linux-kernel - Re: [PATCH] kcov: add __no_sanitize_coverage to fix noinstr for all architectures

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Message-ID: <CANpmjNM_fP-ziy204=1rd52hOO4WfCsxAwQgJ-t1WF_yFo=UzA@mail.gmail.com>
Date:   Wed, 26 May 2021 08:25:13 +0200
From:   Marco Elver <elver@...gle.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] kcov: add __no_sanitize_coverage to fix noinstr for all architectures

On Wed, 26 May 2021 at 03:54, Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
> On Tue, May 25, 2021 at 9:13 PM Marco Elver <elver@...gle.com> wrote:
> >
> > Long story short: this is not fixable without more Clang changes. The
> > only way to do it without a version check would be to introduce
> > no_sanitize_coverage attr to Clang, which we probably shouldn't do,
> > and I didn't want to fight it. ;-)
>
> I am not sure I followed why you would not want to support querying
> for the attributes (if they are intended to be used separately).

Not my decision, but some historical decision in Clang. Somebody
thought "no_sanitize(<string_literal>)" simplifies things. Hence,
Clang only knows about the no_sanitize attribute but not its
"subattributes".

> But regardless of that, why not the feature flag at least then, to be
> consistent with the others?

__has_feature(coverage_sanitizer) does not work either (yet).

> Going back to version checks seems bad -- they should be reserved for
> e.g. known broken versions and things like that. New compiler features
> should come with new feature flags...
>
> In fact, for Clang, I do not see any version checks in code at the
> moment, so this would be the first :(

In this instance it's absolutely required (for now). But if you don't
like it I'll go back to trying to fix Clang more. I'll check with
Clang folks which one we can implement, the feature check or the
attribute check.

> Cheers,
> Miguel