linux-kernel - Re: [PATCH v5 2/6] x86/kasan, mm: introduce generic kasan_populate_zero

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Date:	Tue, 11 Aug 2015 16:41:18 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, Yury <yury.norov@...il.com>,
	Alexey Klimov <klimov.linux@...il.com>,
	Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
	Linus Walleij <linus.walleij@...aro.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	David Keitel <dkeitel@...eaurora.org>,
	Alexander Potapenko <glider@...gle.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 2/6] x86/kasan, mm: introduce generic
 kasan_populate_zero_shadow()

On Tue, Aug 11, 2015 at 05:18:15AM +0300, Andrey Ryabinin wrote:
> --- /dev/null
> +++ b/mm/kasan/kasan_init.c
[...]
> +#if CONFIG_PGTABLE_LEVELS > 3
> +pud_t kasan_zero_pud[PTRS_PER_PUD] __page_aligned_bss;
> +#endif
> +#if CONFIG_PGTABLE_LEVELS > 2
> +pmd_t kasan_zero_pmd[PTRS_PER_PMD] __page_aligned_bss;
> +#endif
> +pte_t kasan_zero_pte[PTRS_PER_PTE] __page_aligned_bss;

Is there any problem if you don't add the #ifs here? Wouldn't the linker
remove them if they are not used?

Original hunk copied here for easy comparison:

> -static int __init zero_pte_populate(pmd_t *pmd, unsigned long addr,
> -				unsigned long end)
> -{
> -	pte_t *pte = pte_offset_kernel(pmd, addr);
> -
> -	while (addr + PAGE_SIZE <= end) {
> -		WARN_ON(!pte_none(*pte));
> -		set_pte(pte, __pte(__pa_nodebug(kasan_zero_page)
> -					| __PAGE_KERNEL_RO));
> -		addr += PAGE_SIZE;
> -		pte = pte_offset_kernel(pmd, addr);
> -	}
> -	return 0;
> -}
[...]
> +static void __init zero_pte_populate(pmd_t *pmd, unsigned long addr,
> +				unsigned long end)
> +{
> +	pte_t *pte = pte_offset_kernel(pmd, addr);
> +	pte_t zero_pte;
> +
> +	zero_pte = pfn_pte(PFN_DOWN(__pa(kasan_zero_page)), PAGE_KERNEL);
> +	zero_pte = pte_wrprotect(zero_pte);
> +
> +	while (addr + PAGE_SIZE <= end) {
> +		set_pte_at(&init_mm, addr, pte, zero_pte);
> +		addr += PAGE_SIZE;
> +		pte = pte_offset_kernel(pmd, addr);
> +	}
> +}

I think there are some differences with the original x86 code. The first
one is the use of __pa_nodebug, does it cause any problems if
CONFIG_DEBUG_VIRTUAL is enabled?

The second is the use of a read-only attribute when mapping
kasan_zero_page on x86. Can it cope with a writable mapping?

If there are no issues, it should be documented in the commit log.

-- 
Catalin
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