Date:   Mon, 29 Jun 2020 15:36:55 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Walter Wu <walter-zh.wu@...iatek.com>,
        Arnd Bergmann <arnd@...db.de>, Daniel Axtens <dja@...ens.net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 2/2] kasan: Update required compiler versions in documentation

On Mon, Jun 29, 2020 at 12:42 PM Marco Elver <elver@...gle.com> wrote:
>
> Updates the recently changed compiler requirements for KASAN. In
> particular, we require GCC >= 8.3.0, and add a note that Clang 11
> supports OOB detection of globals.
>
> Fixes: 7b861a53e46b ("kasan: Bump required compiler version")
> Fixes: acf7b0bf7dcf ("kasan: Fix required compiler version")
> Signed-off-by: Marco Elver <elver@...gle.com>
> ---
>  Documentation/dev-tools/kasan.rst |  7 ++-----
>  lib/Kconfig.kasan                 | 24 +++++++++++++++---------
>  2 files changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index c652d740735d..15a2a53e77b0 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -13,11 +13,8 @@ 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
> -4.9.2 or later for basic support and version 5.0 or later for detection of
> -out-of-bounds accesses for stack and global variables and for inline
> -instrumentation mode (see the Usage section). With Clang it requires version
> -7.0.0 or later and it doesn't support detection of out-of-bounds accesses for
> -global variables yet.
> +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 only supported since Clang 11.
>
>  Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index 89053defc0d9..047b53dbfd58 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -40,6 +40,7 @@ choice
>           software tag-based KASAN (a version based on software memory
>           tagging, arm64 only, similar to userspace HWASan, enabled with
>           CONFIG_KASAN_SW_TAGS).
> +
>           Both generic and tag-based KASAN are strictly debugging features.
>
>  config KASAN_GENERIC
> @@ -51,16 +52,18 @@ config KASAN_GENERIC
>         select STACKDEPOT
>         help
>           Enables generic KASAN mode.
> -         Supported in both GCC and Clang. With GCC it requires version 4.9.2
> -         or later for basic support and version 5.0 or later for detection of
> -         out-of-bounds accesses for stack and global variables and for inline
> -         instrumentation mode (CONFIG_KASAN_INLINE). With Clang it requires
> -         version 3.7.0 or later and it doesn't support detection of
> -         out-of-bounds accesses for global variables yet.
> +
> +         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.
> +
>           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.
>           The performance slowdown is ~x3.
> +
>           For better error detection enable CONFIG_STACKTRACE.
> +
>           Currently CONFIG_KASAN_GENERIC doesn't work with CONFIG_DEBUG_SLAB
>           (the resulting kernel does not boot).
>
> @@ -73,15 +76,19 @@ config KASAN_SW_TAGS
>         select STACKDEPOT
>         help
>           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 version 7.0.0
> +         or later.
> +
>           This mode consumes about 1/16th of available memory at kernel start
>           and introduces an overhead of ~20% for the rest of the allocations.
>           This mode may potentially introduce problems relating to pointer
>           casting and comparison, as it embeds tags into the top byte of each
>           pointer.
> +
>           For better error detection enable CONFIG_STACKTRACE.
> +
>           Currently CONFIG_KASAN_SW_TAGS doesn't work with CONFIG_DEBUG_SLAB
>           (the resulting kernel does not boot).
>
> @@ -107,7 +114,6 @@ config KASAN_INLINE
>           memory accesses. This is faster than outline (in some workloads
>           it gives about x2 boost over outline instrumentation), but
>           make kernel's .text size much bigger.
> -         For CONFIG_KASAN_GENERIC this requires GCC 5.0 or later.
>
>  endchoice
>
> --
> 2.27.0.212.ge8ba1cc988-goog
>

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

Thanks!

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