linux-kernel - Re: [PATCH v7 3/7] kernel: Implement selective syscall userspace redirection

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Message-ID: <87a6vdmedy.fsf@collabora.com>
Date:   Thu, 19 Nov 2020 12:43:05 -0500
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     luto@...nel.org, Rich Felker <dalias@...c.org>
Cc:     tglx@...utronix.de, keescook@...omium.org,
        christian.brauner@...ntu.com, peterz@...radead.org,
        willy@...radead.org, shuah@...nel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kselftest@...r.kernel.org, x86@...nel.org, gofmanp@...il.com,
        kernel@...labora.com
Subject: Re: [PATCH v7 3/7] kernel: Implement selective syscall userspace
 redirection

Gabriel Krisman Bertazi <krisman@...labora.com> writes:

> Introduce a mechanism to quickly disable/enable syscall handling for a
> specific process and redirect to userspace via SIGSYS.  This is useful
> for processes with parts that require syscall redirection and parts that
> don't, but who need to perform this boundary crossing really fast,
> without paying the cost of a system call to reconfigure syscall handling
> on each boundary transition.  This is particularly important for Windows
> games running over Wine.

I raised a discussion about this on libc-alpha, as requested by Florian.
At the moment, there was some back and forth on why the use-case is not
done by seccomp, but a more interesting point about user_notif was
raised by Rich Felker (cc'ed).

SIGSYS, as a signal handler, is limited in what can be done inside it.
Rich suggested the user_notif design is a better solution.  I understand
that from a Wine perspective, SIGSYS suffices for their work, but would
it make sense to extend SUD interface to support a user_notif-like
interface?  Would this be acceptable as future work to be added when/if
needed, or should we design it from the start?

The existing interface could be extended with a flags field as part of
the opcode passed in argument 2, which is currently reserved, and then
return a FD, just like seccomp(2) does.  So it is not like the current
patches couldn't be extended in the future if needed, unless I'm
mistaken.

-- 
Gabriel Krisman Bertazi