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

[Wei Liu <wei.liu2@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.

AIUI old binaries always have shstrtab while it isn't always true for
new ones.

Unfortunately my attempt to figure out the history of this piece of code
is futile.

Wei.

> 
> ~Andrew

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