linux-kernel - Re: [PATCH v5 10/19] x86: LAM compatible non-canonical definition

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Message-ID: <c68330de-c076-45be-beac-147286f2b628@intel.com>
Date: Mon, 25 Aug 2025 14:36:35 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
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Subject: Re: [PATCH v5 10/19] x86: LAM compatible non-canonical definition

On 8/25/25 13:24, Maciej Wieczor-Retman wrote:
> +/*
> + * CONFIG_KASAN_SW_TAGS requires LAM which changes the canonicality checks.
> + */
> +#ifdef CONFIG_KASAN_SW_TAGS
> +static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
> +{
> +	return (vaddr | BIT_ULL(63) | BIT_ULL(vaddr_bits - 1));
> +}
> +#else
>  static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
>  {
>  	return ((s64)vaddr << (64 - vaddr_bits)) >> (64 - vaddr_bits);
>  }
> +#endif

This is the kind of thing that's bound to break. Could we distill it
down to something simpler, perhaps?

In the end, the canonical enforcement mask is the thing that's changing.
So perhaps it should be all common code except for the mask definition:

#ifdef CONFIG_KASAN_SW_TAGS
#define CANONICAL_MASK(vaddr_bits) (BIT_ULL(63) | BIT_ULL(vaddr_bits-1))
#else
#define CANONICAL_MASK(vaddr_bits) GENMASK_UL(63, vaddr_bits)
#endif

(modulo off-by-one bugs ;)

Then the canonical check itself becomes something like:

	unsigned long cmask = CANONICAL_MASK(vaddr_bits);
	return (vaddr & mask) == mask;

That, to me, is the most straightforward way to do it.

I don't see it addressed in the cover letter, but what happens when a
CONFIG_KASAN_SW_TAGS=y kernel is booted on non-LAM hardware?