re: Problem with syscall_disestablish() - PR kern/50430

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

On Tue, 17 Nov 2015, matthew green wrote:

> there's a fairly significant problem with this implementation.
>
> you're only catching callers who use end up going through sy_call()
> and that's not the majority.  mostly they're called directly from
> MD code.  so to fix that, you have to update every platform syscall
> handler.

Ah, I see.  I'll have to investigate further.

> it also slows down normal operation adding branches and data access
> to the call path of every system call.  and bloats struct lwp and
> more.
>
> i don't think it's worth paying that for what feature that isn't
> really useful to anyone.  (sure, it's useful for you when you're
> modifying the calls perhaps, but that comes down to the case of
> don't shoot yourself in the foot and unload something you're
> still using/testing.)

This is actually more oriented to modules that are going to get auto-
unloaded.  For example, we can (attempt to) load a large list of
compat modules in the hope of getting one that can handle an unknown
emulation/execsw.  It's really nice for (most of) those to just "go
away" if they're not really used.  But you need to be able to make
that determination.

> however, it looks like this *should* already be handled by
> platforms that do use sy_call() directly, and could be fixed on
> everywhere else by simply setting l_sysent the same way.  that's
> what the setting of l_sysent does -- "this is the system call i'm
> currently actively running" -- and syscall_disestablish() checks
> this correctly already.

The problem I'm attempting to address is when syscall #1 is blocked,
some signal gets delivered to the lwp, and the signal handler calls some
other syscall #2.  In that case, the single l_sysent gets overwritten
and we now have no record that syscall #1 is still active.


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