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Subject: Re: [PATCH v14 2/4] kasan: allow architectures to provide an outline
 readiness check
To:     Daniel Axtens <dja@axtens.net>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, kasan-dev@googlegroups.com, elver@google.com,
        akpm@linux-foundation.org, andreyknvl@gmail.com
Cc:     linuxppc-dev@lists.ozlabs.org, aneesh.kumar@linux.ibm.com,
        bsingharora@gmail.com,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>
References: <20210617063956.94061-1-dja@axtens.net>
 <20210617063956.94061-3-dja@axtens.net>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <1a8960b7-fcaa-3649-1e8f-01911112209c@csgroup.eu>
Date:   Thu, 17 Jun 2021 08:59:30 +0200
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Le 17/06/2021 à 08:39, Daniel Axtens a écrit :
> Allow architectures to define a kasan_arch_is_ready() hook that bails
> out of any function that's about to touch the shadow unless the arch
> says that it is ready for the memory to be accessed. This is fairly
> uninvasive and should have a negligible performance penalty.
> 
> This will only work in outline mode, so an arch must specify
> ARCH_DISABLE_KASAN_INLINE if it requires this.
> 
> Cc: Balbir Singh <bsingharora@gmail.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> 
> --
> 
> Both previous RFCs for ppc64 - by 2 different people - have
> needed this trick! See:
>   - https://lore.kernel.org/patchwork/patch/592820/ # ppc64 hash series
>   - https://patchwork.ozlabs.org/patch/795211/      # ppc radix series
> 
> I haven't been able to exercise the arch hook error for !GENERIC as I
> don't have a particularly modern aarch64 toolchain or a lot of experience
> cross-compiling with clang. But it does fire for GENERIC + INLINE on x86.
> ---
>   mm/kasan/common.c  | 4 ++++
>   mm/kasan/generic.c | 3 +++
>   mm/kasan/kasan.h   | 8 ++++++++
>   mm/kasan/shadow.c  | 8 ++++++++
>   4 files changed, 23 insertions(+)
> 
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8f450bc28045..b18abaf8c78e 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -449,6 +449,14 @@ static inline void kasan_poison_last_granule(const void *address, size_t size) {
>   
>   #endif /* CONFIG_KASAN_GENERIC */
>   
> +#ifndef kasan_arch_is_ready
> +static inline bool kasan_arch_is_ready(void)	{ return true; }
> +#else
> +#if !defined(CONFIG_KASAN_GENERIC) || !defined(CONFIG_KASAN_OUTLINE)
> +#error kasan_arch_is_ready only works in KASAN generic outline mode!
> +#endif
> +#endif

Would be cleaner and more readable as

+#ifndef kasan_arch_is_ready
+static inline bool kasan_arch_is_ready(void)	{ return true; }
+#elif !defined(CONFIG_KASAN_GENERIC) || !defined(CONFIG_KASAN_OUTLINE)
+#error kasan_arch_is_ready only works in KASAN generic outline mode!
+#endif

> +
>   /*
>    * Exported functions for interfaces called from assembly or from generated
>    * code. Declarations here to avoid warning about missing declarations.
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 082ee5b6d9a1..3c7f7efe6f68 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -73,6 +73,10 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>   {
>   	void *shadow_start, *shadow_end;
>   
> +	/* Don't touch the shadow memory if arch isn't ready */
> +	if (!kasan_arch_is_ready())
> +		return;
> +
>   	/*
>   	 * Perform shadow offset calculation based on untagged address, as
>   	 * some of the callers (e.g. kasan_poison_object_data) pass tagged
> @@ -99,6 +103,10 @@ EXPORT_SYMBOL(kasan_poison);
>   #ifdef CONFIG_KASAN_GENERIC
>   void kasan_poison_last_granule(const void *addr, size_t size)
>   {
> +	/* Don't touch the shadow memory if arch isn't ready */
> +	if (!kasan_arch_is_ready())
> +		return;
> +
>   	if (size & KASAN_GRANULE_MASK) {
>   		u8 *shadow = (u8 *)kasan_mem_to_shadow(addr + size);
>   		*shadow = size & KASAN_GRANULE_MASK;
>