[PATCH] kexec: Add option to fallback to old kexec syscall when kexec file based syscall failed

[Vivek Goyal]

On Fri, Jul 15, 2016 at 04:42:40PM +0200, Petr Tesarik wrote:
> On Fri, 15 Jul 2016 08:51:14 -0400
> Vivek Goyal <vgoyal at redhat.com> wrote:
> 
> > On Fri, Jul 15, 2016 at 09:58:22AM +0200, Petr Tesarik wrote:
> > > On Fri, 15 Jul 2016 07:57:22 +0800
> > > joeyli <jlee at suse.com> wrote:
> > > 
> > > > Hi Vivek
> > > > 
> > > > On Thu, Jul 14, 2016 at 10:53:28AM -0400, Vivek Goyal wrote:
> > > > > On Thu, Jul 14, 2016 at 04:45:11PM +0800, Lee, Chun-Yi wrote:
> > > > > > This patch adds a new "--fallback-kexec" option to give a chance to
> > > > > > fallback to old kexec syscall when file based kexec syscall operation
> > > > > > failed.
> > > > > 
> > > > > I think caller should switch to using different interface if need be. But
> > > > > I don't see much point in providing an option for this in kexec-tools.
> > > > > 
> > > > > Vivek
> > > > >
> > > > 
> > > > OK~ Understood!
> > > > 
> > > > Thanks for Baoquan's and your opinion for this patch.
> > > 
> > > Is there some sort of diagnostics, so a calling script can determine
> > > whether kexec failed, because there's no suppor for kexec_file_load(2)
> > > or for a different reason? 
> > 
> > Will we not get -ENOSYS if kexec_file_load() is not implemented?
> 
> Sure, the kexec code will see a beautiful ENOSYS in errno, but it
> merely prints this message on stderr (possibly with a different error
> string if not linked against glibc):
> 
> kexec_file_load failed: Function not implemented
> 
> ...and exits with status 255 (same for any other error). Which is, um,
> not very friendly to automated error handling...

So what are the options? One should return different error codes as
returned by glibc? But bash or other script will not have a library
to translate it. I guess scripts will have to hard code the meaning
of a particular return code.

BTW, user can always try kexec_file_load() and if it fails, try older
version of syscall and that should work for lot of use cases?

Vivek