On 08/11/2015 06:41 PM, Catalin Marinas wrote:
> 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?
>
__pa_nodebug() should be used before kasan_early_init(), this piece of code
executed far later, so it's ok to use __pa() here.
This was actually a mistake in original code to use __pa_nodebug().
> The second is the use of a read-only attribute when mapping
> kasan_zero_page on x86. Can it cope with a writable mapping?
>
Did you miss this line:
+ zero_pte = pte_wrprotect(zero_pte);
?
> If there are no issues, it should be documented in the commit log.
>
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