linux-kernel - Re: [RFC v2 PATCH 7/7] x86/entry: use int for syscall number; handle all invalid syscall nrs

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Message-ID: <57bf2ff9-11ff-0907-975e-15177df72144@linux.luto.us>
Date:   Thu, 13 May 2021 20:18:37 -0700
From:   Andy Lutomirski <andy@...ux.luto.us>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 PATCH 7/7] x86/entry: use int for syscall number; handle
 all invalid syscall nrs

On 5/13/21 5:38 PM, H. Peter Anvin wrote:
> On 5/12/21 3:22 PM, Thomas Gleixner wrote:
>>>
>>> As far as this being a user ABI change, this is actually a revert to the
>>> original x86-64 ABI; see my message to Ingo.
>>
>> I'm not against that change, but it has to be well justified and the
>> reasoning wants to be in the changelog. You know the drill :)
>>
> 
> FYI:
> 
> So in the process of breaking up and better document this patch, I have
> looked at the syscall_numbering_64 (and have rewritten it to be more
> complete.)
> 
> I found that running it under strace fails, as strace (possibly ptrace,
> possibly the strace binary) causes %rax = 2^32 to be clobbered to zero
> already...
> 
> More motivation, I guess.
> 

Indeed.

I would love to go back in time and switch to long, but there are plenty
of things that use int now.  I suppose we could try to make it long for
real, but seccomp has u32 baked into its ABI.