linux-kernel - Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array

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Message-ID: <20211020194352.7id6nkvtpenmt3p2@treble>
Date:   Wed, 20 Oct 2021 12:43:52 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array

On Wed, Oct 20, 2021 at 09:22:29PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 20, 2021 at 10:09:56AM -0700, Josh Poimboeuf wrote:
> > On Wed, Oct 20, 2021 at 05:46:39PM +0100, Andrew Cooper wrote:
> > > On 20/10/2021 16:57, Josh Poimboeuf wrote:
> > > > On Wed, Oct 20, 2021 at 12:44:50PM +0200, Peter Zijlstra wrote:
> > > >> Stick all the retpolines in a single symbol and have the individual
> > > >> thunks as inner labels, this should guarantee thunk order and layout.
> > > > How so?
> > > >
> > > > Just wondering what the purpose of the array is.  It doesn't seem to be
> > > > referenced anywhere.
> > > 
> > > The array property is what makes:
> > > 
> > > > +	reg = (target - &__x86_indirect_thunk_rax) /
> > > > +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > > 
> > > safe in the next path.
> > 
> > The thunks were already 32-byte aligned.  I don't see how slapping a few
> > unused symbols around them does anything.
> 
> Previously there were 16 (or rather 15 without rsp) separate symbols and
> a toolchain might reasonably expect it could displace them however it
> liked, with disregard for the relative position.
> 
> However, now they're part of a larger symbol. Any change to their
> relative position would disrupt this larger _array symbol and thus not
> be sound.
> 
> This is I think the same reasoning used for data symbols. On their own
> there is no guarantee about their relative position wrt to one aonther,
> but we're still able to do arrays because an array as a whole is a
> single larger symbol.

Makes sense, I think (and good fodder for the commit log).

-- 
Josh