Subject: Re: [PATCH] x86/entry/x32: Check top 32 bits of syscall number on the
 fast path
To: Ben Hutchings <ben@decadent.org.uk>, Andy Lutomirski <luto@kernel.org>
References: <1460940317.9121.56.camel@decadent.org.uk>
 <20160418004731.GB3348@decadent.org.uk>
Cc: X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <5714679B.3040806@zytor.com>
Date: Sun, 17 Apr 2016 21:50:35 -0700
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On 04/17/16 17:47, Ben Hutchings wrote:
> We've always masked off the top 32 bits when x32 is enabled, but
> hopefully no-one relies on that.  Now that the slow path is in C, we
> check all the bits there, regardless of whether x32 is enabled.  Let's
> make the fast path consistent with it.

We have always masked off the top 32 bits *period*.

We have had some bugs where we haven't, because someone has tried to
"optimize" the code and they have been quite serious.  The system call
number is an int, which means the upper 32 bits are undefined on call
entry: we HAVE to mask them.

	-hpa