Re: [PATCH v2 1/2] arm64: Fix kernel address detection of __is_lm_address()

On 1/21/21 3:12 PM, Mark Rutland wrote:

[adding Ard]

Thanks for this, it is related to his patch and I forgot to Cc: him directly.

On Thu, Jan 21, 2021 at 01:19:55PM +0000, Vincenzo Frascino wrote:

quoted

Currently, the __is_lm_address() check just masks out the top 12 bits
of the address, but if they are 0, it still yields a true result.
This has as a side effect that virt_addr_valid() returns true even for
invalid virtual addresses (e.g. 0x0).

When it was added, __is_lm_address() was intended to distinguish valid
kernel virtual addresses (i.e. those in the TTBR1 address range), and
wasn't intended to do anything for addresses outside of this range. See
commit:

  ec6d06efb0bac6cd ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")

... where it simply tests a bit.

So I believe that it's working as intended (though this is poorly
documented), but I think you're saying that usage isn't aligned with
that intent. Given that, I'm not sure the fixes tag is right; I think it
has never had the semantic you're after.

I did not do much thinking on the intended semantics. I based my interpretation
on what you are saying (the usage is not aligned with the intent). Based on what
you are are saying, I will change the patch description removing the "Fix" term.

I had thought the same was true for virt_addr_valid(), and that wasn't
expected to be called for VAs outside of the kernel VA range. Is it
actually safe to call that with NULL on other architectures?

I am not sure on this, did not do any testing outside of arm64.

I wonder if it's worth virt_addr_valid() having an explicit check for
the kernel VA range, instead.

I have no strong opinion either way even if personally I feel that modifying
__is_lm_address() is more clear. Feel free to propose something.

quoted

Fix the detection checking that it's actually a kernel address starting
at PAGE_OFFSET.

Fixes: f4693c2716b35 ("arm64: mm: extend linear region for 52-bit VA configurations")
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
 arch/arm64/include/asm/memory.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 18fce223b67b..e04ac898ffe4 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h

@@ -249,7 +249,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
 /*
  * The linear kernel range starts at the bottom of the virtual address space.
  */
-#define __is_lm_address(addr)	(((u64)(addr) & ~PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
+#define __is_lm_address(addr)	(((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))

If we're going to make this stronger, can we please expand the comment
with the intended semantic? Otherwise we're liable to break this in
future.

Based on your reply on the above matter, if you agree, I am happy to extend the
comment.

Thanks,
Mark.

-- 
Regards,
Vincenzo

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