linux-kernel - Re: Cleaning up numbering for new x86 syscalls?

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Date:   Tue, 20 Nov 2018 09:48:52 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Colascione <dancol@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Rich Felker <dalias@...c.org>
Subject: Re: Cleaning up numbering for new x86 syscalls?

On Mon, Nov 19, 2018 at 04:22:49PM -0800, Andy Lutomirski wrote:
> Hi all-
> 
> We currently have some giant turds in the way that syscalls are
> numbered.  We have the x86_32 table, which is totally sane other than
> some legacy multiplexers.  Then we have the x86_64 table, which is,
> um, demented:
> 
>  - The numbers don't match x86_32.  I have no idea why.
> 
>  - We use bit 30, which triggers in_x32_syscall().  It should have
> been bit 31, bit I digress.
> 
>  - We have this weird set of extra x32 syscalls that start at 512.
> Who wants to bet whether we have no bugs if someone does syscall with,
> say, nr == 512 (i.e. not 512 | BIT(30)) or nr == (16 | BIT(30))?  The
> latter would be non-compat ioctl with in_x32_syscall() set and hence
> in_compat_syscall() set.
> 
>  - Bloody restart_syscall() has a different number on x86_64 and
> x64_32, which is a big mess.
> 
> I propose we consider some subset of the following:
> 
> 1. Introduce restart_syscall_2().  Make its number be 1024.  Maybe
> someday we could start using it instead of restart_syscall().  The
> only issue I can see is programs that allow restart_syscall() using
> seccomp but don't allow the new variant.
>
> 2. Introduce an outright ban on new syscalls with nr < 1024.
> 
> 3. Introduce an outright ban on the addition of new __x32_compat
> syscalls.  If new compat hacks are needed, they can use
> in_compat_syscall(), thank you very much.
> 
> 4. Modify the wrappers of the __x32_compat entries so that they will
> return -ENOSYS if in_x32_syscall() returns false.

This sounds like a great idea independent of all of this.

> 5. Adjust the scripts so that we only have to wire up new syscalls
> once.  They'll have a nr above 1024, and they'll have the same nr on
> all x86 variants.
> 
> Thoughts?

+1. Who wants to do it? :D

Tycho