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

Gabriel Krisman Bertazi
2020-11-19T17:43:32Z

Gabriel Krisman Bertazi <krisman@collabora.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