linux-kernel - Re: System call number masking

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Date:	Mon, 18 Apr 2016 01:45:17 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: System call number masking

On Thu, 2016-04-14 at 10:48 -0700, Andy Lutomirski wrote:
> On Thu, Apr 14, 2016 at 10:22 AM, Ben Hutchings <ben@...adent.org.uk> wrote:
> > 
> > I'm updating my x32-as-boot-time-option patch for 4.6, and I noticed a
> > subtle change in system call number masking on x86_64 as a result of
> > moving the slow path into C.
> > 
> > Previously we would mask out the upper 32 bits before doing anything
> > with the system call number, both on the slow and fast paths, if and
> > only if x32 was enabled.
> I always thought that the old behavior was nonsensical.  The behavior
> should be the same regardless of config options.
[...]

Oops, my C is failing me - ints are sign-extended, not zero-extended,
when promoted to unsigned long.  So the slow path actually does test
the upper 32 bits, and the odd one out is the x32 fast path.

Ben.

-- 
Ben Hutchings
Always try to do things in chronological order;
it's less confusing that way.
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