linux-kernel - Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

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Date:	Tue, 05 Nov 2013 17:17:05 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Olav Haugan <ohaugan@...eaurora.org>
CC:	gregkh@...uxfoundation.org, ngupta@...are.org,
	sjenning@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	minchan@...nel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] staging: zsmalloc: Ensure handle is never 0 on success

Hi Olav,

On 11/05/2013 04:54 PM, Olav Haugan wrote:
> zsmalloc encodes a handle using the page pfn and an object
> index. On some hardware platforms the pfn could be 0 and this
> causes the encoded handle to be 0 which is interpreted as an
> allocation failure.
>
> To prevent this false error we ensure that the encoded handle
> will not be 0 when allocation succeeds.
>
> Change-Id: Ifff930dcf254915b497aec5cb36f152a5e5365d6
> Signed-off-by: Olav Haugan <ohaugan@...eaurora.org>
> ---
>   drivers/staging/zsmalloc/zsmalloc-main.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> index 523b937..0e32c0f 100644
> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> @@ -441,7 +441,7 @@ static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
>   	}
>
>   	handle = page_to_pfn(page) << OBJ_INDEX_BITS;
> -	handle |= (obj_idx & OBJ_INDEX_MASK);
> +	handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);

As suggestion you could use a macro instead of hardcoded 1.

I am not familiar with this code, but if it's a valid test to verify if
the resulting address is page aligned, you might want to set this
offset macro to a page aligned value as well.

>
>   	return (void *)handle;
>   }
> @@ -451,7 +451,7 @@ static void obj_handle_to_location(unsigned long handle, struct page **page,
>   				unsigned long *obj_idx)
>   {
>   	*page = pfn_to_page(handle >> OBJ_INDEX_BITS);
> -	*obj_idx = handle & OBJ_INDEX_MASK;
> +	*obj_idx = (handle & OBJ_INDEX_MASK) - 1;

Ditto.

Br, David Cohen
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