linux-kernel - Re: [PATCH v7 02/11] x86/retpoline: Temporarily disable objtool when CONFIG

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Date:   Tue, 9 Jan 2018 23:58:06 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     Andi Kleen <ak@...ux.intel.com>, 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>,
        Dave Hansen <dave.hansen@...el.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,
        x86@...nel.org
Subject: Re: [PATCH v7 02/11] x86/retpoline: Temporarily disable objtool when
 CONFIG_RETPOLINE=y

On Tue, Jan 09, 2018 at 02:43:08PM +0000, David Woodhouse wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> objtool's assembler currently cannot deal with the code generated by the
> retpoline compiler and throws hundreds of warnings, mostly because it sees
> calls that don't have a symbolic target.
> 
> Exclude all the options that rely on objtool when RETPOLINE is active.
> 
> This mainly means that the kernel has to fallback to use the frame pointer
> unwinder and livepatch is not supported.
> 
> Josh is looking into resolving the issue.

I have a fix brewing for this, in two parts:

- Part 1 will allow objtool to understand the flow *around* the
  retpolines (but not *inside* them).  Which basically means that ORC
  will still get confused if it tries to unwind from inside a retpoline,
  but otherwise it should work fine.  This code is pretty much done,
  just need to do some testing with it first.  This should allow us to
  re-enable objtool and friends: ORC, reliable stacks, livepatch
  consistency model.

- Part 2 will add ORC annotations for inside the retpolines.  This will
  be a little harder, but I have my fingers crossed that it's do-able
  within a week or so.

-- 
Josh