linux-kernel - Re: bisected boot regression post 2.6.25-rc3.. please revert

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Date:	Mon, 3 Mar 2008 23:33:52 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	hans.rosenfeld@....com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: bisected boot regression post 2.6.25-rc3.. please revert

>> so (on 32-bit) ~_PAGE_PSE is ~0x80L is 0xffffff7f, which when cast to
>> 64-bit is 0x00000000ffffff7f,
>
> Actually, it is signed, so this isn't true.  Comments about unsafeness
> still apply.

It turns out that PAGE_SIZE is unsigned.  So this gives us for

	(~PAGE_MASK & ~_PAGE_USER & ~_PAGE_PSE & ~_PAGE_NX)

the types UL, L, L, ULL resp.

The associativity of & is left-to-right, so this in turn becomes

	UL, L, ULL

	UL, ULL

	ULL

and that cast from UL to ULL doesn't sign-extend.


Segher

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