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From: Catalin Marinas <>
To: Peter Collingbourne <>
Cc: Mark Rutland <>,
	Andrey Konovalov <>,
	Mark Brown <>,
	Linux ARM <>,
	Will Deacon <>, Ard Biesheuvel <>,
	Evgenii Stepanov <>
Subject: Re: [PATCH v3 3/3] kasan: arm64: support specialized outlined tag mismatch checks
Date: Tue, 8 Dec 2020 17:49:07 +0000	[thread overview]
Message-ID: <20201208174906.GD13960@gaia> (raw)
In-Reply-To: <>

On Wed, Dec 02, 2020 at 09:12:24PM -0800, Peter Collingbourne wrote:
> By using outlined checks we can achieve a significant code size
> improvement by moving the tag-based ASAN checks into separate
> functions. Unlike the existing CONFIG_KASAN_OUTLINE mode these
> functions have a custom calling convention that preserves most
> registers and is specialized to the register containing the address
> and the type of access, and as a result we can eliminate the code
> size and performance overhead of a standard calling convention such
> as AAPCS for these functions.
> This change depends on a separate series of changes to Clang [1] to
> support outlined checks in the kernel, although the change works fine
> without them (we just don't get outlined checks). This is because the
> flag -mllvm -hwasan-inline-all-checks=0 has no effect until the Clang
> changes land. The flag was introduced in the Clang 9.0 timeframe as
> part of the support for outlined checks in userspace and because our
> minimum Clang version is 10.0 we can pass it unconditionally.
> Outlined checks require a new runtime function with a custom calling
> convention. Add this function to arch/arm64/lib.
> I measured the code size of defconfig + tag-based KASAN, as well
> as boot time (i.e. time to init launch) on a DragonBoard 845c with
> an Android arm64 GKI kernel. The results are below:
>                                code size    boot time
> CONFIG_KASAN_INLINE=y before    92824064      6.18s
> CONFIG_KASAN_INLINE=y after     38822400      6.65s
> CONFIG_KASAN_OUTLINE=y          39215616     11.48s
> We can see straight away that specialized outlined checks beat the
> existing CONFIG_KASAN_OUTLINE=y on both code size and boot time
> for tag-based ASAN.
> As for the comparison between CONFIG_KASAN_INLINE=y before and after
> we saw similar performance numbers in userspace [2] and decided
> that since the performance overhead is minimal compared to the
> overhead of tag-based ASAN itself as well as compared to the code
> size improvements we would just replace the inlined checks with the
> specialized outlined checks without the option to select between them,
> and that is what I have implemented in this patch. But we may make a
> different decision for the kernel such as having CONFIG_KASAN_OUTLINE=y
> turn on specialized outlined checks if Clang is new enough.
> Signed-off-by: Peter Collingbourne <>
> Link:
> Link: [1]
> Link: [2]
> ---
> v3:
> - adopt Mark Rutland's suggested changes
> - move frame record alignment patches behind this one
> v2:
> - use calculations in the stack spills and restores
> - improve the comment at the top of the function
> - add a BTI instruction
>  arch/arm64/include/asm/asm-prototypes.h |  6 ++
>  arch/arm64/include/asm/     | 17 +++++-
>  arch/arm64/lib/Makefile                 |  2 +
>  arch/arm64/lib/kasan_sw_tags.S          | 76 +++++++++++++++++++++++++
>  mm/kasan/tags.c                         |  7 +++
>  scripts/Makefile.kasan                  |  1 +

I can try to queue the series but this patch would need an ack on the
kasan changes.

(also, it may conflict with linux-next which renames tags.c to sw_tags.c
but that's trivial)


linux-arm-kernel mailing list

  reply	other threads:[~2020-12-08 17:50 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-03  5:12 [PATCH v3 1/3] arm64: Change the on_*stack functions to take a size argument Peter Collingbourne
2020-12-03  5:12 ` [PATCH v3 2/3] arm64: stacktrace: Relax frame record alignment requirement to 8 bytes Peter Collingbourne
2020-12-03  5:12 ` [PATCH v3 3/3] kasan: arm64: support specialized outlined tag mismatch checks Peter Collingbourne
2020-12-08 17:49   ` Catalin Marinas [this message]
2021-01-20 12:41     ` Will Deacon
2021-05-13  2:30       ` Peter Collingbourne
2021-01-07 15:20   ` Andrey Konovalov
2020-12-08 10:20 ` [PATCH v3 1/3] arm64: Change the on_*stack functions to take a size argument Mark Rutland

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