linux-kernel - Re: [PATCHv3 6/6] arm64: Add support for CONFIG_DEBUG

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Message-ID: <61391b21-933b-1983-b170-68e7fbc9fee6@redhat.com>
Date:   Fri, 18 Nov 2016 11:17:29 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv3 6/6] arm64: Add support for CONFIG_DEBUG_VIRTUAL

On 11/18/2016 11:05 AM, Mark Rutland wrote:
> On Fri, Nov 18, 2016 at 10:42:56AM -0800, Laura Abbott wrote:
>> On 11/18/2016 09:53 AM, Mark Rutland wrote:
>>> On Thu, Nov 17, 2016 at 05:16:56PM -0800, Laura Abbott wrote:
> 
>>>> +#define __virt_to_phys_nodebug(x) ({					\
>>>>  	phys_addr_t __x = (phys_addr_t)(x);				\
>>>> -	__x & BIT(VA_BITS - 1) ? (__x & ~PAGE_OFFSET) + PHYS_OFFSET :	\
>>>> -				 (__x - kimage_voffset); })
>>>> +	((__x & ~PAGE_OFFSET) + PHYS_OFFSET);				\
>>>> +})
>>>
>>> Given the KASAN failure, and the strong possibility that there's even
>>> more stuff lurking in common code, I think we should retain the logic to
>>> handle kernel image addresses for the timebeing (as x86 does). Once
>>> we've merged DEBUG_VIRTUAL, it will be easier to track those down.
>>
>> Agreed. I might see about adding another option DEBUG_STRICT_VIRTUAL
>> for catching bad __pa vs __pa_symbol usage and keep DEBUG_VIRTUAL for
>> catching addresses that will work in neither case.
> 
> I think it makes sense for DEBUG_VIRTUAL to do both, so long as the
> default behaviour (and fallback after a WARN for virt_to_phys()) matches
> what we currently do. We'll get useful diagnostics, but a graceful
> fallback.
> 

I was suggesting making the WARN optional for having this be more useful
before all the __pa_symbol stuff gets cleaned up. Maybe the WARN won't
actually be a hindrance.

Thanks,
Laura

> I think the helpers I suggested below do that?  Or have I misunderstood,
> and you mean something stricter (e.g. checking whether a lm address is
> is backed by something)?
> 
>>> phys_addr_t __virt_to_phys(unsigned long x)
>>> {
>>> 	WARN(!__is_lm_address(x),
>>> 	     "virt_to_phys() used for non-linear address: %pK\n",
>>> 	     (void*)x);
>>> 	
>>> 	return __virt_to_phys_nodebug(x);
>>> }
>>> EXPORT_SYMBOL(__virt_to_phys);
> 
>>> phys_addr_t __phys_addr_symbol(unsigned long x)
>>> {
>>> 	/*
>>> 	 * This is bounds checking against the kernel image only.
>>> 	 * __pa_symbol should only be used on kernel symbol addresses.
>>> 	 */
>>> 	VIRTUAL_BUG_ON(x < (unsigned long) KERNEL_START ||
>>> 		       x > (unsigned long) KERNEL_END);
>>>
>>> 	return __pa_symbol_nodebug(x);
>>> }
>>> EXPORT_SYMBOL(__phys_addr_symbol);
> 
> Thanks,
> Mark.
>