linux-kernel - Re: [GIT PULL] arm64 fixes for 5.11-rc6

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Message-ID: <CAHk-=wh=1K+i6cd-Y_St3ktJAdrqriXf=ct-DcFUR2GkrraLaA@mail.gmail.com>
Date:   Fri, 29 Jan 2021 14:09:05 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>
Subject: Re: [GIT PULL] arm64 fixes for 5.11-rc6

On Fri, Jan 29, 2021 at 11:03 AM Catalin Marinas
<catalin.marinas@....com> wrote:
>
> arm64 fixes:
>
> - Fix the virt_addr_valid() returning true for < PAGE_OFFSET addresses.

That's a really odd fix.

It went from an incorrect bitwise operation (masking) to an _odd_
bitwise operation (xor).

Yes, PAGE_OFFSET has the bit pattern of all upper bits set, so "(addr
^ PAGE_OFFSET)" by definition reverses the upper bits - and for a
valid case turns them to zero.

But isn't the *logical* thing to do to use a subtract instead? For the
valid cases, the two do the same thing (clear the upper bits), but
just conceptually, isn't the operation that you actually want to do
"(addr - PAGE_OFFSET)"?

IOW, why is it using that odd xor pattern that doesn't make much
sense? I believe it _works_, but it looks very strange to me.

Also, shouldn't _lm_to_phys() do the same? It does that "mask upper
bits" too that was problematic in __is_lm_address(). Again, shouldn't
that logically be a subtract op?

             Linus