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From:   Marco Elver <elver@google.com>
Date:   Wed, 26 May 2021 14:38:21 +0200
Message-ID: <CANpmjNOkX5NFFekezDSntjVBp2mVgxTprjV1QK56DsE2fv-w3A@mail.gmail.com>
Subject: Re: [PATCH] kcov: add __no_sanitize_coverage to fix noinstr for all architectures
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Arnd Bergmann <arnd@arndb.de>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Mark Rutland <mark.rutland@arm.com>
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On Wed, 26 May 2021 at 08:25, Marco Elver <elver@google.com> wrote:
> On Wed, 26 May 2021 at 03:54, Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
> > On Tue, May 25, 2021 at 9:13 PM Marco Elver <elver@google.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.

Ok, let's wait for response to: https://reviews.llvm.org/D103159
If that lands in the LLVM repo I'll change to use
__has_feature(coverage_sanitizer), and send a v2. That __has_feature()
is a bit of a lie though, because fsanitize-coverage has long been
supported, but it just so happens that if we get it, then its
availability implies availability of the no_sanitize("coverage")
attribute.

Thanks,
-- Marco