linux-kernel - Re: [RFC][PATCH] arm64: Add support for CONFIG_DEBUG

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Date:   Fri, 28 Oct 2016 15:07:14 -0700
From:   Laura Abbott <labbott@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] arm64: Add support for CONFIG_DEBUG_VIRTUAL

>>> diff --git a/arch/arm64/mm/physaddr.c b/arch/arm64/mm/physaddr.c
>>> new file mode 100644
>>> index 0000000..6c271e2
>>> --- /dev/null
>>> +++ b/arch/arm64/mm/physaddr.c
>>> @@ -0,0 +1,17 @@
>>> +#include <linux/mm.h>
>>> +
>>> +#include <asm/memory.h>
>>> +
>>> +unsigned long __virt_to_phys(unsigned long x)
>>> +{
>>> +    phys_addr_t __x = (phys_addr_t)x;
>>> +
>>> +    if (__x & BIT(VA_BITS - 1)) {
>>> +        /* The bit check ensures this is the right range */
>>> +        return (__x & ~PAGE_OFFSET) + PHYS_OFFSET;
>>> +    } else {
>>> +        VIRTUAL_BUG_ON(x < kimage_vaddr || x > (unsigned long)_end);
>>
>> IIUC, in (3) you were asking if the last check should be '>' or '>='?
>>
>> To match high_memory, I suspect the latter, as _end doesn't fall within
>> the mapped virtual address space.
>>
>
> I was actually concerned about if _end would be correct with KASLR.
> Ard confirmed that it gets fixed up to be correct. I'll change the
> check to check for >=.
>

While testing this, I found two places with __pa(_end) to get bounds,
one in arm64 code and one in memblock code. x86 gets away with this
because memblock is actually __pa_symbol and x86 does image placement
different and can check against the maximum image size. I think
including _end in __pa_symbol but excluding it from the generic
__virt_to_phys makes sense. It's a bit nicer than doing _end - 1 +
1 everywhere.

Thanks,
Laura