and thus GUE and other UDP encap schemes are taking over]
> 
> Or you might want to forward udp port 4500 from your external IP to a
> dedicated ipsec box or some hardware offload engine... etc.

It's networking you can concoct a scenario to justify anything.

> > So some SoCs which run non-vanilla kernels require hacks to steal
> > ports from the networking stack for use by proprietary firmware.
> > I don't see how merging this patch benefits the community.  
> 
> I think you're failing to account for the fact that the majority of
> Linux users are Android users - there's around 2.5 billion Android
> phones in the wild... - but perhaps you don't consider your users (or
> Android?) to be part of your community?

I don't consider users of non-vanilla kernels to necessarily be a
reason to merge patches upstream, no. They carry literally millions 
of lines of patches out of tree, let them carry this patch, too.
If I can't boot a vanilla kernel on those devices, and clearly there is
no intent by the device manufacturers for me to ever will, why would I
care? Some companies care about upstream, and those should be rewarded
by us taking some of the maintenance off their hands. Some don't:
https://www.youtube.com/watch?v=_36yNWw_07g (link to Linus+nVidia video) 
even tho they sell majority of SoCs for 2.5 billion devices.

> btw. Chrome OS is also Linux based (and if a quick google search is to
> be believed, about 1/7th of the linux desktop/laptop share), but since
> it supports running Android apps, it needs to have all Android
> specific generic kernel changes...
> 
> The reason Android runs non-vanilla kernels is *because* patches like
> this - that make Linux work in the real world - are missing from
> vanilla Linux
> (I can think of a few other networking patches off the top of my head
> where we've been unable to upstream them for no particularly good
> reason).

The way to get those patches upstream is to have a honest discussion
about the use case so people can validate the design. Not by sending
a patch with a 5 page commit message which fails to clearly state the
motivation for the feature :/

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