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SubjectRe: [PATCH 2/2] kasan: Update required compiler versions in documentation
On Mon, Jun 29, 2020 at 12:42 PM Marco Elver <elver@google.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@google.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@google.com>

Thanks!

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