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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 6 Feb 2018 14:52:59 -0800
Message-ID: <CA+55aFxU_vo+JFBZcvE5EOs2D_QHNXBf4G=-KT=3LeP2zjECHw@mail.gmail.com>
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:     Andy Lutomirski <luto@kernel.org>,
        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>,
        "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 1:37 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>
> At that point we're basically just back to the array_ptr() version
> that returned a sanitized pointer to an array element.

.. that one does an extra unnecessary 'andq' instead of the duplicated
cmp.  But at least it avoids comparing that 32-bit integer twice, so
it's probably slightly smaller.

(And your code generation is without the "r" -> "ir" fix for the size argument)

Probably doesn't matter. But a "asm goto" would give you at least
potentially optimal code.

            Linus