Re: [Xen-devel] [PATCH] libxc: elf_kernel loader: Remove check for shstrtab

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, May 15, 2019 at 01:07:03PM +0100, Andrew Cooper wrote:
> On 15/05/2019 12:40, Anthony PERARD wrote:
> > This was probably useful to load ELF Note, but now ELF notes
> > "should live in a PT_NOTE segment" (elfnote.h).
> >
> > With notes living in segment, there are no need for sections, so there
> > is nothing to be stored in the shstrtab.
> >
> > This patch would allow to write a simpler ELF header for an OVMF blob
> > (which isn't an ELF) and allow it to be loaded as a PVH kernel. The
> > header only needs to declare two program segments:
> > - one to tell an ELF loader where to put the blob,
> > - one for a Xen ELFNOTE.
> >
> > The ELFNOTE is to comply to the pvh design which wants the
> > XEN_ELFNOTE_PHYS32_ENTRY to declare a blob as compaptible with the PVH
> > boot ABI.
> >
> > Note that without the ELFNOTE, libxc will load an ELF but with
> > the plain ELF loader, which doesn't check for shstrtab.
> >
> > Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> > ---
> >  tools/libxc/xc_dom_elfloader.c | 9 ---------
> >  1 file changed, 9 deletions(-)
> >
> > diff --git a/tools/libxc/xc_dom_elfloader.c b/tools/libxc/xc_dom_elfloader.c
> > index 82b5f2ee79..b327db219d 100644
> > --- a/tools/libxc/xc_dom_elfloader.c
> > +++ b/tools/libxc/xc_dom_elfloader.c
> > @@ -165,15 +165,6 @@ static elf_negerrnoval xc_dom_parse_elf_kernel(struct xc_dom_image *dom)
> >          return rc;
> >      }
> >  
> > -    /* Find the section-header strings table. */
> > -    if ( ELF_PTRVAL_INVALID(elf->sec_strtab) )
> > -    {
> > -        xc_dom_panic(dom->xch, XC_INVALID_KERNEL, "%s: ELF image"
> > -                     " has no shstrtab", __FUNCTION__);
> > -        rc = -EINVAL;
> > -        goto out;
> > -    }
> 
> This might be fine for newer binaries, but you'll break older ones.
> 
> Instead, you should skip searching for strtab if we've already located
> the Xen notes.

:-(, maybe I should have gone futher on explaining why this check is
useless (and probably at the wrong place, at least now).

The next thing that's done after that check is:
elf_parse_binary()
elf_xen_parse()
Those are located in "xen/common/libelf", and those are the functions
that actually takes care of extracting data from the elf.

elf_xen_parse() first look for Xen ELFNOTE in the program segments
(phdr, PT_NOTE) and skip reading section and strtab if found.

So, libelf already does what you asked for ;-).

The shstrtab are only used to look for legacy __xen_guest section names.
Since ELFNOTEs was used, the name of section aren't looked at.

I hope that help.

Thanks,

-- 
Anthony PERARD

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