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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 6 Feb 2018 12:58:27 -0800
Message-ID: <CA+55aFy2-OMMCatHTXL7wfP7H=4CEKv4Y_pRvfeEOCwxriDP4Q@mail.gmail.com>
Subject: Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read
 under speculation
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        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 12:49 PM, Andy Lutomirski <luto@kernel.org> wrote:
>
> Can you use @cc to make an asm statement that outputs both the masked
> array index and the "if" condition?  I can never remember the syntax,
> but something like:

Yes. Although I'd actually suggest just using an "asm goto" if we
really want to optimize this. Give the "index_mask_nospec()" a third
argument that is the label to jump to for overflow.

Then you can just decide how to implement it best for any particular
architecture (and compiler limitation).

              Linus