linux-kernel - Re: [PATCH 06/12] powerpc/module: Rework is_early_mcount

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Date:	Thu, 25 Feb 2016 21:28:32 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Balbir Singh <bsingharora@...il.com>, linuxppc-dev@...abs.org
Cc:	duwe@....de, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	kamalesh@...ux.vnet.ibm.com, pmladek@...e.com, jeyu@...hat.com,
	jkosina@...e.cz, live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH 06/12] powerpc/module: Rework is_early_mcount_callsite()

On Thu, 2016-02-25 at 10:39 +1100, Balbir Singh wrote:
> On 25/02/16 01:28, Michael Ellerman wrote:
> > is_early_mcount_callsite() needs to detect either the two instruction or
> > the three instruction versions of the _mcount() sequence.
> > 
> > But if we're running a kernel with the two instruction sequence, we need
> > to be careful not to read instruction - 2, otherwise we might fall off
> > the front of a page and cause an oops.
> > 
> > While we're here convert to bool to make the return semantics clear.
> > 
> > Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> > 
> Do we even need to do this anymore?

Yes. Otherwise the code in apply_relocate_add() will see a far call with no nop
slot after it to do the toc restore, and it considers that a bug (which it
usually is, except mcount is special).

As we discussed today I'm hoping we can clean this code up a bit more in the
medium term, but this works for now.

cheers