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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 6 Feb 2018 12:42:24 -0800
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Subject: Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read
 under speculation
To: Dan Williams <dan.j.williams@intel.com>
Cc: Luis Henriques <lhenriques@suse.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>, Greg KH <gregkh@linuxfoundation.org>,
	X86 ML <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>, Alan Cox <alan@linux.intel.com>
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On Tue, Feb 6, 2018 at 12:37 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>
> Are there any compilers that would miscompile:
>
>     mask = 0 - (index < size);
>
> That might be a way to improve the assembly.

Sadly, that is *very* easy to miscompile. In fact, I'd be very
surprised indeed if any compiler worth its name wouldn't combine the
comparison with the conditional branch it accompanies, and just turn
that into a constant. IOW, you'd get

        mask = 0 - (index < size);
        if (index <= size) {
                 ... use mask ..

and the compiler would just turn that into

        if (index <= size) {
                mask = -1;

and be done with it.

               Linus