linux-kernel - Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils

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Date:	Fri, 19 Jun 2015 10:59:36 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils

On Fri, Jun 19, 2015 at 10:10:44AM +0200, Ingo Molnar wrote:
> 
> * Borislav Petkov <bp@...en8.de> wrote:
> 
> > From: Borislav Petkov <bp@...e.de>
> > 
> > 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 <mrkiko.rs@...il.com>
> > Suggested-by: Michael Matz <matz@...e.de>
> > Signed-off-by: Borislav Petkov <bp@...e.de>
> > ---
> >  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.

> 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
make[2]: *** [arch/x86/kernel/head_32.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [arch/x86/kernel] Error 2
make: *** [arch/x86] Error 2
make: *** Waiting for unfinished jobs....

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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