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From: Ard Biesheuvel <ardb@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/2] arm64: Use simpler arithmetics for the linear map macros
Date: Tue, 2 Feb 2021 12:40:36 +0100	[thread overview]
Message-ID: <CAMj1kXFmCpK=Dk6G0TUJbzLLtXSXjf9XpnBUewXHcOpGwmm=4A@mail.gmail.com> (raw)
In-Reply-To: <20210202113609.GA26895@gaia>

On Tue, 2 Feb 2021 at 12:36, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Tue, Feb 02, 2021 at 12:05:10AM +0100, Ard Biesheuvel wrote:
> > On Mon, 1 Feb 2021 at 20:06, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > Because of the tagged addresses, the __is_lm_address() and
> > > __lm_to_phys() macros grew to some harder to understand bitwise
> > > operations using PAGE_OFFSET. Since these macros only accept untagged
> > > addresses, use a simple subtract operation.
> > >
> > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: Ard Biesheuvel <ardb@kernel.org>
> > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > ---
> > >  arch/arm64/include/asm/memory.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > > index 3c1aaa522cbd..ff4732785c32 100644
> > > --- a/arch/arm64/include/asm/memory.h
> > > +++ b/arch/arm64/include/asm/memory.h
> > > @@ -251,9 +251,9 @@ static inline const void *__tag_set(const void *addr, u8 tag)
> > >   * lives in the [PAGE_OFFSET, PAGE_END) interval at the bottom of the
> > >   * kernel's TTBR1 address range.
> > >   */
> > > -#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))
> >
> > Why still subtract PAGE_OFFSET on both sides of the comparison here?
>
> Not sure what you mean (Linus replied on the local variable aspect).
>
> With signed arithmetics, the above would be equivalent to addr < PAGE_END
> but it wouldn't cover the addr < PAGE_OFFSET case. So we rely on the
> unsigned wrap-around to detect this.
>

Never mind, I just got confused. These changes look fine.

For the series,

Acled-by: Ard Biesheuvel <ardb@kernel.org>

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  reply	other threads:[~2021-02-02 11:42 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-01 19:06 [PATCH 0/2] arm64: Further fixes to the linear address checking Catalin Marinas
2021-02-01 19:06 ` [PATCH 1/2] arm64: Do not pass tagged addresses to __is_lm_address() Catalin Marinas
2021-02-01 19:06   ` Catalin Marinas
2021-02-01 19:06 ` [PATCH 2/2] arm64: Use simpler arithmetics for the linear map macros Catalin Marinas
2021-02-01 23:05   ` Ard Biesheuvel
2021-02-01 23:17     ` Linus Torvalds
2021-02-02 11:36     ` Catalin Marinas
2021-02-02 11:40       ` Ard Biesheuvel [this message]
2021-02-01 19:33 ` [PATCH 0/2] arm64: Further fixes to the linear address checking Linus Torvalds
2021-02-02 17:46 ` Catalin Marinas

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