linux-kernel - Re: [PATCH v2] x86: Fix x32 System V message queue syscalls

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Date:   Tue, 1 Aug 2023 02:38:47 +0100
From:   Jessica Clarke <jrtc27@...c27.com>
To:     Harald van Dijk <harald@...awatt.nl>
Cc:     Andy Lutomirski <luto@...nel.org>, Rich Felker <dalias@...c.org>,
        linux-x86_64@...r.kernel.org, Florian Weimer <fweimer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86: Fix x32 System V message queue syscalls

On 1 Aug 2023, at 01:43, Harald van Dijk <harald@...awatt.nl> wrote:
> 
> On 06/12/2020 22:55, Andy Lutomirski wrote:
>> On Sat, Dec 5, 2020 at 4:01 PM Jessica Clarke <jrtc27@...c27.com> wrote:
>>> 
>>> Ping?
>> Can you submit patches implementing my proposal?  One is your existing
>> patch plus fixing struct msghdr, with Cc: stable@...r.kernel.org at
>> the bottom.  The second is a removal of struct msghdr from uapi,
>> moving it into include/inux (no uapi) if needed.  The second should
>> not cc stable.
> 
> Hi,
> 
> This looks like it was forgotten, but it is still needed. Jessica, are you interested in submitting the requested change? If not, would it be okay if I do so? I have been running this locally for a long time now.

Hi,
Please feel free to; sorry that it dropped off my radar. Part of the
issue is my laptop no longer being x86, making it more annoying to test.

> There is one complication that I think has not been mentioned yet: when _GNU_SOURCE is defined, glibc does provide a definition of struct msghdr in <sys/msg.h> with a field "__syscall_slong_t mtype;". This makes it slightly more likely that there is code out there in the wild that works fine with current kernels and would be broken by the fix. Given how rare x32 is, and how rare message queues are, this may still be acceptable, but I am mentioning it just in case this would cause a different approach to be preferred. And whatever is done, a fix should also be submitted to glibc.

Given POSIX is very clear on how msghdr works I think we have to break
whatever oddball code out there might be using this. The alternative is
violating POSIX in a way that makes correct code compile fine but fail
at run time on x32, which is a terrible place to be, especially when
the “fix” is to special-case x32 to go against what POSIX says. I just
can’t see how that’s a good place to stay in, even if something might
break when we fix this bug.

Thanks,
Jess

> (musl define struct msghdr as well, but defines mtype unconditionally as having type long, so if this approach is still preferred, needs no changes.)
> 
> Cheers,
> Harald van Dijk