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

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Date:	Thu, 7 Feb 2008 01:11:19 -0800
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
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

On Feb 7, 2008 1:04 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * 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 tr, 32-bitim_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.

so you mean gcc will do some optimization to make

+               trim_start = highest_pfn;
+               trim_start <<= PAGE_SHIFT;

to be

+               trim_start = highest_pfn << PAGE_SHIFT;

looks scary...

then gcc need to be fixed.

YH
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