linux-kernel - Re: [PATCH 11/13] kexec-bzImage: Support for loading bzImage using 64bit entry

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Date:	Tue, 24 Jun 2014 13:31:25 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	ebiederm@...ssion.com, hpa@...or.com, mjg59@...f.ucam.org,
	greg@...ah.com, jkosina@...e.cz, dyoung@...hat.com,
	chaowang@...hat.com, bhe@...hat.com, akpm@...ux-foundation.org,
	gong.chen@...ux.intel.com
Subject: Re: [PATCH 11/13] kexec-bzImage: Support for loading bzImage using
 64bit entry

On Sun, Jun 15, 2014 at 06:35:15PM +0200, Borislav Petkov wrote:

[..]
> > +int kexec_setup_initrd(struct boot_params *params,
> > +		unsigned long initrd_load_addr, unsigned long initrd_len)
> > +{
> > +	params->hdr.ramdisk_image = initrd_load_addr & 0xffffffffUL;
> > +	params->hdr.ramdisk_size = initrd_len & 0xffffffffUL;
> 
> We have more readable GENMASK* macros for contiguous masks. This one
> will then look like:
> 
> 	params->hdr.ramdisk_image = initrd_load_addr & GENMASK(31, 0);
> 	params->hdr.ramdisk_size = initrd_len & GENMASK(31, 0);
> 
> and this way we know exactly about which bits are we talking about. :)

[ CC gong.chen@...ux.intel.com ]

GENMASK(31,0) outputs compilation warning.

arch/x86/kernel/machine_kexec.c: In function ‘kexec_setup_initrd’:
arch/x86/kernel/machine_kexec.c:30:2: warning: left shift count >= width
of type [enabled by default]
  params->hdr.ramdisk_image = initrd_load_addr & GENMASK(31, 0);
  ^
arch/x86/kernel/machine_kexec.c:31:2: warning: left shift count >= width
of type [enabled by default]
  params->hdr.ramdisk_size = initrd_len & GENMASK(31, 0);
  ^
arch/x86/kernel/machine_kexec.c: In function ‘kexec_setup_cmdline’:
arch/x86/kernel/machine_kexec.c:52:2: warning: left shift count >= width
of type [enabled by default]
  cmdline_low_32 = cmdline_ptr_phys & GENMASK(31, 0);

I think problem is that we shift 1 by 32 bits in this case (31 - 0 + 1) and
that overflows the size of unsigned. So there is this corner case where 
it does not seem to work (or atleast outputs warning).

Thanks
Vivek
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