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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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.

~Andrew

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