linux-kernel - Re: [PATCH v3 05/14] x86: Reset tag for virtual to physical address conversions

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Message-ID: <a7imdoiu4dpypmw2rp25j3vmmuvvzhg6bcx5wtt2dgukg5u7cz@nuyvc77cdmvb>
Date: Wed, 9 Apr 2025 09:36:32 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
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Subject: Re: [PATCH v3 05/14] x86: Reset tag for virtual to physical address
 conversions

On 2025-04-04 at 09:42:55 -0700, Dave Hansen wrote:
>On 4/4/25 06:14, Maciej Wieczor-Retman wrote:
>> +#ifdef CONFIG_KASAN_SW_TAGS
>> +#define page_to_virt(x)	({									\
>> +	__typeof__(x) __page = x;								\
>> +	void *__addr = __va(page_to_pfn((__typeof__(x))__tag_reset(__page)) << PAGE_SHIFT);	\
>> +	(void *)__tag_set((const void *)__addr, page_kasan_tag(__page));			\
>> +})
>> +#endif
>
>Is this #ifdef needed?
>
>I thought there were stub versions of all of those tag functions. So it
>should be harmless to use this page_to_virt() implementation with or
>without KASAN. Right?
>
>I'm also confused by the implementation. This is one reason why I rather
>dislike macros. Why does this act like the type of 'x' is variable?
>Isn't it always a 'struct page *'? If so, then why all of the
>__typeof__()'s?
>
>Are struct page pointers _ever_ tagged? If they are, then doesn't
>page_to_pfn() need to handle untagging as well? If they aren't, then
>there's no reason to __tag_reset() in here.
>
>What was the thinking behind this cast:
>
>	(const void *)__addr
>
>?
>
>Are any of these casts _doing_ anything? I'm struggling to find anything
>wrong with:
>
>#define page_to_virt(x)	({
>	void *__addr = __va(page_to_pfn(__page) << PAGE_SHIFT);
>	__tag_set(__addr, page_kasan_tag(x))
>})
>
>... which made me look back at:
>
>	static inline const void *__tag_set(const void *addr, u8 tag)
>
>from patch 3. I don't think the 'const' makes any sense on the return
>value here. Surely the memory pointed at by a tagged pointer doesn't
>need to be const. Why should the tag setting function be returning a
>const pointer?
>
>I can see why it would *take* a const pointer since it's not modifying
>the memory, but I don't see why it is returning one.
>

Right, yes, both your page_to_virt() and removing the const from __tag_set()
return seem to be working just fine. Thanks for pointing it out.

With the macros I was trying to do what the arm64 implementation did assuming it
had some significance that wasn't written down anywhere.

I recall lack of the const thing was giving me a compilation error a while
back but now it's gone. And I didn't think much of it since arm had the same
thing.

If Andrey Konovalov is reading this: is there a reason the __tag_set() on arm64
returns a const pointer? And the page_to_virt() does the __typeof__()?

-- 
Kind regards
Maciej Wieczór-Retman