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

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

On 16/05/2019 14:23, Wei Liu wrote:
> On Wed, May 15, 2019 at 01:55:30PM +0100, Anthony PERARD wrote:
>> 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.
>>
> Andrew, do you still have concern?

If libelf goes on to DTRT then fine, but this full explanation needs to
be in the commit message.

~Andrew

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