Messages in this thread | | | Date | Thu, 7 Feb 2008 01:11:19 -0800 | From | "Yinghai Lu" <> | Subject | Re: [PATCH][Regression] x86, 32-bit: trim memory not covered by wb mtrrs - FIX |
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On Feb 7, 2008 1:04 AM, Ingo Molnar <mingo@elte.hu> wrote: > > * Yinghai Lu <yhlu.kernel@gmail.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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