linux-kernel - Re: [PATCH v2 22/37] arm64: mte: Add in-kernel MTE helpers

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Message-ID: <20200917134653.GB10662@gaia>
Date:   Thu, 17 Sep 2020 14:46:53 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        kasan-dev@...glegroups.com,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Elena Petrova <lenaptr@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 22/37] arm64: mte: Add in-kernel MTE helpers

On Tue, Sep 15, 2020 at 11:16:04PM +0200, Andrey Konovalov wrote:
> diff --git a/arch/arm64/include/asm/mte-helpers.h b/arch/arm64/include/asm/mte-helpers.h
> new file mode 100644
> index 000000000000..5dc2d443851b
> --- /dev/null
> +++ b/arch/arm64/include/asm/mte-helpers.h
> @@ -0,0 +1,48 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2020 ARM Ltd.
> + */
> +#ifndef __ASM_MTE_ASM_H
> +#define __ASM_MTE_ASM_H
> +
> +#define __MTE_PREAMBLE		".arch armv8.5-a\n.arch_extension memtag\n"

Because of how the .arch overrides a previous .arch, we should follow
the ARM64_ASM_PREAMBLE introduced in commit 1764c3edc668 ("arm64: use a
common .arch preamble for inline assembly"). The above should be
something like:

#define __MTE_PREAMBLE	ARM64_ASM_PREAMBLE ".arch_extension memtag"

with the ARM64_ASM_PREAMBLE adjusted to armv8.5-a if available.

> +#define MTE_GRANULE_SIZE	UL(16)
> +#define MTE_GRANULE_MASK	(~(MTE_GRANULE_SIZE - 1))
> +#define MTE_TAG_SHIFT		56
> +#define MTE_TAG_SIZE		4
> +#define MTE_TAG_MASK		GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
> +#define MTE_TAG_MAX		(MTE_TAG_MASK >> MTE_TAG_SHIFT)

In v1 I suggested we keep those definitions in mte-def.h (or
mte-hwdef.h) so that they can be included in cache.h. Anything else
should go in mte.h, I don't see the point of two headers for various MTE
function prototypes.

> +
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/types.h>
> +
> +#ifdef CONFIG_ARM64_MTE
> +
> +#define mte_get_ptr_tag(ptr)	((u8)(((u64)(ptr)) >> MTE_TAG_SHIFT))

I wonder whether this could also be an inline function that takes a void
*ptr.

> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 52a0638ed967..e238ffde2679 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -72,6 +74,52 @@ int memcmp_pages(struct page *page1, struct page *page2)
>  	return ret;
>  }
>  
> +u8 mte_get_mem_tag(void *addr)
> +{
> +	if (system_supports_mte())
> +		asm volatile(ALTERNATIVE("ldr %0, [%0]",
> +					 __MTE_PREAMBLE "ldg %0, [%0]",
> +					 ARM64_MTE)
> +			     : "+r" (addr));

This doesn't do what you think it does. LDG indeed reads the tag from
memory but LDR loads the actual data at that address. Instead of the
first LDR, you may want something like "mov %0, #0xf << 56" (and use
some macros to avoid the hard-coded 56).

> +
> +	return 0xF0 | mte_get_ptr_tag(addr);
> +}
> +
> +u8 mte_get_random_tag(void)
> +{
> +	u8 tag = 0xF;
> +	u64 addr = 0;
> +
> +	if (system_supports_mte()) {
> +		asm volatile(ALTERNATIVE("add %0, %0, %0",
> +					 __MTE_PREAMBLE "irg %0, %0",
> +					 ARM64_MTE)
> +			     : "+r" (addr));

What was the intention here? The first ADD doubles the pointer value and
gets a tag out of it (possibly doubled as well, depends on the carry
from bit 55). Better use something like "orr %0, %0, #0xf << 56".

> +
> +		tag = mte_get_ptr_tag(addr);
> +	}
> +
> +	return 0xF0 | tag;

This function return seems inconsistent with the previous one. I'd
prefer the return line to be the same in both.

> +}
> +
> +void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
> +{
> +	void *ptr = addr;
> +
> +	if ((!system_supports_mte()) || (size == 0))
> +		return addr;
> +
> +	/* Make sure that size is aligned. */
> +	WARN_ON(size & (MTE_GRANULE_SIZE - 1));
> +
> +	tag = 0xF0 | (tag & 0xF);

No point in tag & 0xf, the top nibble doesn't matter as you or 0xf0 in.

> +	ptr = (void *)__tag_set(ptr, tag);
> +
> +	mte_assign_mem_tag_range(ptr, size);
> +
> +	return ptr;
> +}
> +
>  static void update_sctlr_el1_tcf0(u64 tcf0)
>  {
>  	/* ISB required for the kernel uaccess routines */
> diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
> index 03ca6d8b8670..cc2c3a378c00 100644
> --- a/arch/arm64/lib/mte.S
> +++ b/arch/arm64/lib/mte.S
> @@ -149,3 +149,20 @@ SYM_FUNC_START(mte_restore_page_tags)
>  
>  	ret
>  SYM_FUNC_END(mte_restore_page_tags)
> +
> +/*
> + * Assign allocation tags for a region of memory based on the pointer tag
> + *   x0 - source pointer
> + *   x1 - size
> + *
> + * Note: size must be non-zero and MTE_GRANULE_SIZE aligned
> + */
> +SYM_FUNC_START(mte_assign_mem_tag_range)
> +	/* if (src == NULL) return; */
> +	cbz	x0, 2f
> +1:	stg	x0, [x0]
> +	add	x0, x0, #MTE_GRANULE_SIZE
> +	sub	x1, x1, #MTE_GRANULE_SIZE
> +	cbnz	x1, 1b
> +2:	ret
> +SYM_FUNC_END(mte_assign_mem_tag_range)

I thought Vincenzo agreed to my comments on the previous version w.r.t.
the fist cbz and the last cbnz:

https://lore.kernel.org/linux-arm-kernel/921c4ed0-b5b5-bc01-5418-c52d80f1af59@arm.com/

-- 
Catalin