kexec does not work for kernel version with patch level >= 256

[Baoquan He]

On 03/31/21 at 11:04am, Patrick Sung wrote:
> On Wed, Mar 31, 2021 at 10:47 AM Baoquan He <bhe at redhat.com> wrote:
> >
> > On 03/24/21 at 12:28pm, Patrick Sung wrote:
> > > Hello all,
> > >
> > > I am using the 4.9 long term kernel which is currently at 4.9.262.
> > > When using this kernel with kexec-tools it prints out this error
> > >
> > >   Unsupported utsname.release: 4.9.262
> > >   Cannot load <kdump images path>
> > >
> > > A quick search in the code shows that kexec/kernel_version.c doing this check:
> > >
> > >   if (major >= 256 || minor >= 256 || patch >= 256) {
> > >
> > > and also in kexec/kexec.h
> > >   #define KERNEL_VERSION(major, minor, patch) \
> > >     (((major) << 16) | ((minor) << 8) | patch)
> >
> > Yeah, this seems to be a good catch. The existing longterm kenrel 4.9.262
> > does cause the problem. I am not very sure about the longterm kernel
> > version numbering, maybe we can leave 16 bits for for patch number to
> > avoid the longterm kernel issue?
> >
> > Is there document telling the longterm kernel version numbering, or any
> > pointer?
> >
> Actually I found that the mainline kernel clamp the "patch" version to 255
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/Makefile?id=9b82f13e7ef316cdc0a8858f1349f4defce3f9e0

Yeah, mainline kernel use below formula to construct kernel version.
Seems longterm kernel takes a different way. While it's understandable
that Longterm kernel using a larger patch number since it will evolve
evolve for a longer time to get in bug fixes. Maybe we should enlarge
patch number to 16 bits?

echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) +  \                                                                        
        ((c) > 255 ? 255 : (c)))';

By the way, it calls 'a' version number, 'b' patch level, 'c' sub-level
in Makefile of kernel?

> 
> > Thanks
> > Baoquan
> >
> > >
> > > which explains the reason for the range check in kernel_version.c
> > >
> > > Increasing the number of bits allowed in "patch" seems to fix the issue.
> > >
> > > Thanks,
> > > Patrick
> > >
> > > _______________________________________________
> > > kexec mailing list
> > > kexec at lists.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/kexec
> > >
> >
>