Re: io_uring vs in_compat_syscall()

On Tue, Jul 21, 2020 at 10:25:24AM -0700, Andy Lutomirski wrote:

On Tue, Jul 21, 2020 at 7:34 AM Christoph Hellwig [off-list ref] wrote:

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On Tue, Jul 21, 2020 at 07:31:02AM -0700, Andy Lutomirski wrote:

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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.

I was imagining using a flag.  Some of the net code uses
MSG_CMSG_COMPAT for this purpose.

Killing that nightmarish monster is what actually got me into looking
io_uring and starting this thread.

I agree that MSG_CMSG_COMPAT is nasty, but I think the concept is
sound -- rather than tracking whether we're compat by using a
different function or a per-thread variable, actually explicitly
tracking the mode seems sensible.

I very strongly disagree.  Two recent projects I did was to remove
the compat_exec mess, and the compat get/setsockopt mess, and each
time it removed hundreds of lines of code duplicating native
functionality, often in slightly broken ways.  We need a generic
out of band way to transfer the information down and just check in
in a few strategic places, and in_compat_syscall() does the right
thing for that.

If we're going to play in_compat_syscall() games, let's please make
io_uring_enter() return -EINVAL if in_compat_syscall() != ctx->compat.

That sounds like a plan, but still doesn't help with submissions from
the offload WQ or the sqpoll thread.