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

[Alexandre Chartre]

On 4/2/20 5:54 PM, Josh Poimboeuf wrote:
> On Thu, Apr 02, 2020 at 05:04:07PM +0200, Peter Zijlstra wrote:
> > 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:
> >
> > > > > +ÃÂÃÂÃÂ sec = find_section_by_name(file->elf,
> > > > > +ÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂÃÂ ".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?
>
> In general I prefer to keep it simple, and keep the annotations to a
> minimum.  And I don't think this warning has ever found anything useful.
> So I'd be inclined to say just allow them and automatically detect them.
>
> However the fact that arm64 asm actually uses them worries me a bit.
>
> So for me it kind of hinges on whether arm64 has a legitimate use case
> for them, or if the warning actually points to smelly code.

Then what I can do is:
- by default, automatically detect and validate intra-function calls
- remove the INTRA_FUNCTION_CALL annotation
- add an objtool option to print a warning about intra-function calls
  (but still validate such calls).

This way by default intra-function calls are automatically detected and
validated. But you still have the option to print them out if you want
to check if there are any intra-function calls.

alex.