linux-kernel - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Date:   Fri, 26 Jan 2018 07:57:35 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Alan Cox <alan@...ux.intel.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Fri, Jan 26, 2018 at 6:24 AM, Alan Cox <alan@...ux.intel.com> wrote:
>> NetBSD (and the other BSD?) defines a structure for the arguments to
>> each syscall.
>
> Goes back to v7 or so but they put the syscall arguments into the uarea
> so that no pointers were needed (uarea being a per process mapping at a
> fixed address) in order to also reduce pointer dereferencing costs (not
> that those matter much on modern processors)
>

I gave the rearrangement like this a try yesterday and it's a bit of a
mess.  Part of the problem is that there are a bunch of pieces of code
that expect sys_xyz() to be actual callable functions.  The best way
to deal with that is probably to switch to calling normal functions.