Re: Edited kexec_load(2) [kexec_file_load()] man page for review

[Scot Doyle <lkml14@scotdoyle.com>]

On Thu, 29 Jan 2015, Michael Kerrisk (man-pages) wrote:
> On 29 January 2015 at 02:27, Scot Doyle <lkml14@scotdoyle.com> wrote:
> > On Wed, 28 Jan 2015, Vivek Goyal wrote:
> >> On Wed, Jan 28, 2015 at 10:10:59PM +0000, Scot Doyle wrote:
> >> > On Wed, 28 Jan 2015, Vivek Goyal wrote:
> >> > > On Wed, Jan 28, 2015 at 09:14:03PM +0000, Scot Doyle wrote:
> >> > > > When I tested, kexec_file_load required CONFIG_RELOCATABLE. Is the same
> >> > > > true for kexec_load? Would it make sense to note this in the man pages
> >> > > > along with the need for CONFIG_KEXEC_FILE, etc? Or as an error message?
> >> > >
> >> > > Hmm.., I can't see an explicity dependency between RELOCATABLE and
> >> > > KEXEC. Both KEXEC and KEXEC_FILE should be able to load a kernel
> >> > > even if it had RELOCATABLE=n.
> >> > >
> >> > > Just that kernel will run from the address it has been built for.
> >> > >
> >> > > Thanks
> >> > > Vivek
> >> >
> >> > Confusing, right? kexec_file_load returns -ENOEXEC and dmesg says
> >> > "kexec-bzImage64: XLF_CAN_BE_LOADED_ABOVE_4G is not set." which leads to
> >> > arch/x86/boot/header.S line 396:
> >> >
> >> > #if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64)
> >> >    /* kernel/boot_param/ramdisk could be loaded above 4g */
> >> > # define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
> >> > #else
> >> > # define XLF1 0
> >> > #endif
> >>
> >> Ah, this one. Actually generic kexec file loading implementation does not
> >> impose this restriction. It is the image specific loader part which
> >> decides what kind of bzImage it can load.
> >>
> >> Current implementation (kexec-bzimage64.c), is only supporting loading
> >> bzImages which are 64bit and can be loaded above 4G. This simplifies
> >> the implementation of loader.
> >>
> >> But there is nothing which prevents one from implementing other image
> >> loaders.
> >>
> >> So instead of saying that kexec_file_load() depends on CONFIG_RELOCATABLE,
> >> it might be better to say in man page that currently this system call
> >> supports only loading a bzImage which is 64bit and which can be loaded
> >> above 4G too.
> >>
> >> Thanks
> >> Vivek
> >
> > Thanks, I agree, and think it would make sense to list them as part of the
> > page's ENOEXEC error.
> 
> Scott, could you then phras a couple of sentences that capture thge
> details, so I can add it to the ENOEXEC error?
> 
> Thanks,
> 
> Michael

Yes, maybe something like "kernel_fd does not refer to an open file, or 
the file type is not supported. Currently, the file must be a bzImage
and contain an x86 kernel loadable above 4G in memory (see 
Documentation/x86/boot.txt)."?

boot.txt explains that loading above 4G implies 64-bit and is specified 
via a bit in xloadflags added in Linux 3.8.