Subject: Re: [PATCH v3 1/3] kasan: use separate (un)poison implementation for integrated init

From: Marco Elver <[email-masked]>
Date: Wed, 26 May 2021 12:12:22 +0200

On Wed, May 12, 2021 at 01:09PM -0700, Peter Collingbourne wrote:
[...] 
> +void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags);
> +void kasan_free_pages(struct page *page, unsigned int order);
> +
>  #else /* CONFIG_KASAN_HW_TAGS */
>  
>  static inline bool kasan_enabled(void)
>  {
> +#ifdef CONFIG_KASAN
>  	return true;
> +#else
> +	return false;
> +#endif
>  }

Just

	return IS_ENABLED(CONFIG_KASAN);

>  static inline bool kasan_has_integrated_init(void)
> @@ -113,8 +113,30 @@ static inline bool kasan_has_integrated_init(void)
>  	return false;
>  }
>  
> +static __always_inline void kasan_alloc_pages(struct page *page,
> +					      unsigned int order, gfp_t flags)
> +{
> +	/* Only available for integrated init. */
> +	BUILD_BUG();
> +}
> +
> +static __always_inline void kasan_free_pages(struct page *page,
> +					     unsigned int order)
> +{
> +	/* Only available for integrated init. */
> +	BUILD_BUG();
> +}

This *should* always work, as long as the compiler optimizes everything
like we expect.

But: In this case, I think this is sign that the interface design can be
improved. Can we just make kasan_{alloc,free}_pages() return a 'bool
__must_check' to indicate if kasan takes care of init?

The variants here would simply return kasan_has_integrated_init().

That way, there'd be no need for the BUILD_BUG()s and the interface
becomes harder to misuse by design.

Also, given that kasan_{alloc,free}_pages() initializes memory, this is
an opportunity to just give them a better name. Perhaps

	/* Returns true if KASAN took care of initialization, false otherwise. */
	bool __must_check kasan_alloc_pages_try_init(struct page *page, unsigned int order, gfp_t flags);
	bool __must_check kasan_free_pages_try_init(struct page *page, unsigned int order);

[...]
> -	init = want_init_on_free();
> -	if (init && !kasan_has_integrated_init())
> -		kernel_init_free_pages(page, 1 << order);
> -	kasan_free_nondeferred_pages(page, order, init, fpi_flags);
> +	if (kasan_has_integrated_init()) {
> +		if (!skip_kasan_poison)
> +			kasan_free_pages(page, order);

I think kasan_free_pages() could return a bool, and this would become

	if (skip_kasan_poison || !kasan_free_pages(...)) {
		...

> +	} else {
> +		bool init = want_init_on_free();
> +
> +		if (init)
> +			kernel_init_free_pages(page, 1 << order);
> +		if (!skip_kasan_poison)
> +			kasan_poison_pages(page, order, init);
> +	}
>  
>  	/*
>  	 * arch_free_page() can make the page's contents inaccessible.  s390
> @@ -2324,8 +2324,6 @@ static bool check_new_pages(struct page *page, unsigned int order)
>  inline void post_alloc_hook(struct page *page, unsigned int order,
>  				gfp_t gfp_flags)
>  {
> -	bool init;
> -
>  	set_page_private(page, 0);
>  	set_page_refcounted(page);
>  
> @@ -2344,10 +2342,16 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>  	 * kasan_alloc_pages and kernel_init_free_pages must be
>  	 * kept together to avoid discrepancies in behavior.
>  	 */
> -	init = !want_init_on_free() && want_init_on_alloc(gfp_flags);
> -	kasan_alloc_pages(page, order, init);
> -	if (init && !kasan_has_integrated_init())
> -		kernel_init_free_pages(page, 1 << order);
> +	if (kasan_has_integrated_init()) {
> +		kasan_alloc_pages(page, order, gfp_flags);

It looks to me that kasan_alloc_pages() could return a bool, and this
would become

	if (!kasan_alloc_pages(...)) {
		...

> +	} else {
> +		bool init =
> +			!want_init_on_free() && want_init_on_alloc(gfp_flags);
> +

[ No need for line-break (for cases like this the kernel is fine with up
to 100 cols if it improves readability). ]

> +		kasan_unpoison_pages(page, order, init);
> +		if (init)
> +			kernel_init_free_pages(page, 1 << order);
> +	}

Thoughts?

Thanks,
-- Marco

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