Re: [PATCH v5 02/10] x86: Compile-time asm code validation

Josh Poimboeuf
2015-06-10T17:53:30Z

On Wed, Jun 10, 2015 at 10:21:36AM -0700, Andy Lutomirski wrote:
> On Jun 10, 2015 5:07 AM, "Josh Poimboeuf" <jpoimboe@redhat.com> wrote:
> >
> > Add a new CONFIG_ASM_VALIDATION option which adds an asmvalidate host
> > tool which runs on every compiled .S file. Its goal is to enforce sane
> > rules on all asm code, so that stack debug metadata (frame/back chain
> > pointers and/or DWARF CFI metadata) can be made reliable.
> >
> > It enforces the following rules:
> >
> > 1. Each callable function must be annotated with the ELF STT_FUNC type.
> > This is typically done using the ENTRY/ENDPROC macros. If
> > asmvalidate finds a return instruction outside of a function, it
> > flags an error, since that usually indicates callable code which
> > should be annotated accordingly.
> >
> > 2. Each callable function must never leave its own bounds (i.e. with a
> > jump to outside the function) except when returning.
>
> Won't that break with sibling/tail calls?

Yes, asmvalidate will flag a warning for tail calls.

> GCC can generate those, and the ia32_ptregs_common label is an example
> of such a thing.
>
> I'd rather have the script understand tail calls and possibly require
> that ia32_ptregs_common have a dummy frame pointer save in front
> before the label if needed.

Why do you prefer tail calls there? See patch 3 for how I handled that
for ia32_ptregs_common (I duplicated the code with macros).

I think adding support for tail calls in the tooling would be tricky.
So I'm just trying to figure out if there's a good reason to keep them.

--
Josh