linux-kernel - Re: __X32_SYSCALL_BIT being defined as UL constant breaks userspace

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Message-ID: <alpine.DEB.2.22.394.2004090945110.31491@tglase-nb.lan.tarent.de>
Date:   Thu, 9 Apr 2020 09:47:51 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, 954294@...s.debian.org
Subject: Re: __X32_SYSCALL_BIT being defined as UL constant breaks
 userspace

On Wed, 8 Apr 2020, Andy Lutomirski wrote:

> One might reasonably ask whether it makes sense for syscall nrs to be
> signed at all.

It doesn’t, but it’s probably this way for hysteric raisins.

> But regardless, this breaks userspace and we should fix it.  I can
> whip up a patch to split it into X32_SYSCALL_BIT (unsigned long) and
> __X32_SYSCALL_BIT (uapi, int).  Thomas, etc, does this seem

This would help with the issue, thanks.

> reasonable?  (For those not following all the machinations, this
> change caused some userspace build failures in libseccomp and/or
> systemd for reasons that are vaguely silly.)

Not very silly:

	printf("%d\n", __NR_mmap);

That with -Wformat and -Werror (or -Werror=format) rightfully warns,
as the compiler cannot, on x32 (where int=long), detect that, on
architectures where int≠long the constant is int.

Even worse, switching userspace to unsigned long globally would
completely hose this on LP64 architectures.

So this is, in my opinion, completely justified. (Disclaimer: I’m
not affiliated with systemd and not even running it except udev.)

bye,
//mirabilos
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