On Thu, Dec 19, 2019 at 10:17 PM Neil Horman [off-list ref] wrote:

quoted

As I understand it, those utilities keep the ports reserved by binding
to them so that no other process can. This doesn't work for Android
because there are conformance tests that probe the device from the
network and check that there are no open ports.

But you can address that with some augmentation to portreserve (i.e. just have
it add an iptables rule to drop frames on that port, or respond with a port
unreachable icmp message)

There are also tests that run on device by inspecting
/proc/net/{tcp,udp} to check that there are no open sockets. We'd have
to change them as well.

But sure. It's not impossible to do this in userspace. We wouldn't use
portreserve itself because the work to package it and make it work on
Android (which has no /etc/services file), would likely be greater
than just adding the code to an existing Android daemon (and because
the reaction of the portreserve maintainers might be similar to yours:
"you don't need to add code to portreserve for this, just use a script
that shells out to iptables").

But in any case, the result would be more complicated to use and
maintain, and it would likely also be less realistic, such that a
sophisticated conformance test might still find that the port was
actually bound. Other users of the kernel wouldn't get to use this
sysctl, and the userspace code can't be easily reused in other
open-source projects, so the community gets nothing useful. That
doesn't seem great.

Or, we could take this patch and maintain it in the Android kernel
tree. Android kernels get a tiny bit further from mainline. Other uses
of the kernel wouldn't get to use this sysctl, and again the community
gets nothing useful. That doesn't seem great either.