linux-kernel - Re: [PATCH] Ensure that walk_page_range()'s start and end are page-aligned

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Date:	Fri, 24 Feb 2012 12:55:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Smith <danms@...ibm.com>
Cc:	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dave@...ux.vnet.ibm.com
Subject: Re: [PATCH] Ensure that walk_page_range()'s start and end are
 page-aligned

On Fri, 24 Feb 2012 11:19:25 -0800
Dan Smith <danms@...ibm.com> wrote:

>
> ...
>
>     The inner function walk_pte_range() increments "addr" by PAGE_SIZE after
>     each pte is processed, and only exits the loop if the result is equal to
>     "end". Current, if either (or both of) the starting or ending addresses
>     passed to walk_page_range() are not page-aligned, then we will never
>     satisfy that exit condition and begin calling the pte_entry handler with
>     bad data.
>     
>     To be sure that we will land in the right spot, this patch checks that
>     both "addr" and "end" are page-aligned in walk_page_range() before starting
>     the traversal.
>     
> ...
>
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -196,6 +196,11 @@ int walk_page_range(unsigned long addr, unsigned long end,
>  	if (addr >= end)
>  		return err;
>  
> +	if (WARN_ONCE((addr & ~PAGE_MASK) || (end & ~PAGE_MASK),
> +		      "address range is not page-aligned")) {
> +		return -EINVAL;
> +	}
> +
>  	if (!walk->mm)
>  		return -EINVAL;

Well...  why should we apply the patch?  Is there some buggy code which
is triggering the problem?  Do you intend to write some buggy code to
trigger the problem?  ;) 

IOW, what benefit is there to this change?

Also, as it's a developer-only thing we should arrange for the overhead
to vanish when CONFIG_DEBUG_VM=n?
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