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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 25 Jan 2018 13:02:25 -0800
Message-ID: <CALCETrVUY9p9jMNhuBgM3mWATsr1Fc_ZWc=bjidBkLw7bq5Vdg@mail.gmail.com>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Alan Cox <alan@linux.intel.com>, Jann Horn <jannh@google.com>,
        Samuel Neves <samuel.c.p.neves@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Borislav Petkov <bp@alien8.de>
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On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Jan 22, 2018 at 10:55 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> Honestly, I'd rather get rid of the fast-path entirely. Compared to
>> all the PTI mess, it's not even noticeable.
>
> So I looked at how that would be.
>
> Patch attached. Not really "tested", but I'm running the kernel with
> this patch now, and 'strace' etc works, and honestly, it seems very
> obvious.
>
> Also, code generation for 'do_syscall_64()' does not look horrible. In
> fact, it doesn't look all that much worse than the fast-path ever did.
>
> So the biggest impact of this is the extra register saves
> (SAVE_EXTRA_REGS) from setting up the full ptregs. And honestly, I
> hate how that stupid macro still uses "movq reg,off(%rsp)" instead of
> "pushq %reg".
>
> Considering the diffstat:
>
>  2 files changed, 2 insertions(+), 121 deletions(-)
>
> and how those 100+ lines are nasty assembly code, I do think we should
> just do it.

Feel free to Acked-by: Andy Lutomirski <luto@kernel.org> that patch.

Or I can grab it and send it to -tip.

Re: the trampoline not showing up: if I find some time, I'll try to
wire it up correctly in kallsyms.

--Andy