linux-kernel - Re: io_uring vs in_compat

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Message-ID: <20200721070709.GB11432@lst.de>
Date:   Tue, 21 Jul 2020 09:07:09 +0200
From:   Christoph Hellwig <hch@....de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: io_uring vs in_compat_syscall()

On Mon, Jul 20, 2020 at 10:28:55AM -0700, Andy Lutomirski wrote:
> > Sure, I'd consider that implementation detail for the actual patch(es)
> > for this issue.
> 
> There’s a corner case, though: doesn’t io_uring submission frequently do the work synchronously in the context of the calling thread?

Yes.

> If so, can a thread do a 64-bit submit with 32-bit work or vice versa?

In theory you could share an fd created in a 32-bit thread to a 64-bit
thread or vice versa, but I think at that point you absolutely are in
"you get to keep the pieces" land.

> Sometimes I think that in_compat_syscall() should have a mode in which calling it warns (e.g. not actually in a syscall when doing things in io_uring).  And the relevant operations should be properly wired up to avoid global state like this.

What do you mean with "properly wired up".  Do you really want to spread
->compat_foo methods everywhere, including read and write?  I found
in_compat_syscall() a lot small and easier to maintain than all the
separate compat cruft.