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

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Message-ID: <9ef0a773-71f0-c1d6-b67e-ccf7d8bcbbe6@arm.com>
Date:   Thu, 17 Sep 2020 17:17:00 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.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 9/17/20 2:46 PM, Catalin Marinas wrote:
>> 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).
>

Seems I can't encode a shift of 56 neither in mov nor in orr. I propose to
replace both with an and of the address with itself.
This should not change anything.

Thoughts?

>> +
>> +	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".
> 

-- 
Regards,
Vincenzo