From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754712AbbFSKqn (ORCPT ); Fri, 19 Jun 2015 06:46:43 -0400 Received: from mail-wi0-f181.google.com ([209.85.212.181]:38635 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750868AbbFSKqd (ORCPT ); Fri, 19 Jun 2015 06:46:33 -0400 Date: Fri, 19 Jun 2015 12:46:28 +0200 From: Ingo Molnar To: Borislav Petkov Cc: LKML , Thomas Gleixner , "H. Peter Anvin" , Linus Torvalds Subject: Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils Message-ID: <20150619104628.GA16374@gmail.com> References: <1434700295-17721-1-git-send-email-bp@alien8.de> <20150619081044.GA32169@gmail.com> <20150619085936.GA17826@pd.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150619085936.GA17826@pd.tnic> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Borislav Petkov wrote: > On Fri, Jun 19, 2015 at 10:10:44AM +0200, Ingo Molnar wrote: > > > > * Borislav Petkov wrote: > > > > > From: Borislav Petkov > > > > > > When building the kernel with 32-bit binutils built with support only > > > for the i386 target, we get the following warning: > > > > > > arch/x86/kernel/head_32.S:66: Warning: shift count out of range (32 is not between 0 and 31) > > > > > > The problem is that in that case, binutils' internal type representation > > > is 32-bit wide and the shift range overflows. > > > > > > In order to fix this, manipulate the shift expression which creates the > > > 4GiB constant to not overflow the shift count. > > > > > > Reported-and-tested-by: Enrico Mioso > > > Suggested-by: Michael Matz > > > Signed-off-by: Borislav Petkov > > > --- > > > arch/x86/kernel/head_32.S | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S > > > index 53eeb226657c..c601d1de5ae5 100644 > > > --- a/arch/x86/kernel/head_32.S > > > +++ b/arch/x86/kernel/head_32.S > > > @@ -63,8 +63,8 @@ > > > #endif > > > > > > /* Number of possible pages in the lowmem region */ > > > -LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT) > > > - > > > +LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT) > > > + > > > > Hm, so if internally GAS uses 64-bit types, couldn't we 'make sure' it's a 64-bit > > type, by using something like 31LL? Assuming GAS understands that? > > I don't think it does. > > This fix is only when gas is compiled with the i386 target which uses solely > 32-bit sized types for internal representation. The warning won't fire on gas > built with multiple targets support even if you build with -m32. Ugh, nasty. > > Or if the internal representation is 32 bits, then 2<<31 is just a fancy way of > > saying '0', right? > > > > So this could be written as: > > > > LOWMEM_PAGES = (-__PAGE_OFFSET >> PAGE_SHIFT) > > > > or: > > > > LOWMEM_PAGES = ((0 - __PAGE_OFFSET) >> PAGE_SHIFT) > > > > right? > > Yes, you can do that only when the internal representation is a 32-bit > type. If it is not and you still do an -m32 build, you then want the > 4Gib value there and not a 0: > > arch/x86/kernel/head_32.S: Assembler messages: > arch/x86/kernel/head_32.S:227: Error: value of 18014395285110787 too large for field of 4 bytes at 146 Grumble. I guess the workaround is OK then because I cannot think of any cleaner solution - but I'd suggest to put a comment there at minimum, to explain what it's about. ... and chances are that GAS might start warning about 2<<31 in the future as well. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/