Testing Emulation Syscalls

[Theodore Preduta <theo%pta.gg@localhost>]

This comes somewhat as a "part 2" to
https://mail-index.netbsd.org/tech-kern/2023/06/21/msg028926.html

Given the responses to that thread, I decided to add native stubs
for epoll (the fact epoll is widespread alone justifies it, but it has
already had some negative side effects, see:
https://mail-index.netbsd.org/source-changes-d/2023/07/30/msg013999.html).

However, there are other syscalls, namely inotify, where it can't really
be justified, but the code still deserves tests.  As such I'm looking
for a way to test emulation syscalls with ATF.

One idea (mentioned in the original thread) would be to introduce a
syscall along the lines of

	int emul_syscall(const char *emul_name, int number, ...)

which executes a single syscall.  The flaw with this idea is that state
may need to be stored across syscalls in struct linux_emuldata, but I
don't know how this interface could accommodate this.

Another idea would be to introduce a syscall along the lines of

	int setemul(const char *emul_name)

which would switch the syscall table dynamically so that the test case
could be run under emulation (preserving emuldata state) and then switch
back to report the result.  (And then individual syscalls would be
called via __syscall(2).)

Both of these ideas have security security implications.  So they should
be limited to root (or perhaps a new kauth capability).

A third idea would be to figure out a way to compile ATF tests directly
as Linux binaries.  That way no new syscall would be needed, but this
will likely cause pain when trying to distribute the tests.

Once again... thoughts?

Theo(dore)