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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Fri, 28 Aug 2020 12:54:15 +0200
Message-ID: <CAAeHK+zyjKWrSU-udVuqLN1i2c0bxNTMVirGjaRfXN=opn6spw@mail.gmail.com>
Subject: Re: [PATCH] compiler-clang: add build check for clang 10.0.1
To:     Marco Elver <elver@google.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Thu, Aug 27, 2020 at 9:02 PM Marco Elver <elver@google.com> wrote:
>
> On Wed, Aug 26, 2020 at 02:42PM -0700, Nathan Chancellor wrote:
> > On Wed, Aug 26, 2020 at 01:14:19PM -0700, Nick Desaulniers wrote:
> > > During Plumbers 2020, we voted to just support the latest release of
> > > Clang for now.  Add a compile time check for this.
> > >
> > > Older clang's may work, but we will likely drop workarounds for older
> > > versions.
> >
> > I think this part of the commit message is a little wishy-washy. If we
> > are breaking the build for clang < 10.0.1, we are not saying "may work",
> > we are saying "won't work". Because of this, we should take the
> > opportunity to clean up behind us and revert/remove parts of:
> >
> > 87e0d4f0f37f ("kbuild: disable clang's default use of -fmerge-all-constants")
> > b0fe66cf0950 ("ARM: 8905/1: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer")
> > b9249cba25a5 ("arm64: bti: Require clang >= 10.0.1 for in-kernel BTI support")
> > 3acf4be23528 ("arm64: vdso: Fix compilation with clang older than 8")
> >
> > This could be a series or a part of this commit, I do not have a
> > strong preference. If we are not going to clean up behind us, this
> > should be a warning and not an error.
>
> There are also some other documentation that would go stale. We probably
> have to change KASAN docs to look something like the below.
>
> I wish we could also remove the "but detection of out-of-bounds accesses
> for global variables is only supported since Clang 11", but Clang 10 is
> a vast improvement so I'm not complaining. :-)
>
> Acked-by: Marco Elver <elver@google.com>
>
> Thanks,
> -- Marco
>
> ------ >8 ------
>
> From 13d03b55c69dec813d94c1481dcb294971f164ef Mon Sep 17 00:00:00 2001
> From: Marco Elver <elver@google.com>
> Date: Thu, 27 Aug 2020 20:56:34 +0200
> Subject: [PATCH] kasan: Remove mentions of unsupported Clang versions
>
> Since the kernel now requires at least Clang 10.0.1, remove any mention
> of old Clang versions and simplify the documentation.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  Documentation/dev-tools/kasan.rst | 4 ++--
>  lib/Kconfig.kasan                 | 9 ++++-----
>  2 files changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 38fd5681fade..4abc84b1798c 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -13,10 +13,10 @@ KASAN uses compile-time instrumentation to insert validity checks before every
>  memory access, and therefore requires a compiler version that supports that.
>
>  Generic KASAN is supported in both GCC and Clang. With GCC it requires version
> -8.3.0 or later. With Clang it requires version 7.0.0 or later, but detection of
> +8.3.0 or later. Any supported Clang version is compatible, but detection of
>  out-of-bounds accesses for global variables is only supported since Clang 11.
>
> -Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.
> +Tag-based KASAN is only supported in Clang.
>
>  Currently generic KASAN is supported for the x86_64, arm64, xtensa, s390 and
>  riscv architectures, and tag-based KASAN is supported only for arm64.
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index 047b53dbfd58..033a5bc67ac4 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -54,9 +54,9 @@ config KASAN_GENERIC
>           Enables generic KASAN mode.
>
>           This mode is supported in both GCC and Clang. With GCC it requires
> -         version 8.3.0 or later. With Clang it requires version 7.0.0 or
> -         later, but detection of out-of-bounds accesses for global variables
> -         is supported only since Clang 11.
> +         version 8.3.0 or later. Any supported Clang version is compatible,
> +         but detection of out-of-bounds accesses for global variables is
> +         supported only since Clang 11.
>
>           This mode consumes about 1/8th of available memory at kernel start
>           and introduces an overhead of ~x1.5 for the rest of the allocations.
> @@ -78,8 +78,7 @@ config KASAN_SW_TAGS
>           Enables software tag-based KASAN mode.
>
>           This mode requires Top Byte Ignore support by the CPU and therefore
> -         is only supported for arm64. This mode requires Clang version 7.0.0
> -         or later.
> +         is only supported for arm64. This mode requires Clang.
>
>           This mode consumes about 1/16th of available memory at kernel start
>           and introduces an overhead of ~20% for the rest of the allocations.
> --
> 2.28.0.297.g1956fa8f8d-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@google.com>

Thanks!