linux-kernel - Re: [PATCH 3/7] objtool: Add support for intra-function calls

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Date:   Thu, 2 Apr 2020 17:04:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Julien Thierry <jthierry@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH 3/7] objtool: Add support for intra-function calls

On Thu, Apr 02, 2020 at 03:24:45PM +0200, Alexandre Chartre wrote:
> On 4/2/20 2:53 PM, Julien Thierry wrote:
> > On 4/2/20 9:22 AM, Alexandre Chartre wrote:

> > > +    sec = find_section_by_name(file->elf,
> > > +                   ".rela.discard.intra_function_call");
> > 
> > I'm wondering, do we really need to annotate the intra_function_call
> > and group the in a section?
> > 
> > Would it be a problem to consider all (static) call instructions with
> > a destination that is not the start offset of a symbol to be an
> > intra-function call (and set insn->intra_function_call and
> > insn->jump_dest accordingly)?
> 
> Correct, we could automatically detect intra-function calls instead of
> having to annotate them. However, I choose to annotate them because I don't
> think that's not an expected construct in a "normal" code flow (at least
> on x86). So objtool would still issue a warning on intra-function calls
> by default, and you can annotate them to indicate if they are expected.

I wondered the same thing when reading the patch. I'm confliected on
this. On the one hand auto-detecting this seems like an excellent idea.

If/when the compiler generates them, they had better be okay too.

Josh?