Re: [PATCH 1/1] Allow i386 crash kernels to handle x86_64 dumps

[Vivek Goyal <vgoyal@in.ibm.com>]

On Thu, Mar 15, 2007 at 02:07:56PM +0900, Horms wrote:
> On Thu, Mar 15, 2007 at 10:25:36AM +0530, Vivek Goyal wrote:
> > On Thu, Mar 15, 2007 at 10:46:38AM +0900, Horms wrote:
> > > On Wed, Mar 14, 2007 at 05:00:09PM +0000, Ian Campbell wrote:
> > > > The specific case I am encountering is kdump under Xen with a 64 bit
> > > > hypervisor and 32 bit kernel/userspace. The dump created is a 64 bit due
> > > > to the hypervisor but the dump kernel is 32 bit to match the domain 0
> > > > kernel.
> > > > 
> > > > It's possibly less likely to be useful in a purely native scenario but I
> > > > see no reason to disallow it.
> > > 
> > > For native Linux, would this cover the case where the pre-crash kernel
> > > is 64bit and the crashdump (post-crash) kernel is 32bit?
> > > 
> > 
> > I think so. Though I have never tried this.
> > 
> > > > Signed-off-by: Ian Campbell <ian.campbell@xensource.com>
> > > > 
> > > > --- pristine-linux-2.6.18/include/asm-i386/elf.h	2006-09-20 04:42:06.000000000 +0100
> > > > +++ linux-2.6.18-xen/include/asm-i386/elf.h	2007-03-14 16:42:30.000000000 +0000
> > > > @@ -36,7 +36,7 @@
> > > >   * This is used to ensure we don't load something for the wrong architecture.
> > > >   */
> > > >  #define elf_check_arch(x) \
> > > > -	(((x)->e_machine == EM_386) || ((x)->e_machine == EM_486))
> > > > +	(((x)->e_machine == EM_386) || ((x)->e_machine == EM_486) || ((x)->e_machine == EM_X86_64))
> > 
> > But I think changing this macro might run into issues. It is being used at
> > few places in kernel, for example while loading module. This will essentially
> > mean that we allow loading 64bit x86_64 modules on 32bit i386 systems?
> > 
> > Similarly, load_elf_interp() is using it, again will we allow loading a 
> > interp written for X86_64 on a 32bit i386 machine?
> > 
> > Should we create a separate macro something like elf_check_allowed_arch(),
> > to take care of such corner cases?
> 
> That sounds reasonable to me. Though perhaps it could just be
> kexec_elf_check_arch() for now, as I don't think there are any
> other consumers of it.

Kexec will also not allow loading an x86_64 kernel on a 32bit machine.
So how about something like vmcore_elf_allowed_cross_arch()? Vmcore code
can continue to check elf_check_arch() and if that fails it can invoke
vmcore_elf_allowed_cross_arch() to find out what cross arch are allowed
for vmcore.

Thanks
Vivek