Re: [PATCH] kcov: fix generic Kconfig dependencies if ARCH_WANTS_NO_INSTR

[Marco Elver <elver@google.com>]

On Wed, 1 Dec 2021 at 16:57, Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Marco,
>
> On Wed, Dec 01, 2021 at 04:26:04PM +0100, Marco Elver wrote:
> > Until recent versions of GCC and Clang, it was not possible to disable
> > KCOV instrumentation via a function attribute. The relevant function
> > attribute was introduced in 540540d06e9d9 ("kcov: add
> > __no_sanitize_coverage to fix noinstr for all architectures").
> >
> > x86 was the first architecture to want a working noinstr, and at the
> > time no compiler support for the attribute existed yet. Therefore,
> > 0f1441b44e823 ("objtool: Fix noinstr vs KCOV") introduced the ability to
> > NOP __sanitizer_cov_*() calls in .noinstr.text.
> >
> > However, this doesn't work for other architectures like arm64 and s390
> > that want a working noinstr per ARCH_WANTS_NO_INSTR.
> >
> > At the time of 0f1441b44e823, we didn't yet have ARCH_WANTS_NO_INSTR,
> > but now we can move the Kconfig dependency checks to the generic KCOV
> > option. KCOV will be available if:
> >
> >       - architecture does not care about noinstr, OR
> >       - we have objtool support (like on x86), OR
> >       - GCC is 12.0 or newer, OR
> >       - Clang is 13.0 or newer.
>
> I agree this is the right thing to do, but since GCC 12.0 isn't out yet (and
> only x86 has objtool atm) this will prevent using KCOV with a released GCC on
> arm64 and s390, which would be unfortunate for Syzkaller.
>
> AFAICT the relevant GCC commit is:
>
>    https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cec4d4a6782c9bd8d071839c50a239c49caca689
>
> Currently we mostly get away with disabling KCOV for while compilation units,
> so maybe it's worth waiting for the GCC 12.0 release, and restricting things
> once that's out?

An alternative would be to express 'select ARCH_WANTS_NO_INSTR' more
precisely, say with an override or something. Because as-is,
ARCH_WANTS_NO_INSTR then doesn't quite reflect reality on arm64
(yet?).

But it does look simpler to wait, so I'm fine with that. I leave it to you.

Thanks,
-- Marco