linux-kernel - Re: [PATCH v3 03/13] x86/retpoline/entry: Convert entry assembler indirect jumps

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Date:   Thu, 4 Jan 2018 14:49:27 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Dave Hansen <dave.hansen@...el.com>, <ak@...ux.intel.com>
CC:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, <tglx@...utronix.de>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v3 03/13] x86/retpoline/entry: Convert entry assembler
 indirect jumps

On Thu, 2018-01-04 at 06:46 -0800, Dave Hansen wrote:
> On 01/04/2018 06:37 AM, David Woodhouse wrote:
> > KPTI complicates this a little; the one in entry_SYSCALL_64_trampoline
> > can't just jump to the thunk because the thunk isn't mapped. So it gets
> > its own copy of the thunk, inline.
> 
> This one call site isn't too painful, of course.
> 
> But, is there anything keeping us from just sticking the thunk in the
> entry text section where it would be available while still in the
> trampoline?

Not really. Since we have a thunk per register and the trampoline is
limited in size, if you wanted to put just the *one* thunk that's
needed into the trampoline then it's slightly icky, but all perfectly
feasible.
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