linux-kernel - Re: [PATCH][Regression] x86, 32-bit: trim memory not covered by wb mtrrs

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Date:	Thu, 7 Feb 2008 10:04:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	Balaji Rao <balajirrao@...il.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>, jesse.barnes@...el.com,
	ak@...e.de, Harvey Harrison <harvey.harrison@...il.com>
Subject: Re: [PATCH][Regression] x86, 32-bit: trim memory not covered by wb
	mtrrs - FIX


* Yinghai Lu <yhlu.kernel@...il.com> wrote:

> minor difference
> +               trim_start = highest_pfn << PAGE_SHIFT;
> +               trim_size = end_pfn << PAGE_SHIFT;
> 
> could cause some problem with 32 bit kernel when mem > 4g. becase 
> highest_pfn and end_pfn is unsigned long aka 32 bit ...could overflow.
> 
> so need to assign thtem to trim_start/trim_end at first
> or
> +               trim_start = (u64)highest_pfn << PAGE_SHIFT;
> +               trim_size = (u64)end_pfn << PAGE_SHIFT;

indeed ...

i think the 64-bit behavior of gcc is inherently dangerous, we had 
numerous subtle bugs in that area.

i think perhaps Sparse should be extended to warn about this. I think 
any case where on _32-bit_ we have an 'unsigned long' that is shifted to 
the left by any significant amount is clearly in danger of overflowing. 
_Especially_ when the lvalue is 64-bit!

or in other words, on any such construct:

   64-bit lvalue = ... 32-bit value

we should enforce _explicit_ (u64) conversions.

	Ingo
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