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SubjectRe: [PATCH][Regression] x86, 32-bit: trim memory not covered by wb mtrrs - FIX

* 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 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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