Re: [PATCH v11 1/6] kasan: allow an architecture to disable inline instrumentation

From: Marco Elver <elver@google.com>
To: Daniel Axtens <dja@axtens.net>
Cc: LKML <linux-kernel@vger.kernel.org>,
	 Linux Memory Management List <linux-mm@kvack.org>,
	linuxppc-dev@lists.ozlabs.org,
	 kasan-dev <kasan-dev@googlegroups.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	 aneesh.kumar@linux.ibm.com, Balbir Singh <bsingharora@gmail.com>
Subject: Re: [PATCH v11 1/6] kasan: allow an architecture to disable inline instrumentation
Date: Mon, 22 Mar 2021 09:18:55 +0100	[thread overview]
Message-ID: <CANpmjNOygpN7Aifd_+ycvxA+zNvyb9rF7mTA_yCfjbRK9evLGA@mail.gmail.com> (raw)
In-Reply-To: <20210319144058.772525-2-dja@axtens.net>

On Fri, 19 Mar 2021 at 15:41, Daniel Axtens <dja@axtens.net> wrote:
>
> For annoying architectural reasons, it's very difficult to support inline
> instrumentation on powerpc64.
>
> Add a Kconfig flag to allow an arch to disable inline. (It's a bit
> annoying to be 'backwards', but I'm not aware of any way to have
> an arch force a symbol to be 'n', rather than 'y'.)
>
> We also disable stack instrumentation in this case as it does things that
> are functionally equivalent to inline instrumentation, namely adding
> code that touches the shadow directly without going through a C helper.
>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> ---
>  lib/Kconfig.kasan | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index cffc2ebbf185..7e237dbb6df3 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -12,6 +12,9 @@ config HAVE_ARCH_KASAN_HW_TAGS
>  config HAVE_ARCH_KASAN_VMALLOC
>         bool
>
> +config ARCH_DISABLE_KASAN_INLINE
> +       def_bool n
> +

Does just "bool" work here?

>  config CC_HAS_KASAN_GENERIC
>         def_bool $(cc-option, -fsanitize=kernel-address)
>
> @@ -130,6 +133,7 @@ config KASAN_OUTLINE
>
>  config KASAN_INLINE
>         bool "Inline instrumentation"
> +       depends on !ARCH_DISABLE_KASAN_INLINE
>         help
>           Compiler directly inserts code checking shadow memory before
>           memory accesses. This is faster than outline (in some workloads
> @@ -142,6 +146,7 @@ config KASAN_STACK
>         bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
>         depends on KASAN_GENERIC || KASAN_SW_TAGS
>         default y if CC_IS_GCC
> +       depends on !ARCH_DISABLE_KASAN_INLINE

Minor, but perhaps this 'depends on' line could be moved up 1 line to
be grouped with the other 'depends on'.

>         help
>           The LLVM stack address sanitizer has a know problem that
>           causes excessive stack usage in a lot of functions, see
> @@ -154,6 +159,9 @@ config KASAN_STACK
>           but clang users can still enable it for builds without
>           CONFIG_COMPILE_TEST.  On gcc it is assumed to always be safe
>           to use and enabled by default.
> +         If the architecture disables inline instrumentation, this is
> +         also disabled as it adds inline-style instrumentation that
> +         is run unconditionally.
>
>  config KASAN_SW_TAGS_IDENTIFY
>         bool "Enable memory corruption identification"
> --
> 2.27.0
>
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