Re: [RFC][PATCH 3/5] objtool: Add support for relocations without addends

From: Matt Helsley <mhelsley@vmware.com>
To: Julien Thierry <jthierry@redhat.com>
Cc: <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Miroslav Benes <mbenes@suse.cz>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC][PATCH 3/5] objtool: Add support for relocations without addends
Date: Thu, 14 May 2020 14:09:42 -0700	[thread overview]
Message-ID: <20200514210942.GK9040@rlwimi.vmware.com> (raw)
In-Reply-To: <61c35ce2-47b3-1f62-d4ec-da9984289c5d@redhat.com>

On Wed, May 13, 2020 at 05:55:21PM +0100, Julien Thierry wrote:
> 
> 
> On 5/13/20 5:26 PM, Matt Helsley wrote:
> > On Tue, May 12, 2020 at 06:04:50PM +0100, Julien Thierry wrote:
> > > Hi Matt,
> > > 
> > > On 5/11/20 6:35 PM, Matt Helsley wrote:
> > > > Currently objtool only collects information about relocations with
> > > > addends. In recordmcount, which we are about to merge into objtool,
> > > > some supported architectures do not use rela relocations. Since
> > > > object files use one or the other the list can be reused.
> > > > 
> > > > Signed-off-by: Matt Helsley <mhelsley@vmware.com>
> > > > ---
> > > >    tools/objtool/elf.c | 55 ++++++++++++++++++++++++++++++++++++---------
> > > >    tools/objtool/elf.h |  5 ++++-

<snip>

> > I'm thinking it's simpler with fewer variables. I don't think
> > moving the cast into the switch cases makes it any clearer. It's also
> > odd because we'll keep re-initializing relas or rels to rel_buf each loop
> > iteration. Finally, this approach has the advantage that, when reviewing
> > the patch, it's clear that the original code handling RELA relocation entries
> > isn't changing -- you can see it's just shifting into one of the cases
> > (below).
> > 
> > Do you still prefer introducing rel_buf?
> 
> On a completely personal taste, yes. I do not like having two local
> variables in the same scope pointing at the same data but with an implied
> "you should only use one or the other under the right circumstances".
> 
> But my main concern was having an allocation of a certain size and then
> modifying the size (might have been valid if sizeof(GElf_Rel) <=
> sizeof(GElf_Rela), but I must admit I did not bother to check). Since you've
> addressed that issue, the rest is just a matter of taste so better left to
> the maintainers.

OK. As a sort of tie-breaker I tried to quickly find an analogous piece of code
in objtool which I could use as a guide on maintainer preference. The
only place I see void pointers being used is as keys for comparison
functions. So my guess and preference is to not use a void pointer here.

Cheers,
	-Matt Helsley