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

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Message-ID: <20200917170752.GS10662@gaia>
Date:   Thu, 17 Sep 2020 18:07:52 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        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 Thu, Sep 17, 2020 at 05:17:00PM +0100, Vincenzo Frascino wrote:
> 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.

Then use a NOP.

-- 
Catalin