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

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Message-ID: <87wo6pp0ca.fsf@nanos.tec.linutronix.de>
Date:   Thu, 09 Apr 2020 09:47:17 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Thorsten Glaser <t.glaser@...ent.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, 954294@...s.debian.org
Subject: Re: __X32_SYSCALL_BIT being defined as UL constant breaks userspace

Andy Lutomirski <luto@...nel.org> writes:
> On Wed, Apr 8, 2020 at 7:34 AM Thorsten Glaser <t.glaser@...ent.de> wrote:
>> asm/unistd_x32.h:#define __NR_mmap (__X32_SYSCALL_BIT + 9)
>>
>> This construct is, thankfully, still usable in something like
>>         #if (__NR_mmap > __NR_somethingelse)
>> but as __X32_SYSCALL_BIT is no longer int its type also isn’t.
>>
>> Therefore I ask you to revert this change, bringing x32 closer
>> to all other architectures.
>>
>
> One might reasonably ask whether it makes sense for syscall nrs to be
> signed at all.
>
> 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
> 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.)

Yes.