linux-kernel - Re: [PATCH v2] ARM: mm: fix size rounding-down of arm_add

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Date:	Mon, 22 Dec 2014 10:09:32 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Masahiro Yamada <yamada.m@...panasonic.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	Olof Johansson <olof@...om.net>,
	Laura Abbott <lauraa@...eaurora.org>,
	Daniel Thompson <daniel.thompson@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: mm: fix size rounding-down of arm_add_memory() function

On 22 December 2014 at 04:17, Masahiro Yamada <yamada.m@...panasonic.com> wrote:
> The current rounding of "size" is wrong:
>
>  - If "start" is sufficiently near the next page boundary, "size"
>    is decremented by more than enough and the last page is lost.
>
>  - If "size" is sufficiently small, it is wrapped around and gets
>    a bogus value.
>
> Signed-off-by: Masahiro Yamada <yamada.m@...panasonic.com>
> Acked-by: Geert Uytterhoeven <geert@...ux-m68k.org>

Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>


> ---
>
> Changes in v2:
>  - Avoid wrap around of "size"
>
>  arch/arm/kernel/setup.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index f9c8639..d13f185 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -657,10 +657,13 @@ int __init arm_add_memory(u64 start, u64 size)
>
>         /*
>          * Ensure that start/size are aligned to a page boundary.
> -        * Size is appropriately rounded down, start is rounded up.
> +        * Size is rounded down, start is rounded up.
>          */
> -       size -= start & ~PAGE_MASK;
>         aligned_start = PAGE_ALIGN(start);
> +       if (aligned_start > start + size)
> +               size = 0;
> +       else
> +               size -= aligned_start - start;
>
>  #ifndef CONFIG_ARCH_PHYS_ADDR_T_64BIT
>         if (aligned_start > ULONG_MAX) {
> --
> 1.9.1
>
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