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From:   Marco Elver <elver@google.com>
Date:   Tue, 15 Jun 2021 09:46:16 +0200
Message-ID: <CANpmjNOa-a=M-EgdkneiWDD0eCF-DELjMFxAeJzGQz6AgCdNWg@mail.gmail.com>
Subject: Re: [PATCH v12 1/6] kasan: allow an architecture to disable inline instrumentation
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>
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On Tue, 15 Jun 2021 at 03:47, 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 | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index cffc2ebbf185..935814f332a7 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -12,6 +12,15 @@ config HAVE_ARCH_KASAN_HW_TAGS
>  config HAVE_ARCH_KASAN_VMALLOC
>         bool
>
> +# Sometimes an architecture might not be able to support inline instrumentation
> +# but might be able to support outline instrumentation. This option allows an
> +# arch to prevent inline and stack instrumentation from being enabled.

This comment could be moved into 'help' of this new config option.

> +# ppc64 turns on virtual memory late in boot, after calling into generic code
> +# like the device-tree parser, so it uses this in conjuntion with a hook in
> +# outline mode to avoid invalid access early in boot.

I think the ppc64-related comment isn't necessary and can be moved to
arch/ppc64 somewhere, if there isn't one already.

> +config ARCH_DISABLE_KASAN_INLINE
> +       bool
> +
>  config CC_HAS_KASAN_GENERIC
>         def_bool $(cc-option, -fsanitize=kernel-address)
>
> @@ -130,6 +139,7 @@ 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
> @@ -141,6 +151,7 @@ endchoice
>  config KASAN_STACK
>         bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
>         depends on KASAN_GENERIC || KASAN_SW_TAGS
> +       depends on !ARCH_DISABLE_KASAN_INLINE
>         default y if CC_IS_GCC
>         help
>           The LLVM stack address sanitizer has a know problem that
> @@ -154,6 +165,9 @@ 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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