On 2025-08-26 at 19:46:19 -0500, Samuel Holland wrote:

Hi Maciej,

On 2025-08-26 3:08 AM, Maciej Wieczor-Retman wrote:

quoted

On 2025-08-25 at 14:36:35 -0700, Dave Hansen wrote:

quoted

On 8/25/25 13:24, Maciej Wieczor-Retman wrote:

quoted

+/*
+ * 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.

Thanks, I'll try something like this. I will also have to investigate what
Samuel brought up that KVM possibly wants to pass user addresses to this
function as well.

quoted

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?

That's a good point, I need to add it to the cover letter. On non-LAM hardware
the kernel just doesn't boot. Disabling KASAN in runtime on unsupported hardware
isn't that difficult in outline mode, but I'm not sure it can work in inline
mode (where checks into shadow memory are just pasted into code by the
compiler).

On RISC-V at least, I was able to run inline mode with missing hardware support.
The shadow memory is still allocated, so the inline tag checks do not fault. And
with a patch to make kasan_enabled() return false[1], all pointers remain
canonical (they match the MatchAllTag), so the inline tag checks all succeed.

[1]:
https://lore.kernel.org/linux-riscv/20241022015913.3524425-3-samuel.holland@sifive.com/ (local)

Thanks, that should work :)

I'll test it and apply to the series.

Regards,
Samuel

quoted

Since for now there is no compiler support for the inline mode anyway, I'll try to
disable KASAN on non-LAM hardware in runtime.

-- 
Kind regards
Maciej Wieczór-Retman