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

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Date:   Wed, 8 Apr 2020 22:00:27 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thorsten Glaser <t.glaser@...ent.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, 954294@...s.debian.org
Subject: Re: __X32_SYSCALL_BIT being defined as UL constant breaks userspace

On Wed, Apr 8, 2020 at 7:34 AM Thorsten Glaser <t.glaser@...ent.de> wrote:
>
> Hello,
>
> I’m writing to you because your name shows up on this:
>
> commit 45e29d119e9923ff14dfb840e3482bef1667bbfb
> Author: Andy Lutomirski <luto@...nel.org>
> Date:   Wed Jul 3 13:34:05 2019 -0700
>
>     x86/syscalls: Make __X32_SYSCALL_BIT be unsigned long
>
>     Currently, it's an int.  This is bizarre.  Fortunately, the code using it
>     still works: ~__X32_SYSCALL_BIT is also int, so, if nr is unsigned long,
>     then C kindly sign-extends the ~__X32_SYSCALL_BIT part, and it actually
>     results in the desired value.
>
>     This is far more subtle than it deserves to be.  Syscall numbers are, for
>     all practical purposes, unsigned long, so make __X32_SYSCALL_BIT be
>     unsigned long.
>
>     Signed-off-by: Andy Lutomirski <luto@...nel.org>
>     Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>     Link: https://lkml.kernel.org/r/99b0d83ad891c67105470a1a6b63243fd63a5061.1562185330.git.luto@kernel.org
>
> This commit changed an uapi header, breaking userspace. Long debugging
> story (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=954294 if you
> are interested) short, it goes like this:
>
> libseccomp exposes an interface SCMP_SYS() which is designed to expand
> to an int and be usable in cpp context. It redirects to the __NR_*
> constants from <asm/unistd.h>.
>
> Example: SCMP_SYS(mmap) becomes __NR_mmap or __NR_mmap2 (depending on
> the architecture).
>
> Now, most architectures define __NR_mmap as a mere integer number:
>
> asm/unistd_32.h:#define __NR_mmap 90
> asm/unistd_64.h:#define __NR_mmap 9
>
> x32 differs:
>
> 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.)

> > Syscall numbers are, for
> > all practical purposes, unsigned long
>
> Yes, except for the one purpose of the C data type of the
> syscall constants exposed to userspace, they are.
>
> Feel free to handle __X32_SYSCALL_BIT differently in the
> kernel (although even there it *will* introduce subtle
> differences from other architectures), but please keep it
> as int as visible from userspace.
>
> Thanks in advance,
> //mirabilos
>
> PS: Please keep both me *and* the Debian bug in Cc, but
>     feel free to forward to relevant lists and persons;
>     I’m unsure where exactly to write to about this.
>
> @bwh: commit 45e29d119e9923ff14dfb840e3482bef1667bbfb is
>     literally just this…
>         -#define __X32_SYSCALL_BIT      0x40000000
>         +#define __X32_SYSCALL_BIT      0x40000000UL
>     … so can you please revert it in Debian in the meantime,
>     even if you said you won’t spend time on this?
> --
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