Re: xen (multiboot) binary gets corrupted ELF notes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2023 15:12, Marek Marczykowski-Górecki wrote:
> On Mon, Feb 13, 2023 at 01:53:21PM +0100, Jan Beulich wrote:
>> On 13.02.2023 12:14, Marek Marczykowski-Górecki wrote:
>>> Hi,
>>>
>>> I'm getting some ELF note issues on multiboot binary
>>> specifically:
>>> xen/xen: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), 
>>> statically linked, BuildID[sha1]=f7d2c37a4ad599b268f2f2d94bff3776d12649b3, 
>>> bad note description size 0xc0010001, stripped
>>>
>>> readelf additionally says:
>>>
>>>     Displaying notes found in: .note
>>>       Owner                Data size        Description
>>>       GNU                  0x00000014       NT_GNU_BUILD_ID (unique build 
>>> ID bitstring)
>>>         Build ID: c5825a0d08edc4d11b1138fedca6b14ce8ba7302
>>>       (NONE)               0x00000004       Unknown note type: (0x00000020)
>>>        description data: 05 00 00 00 
>>>     readelf: xen/xen: Warning: note with invalid namesz and/or descsz found 
>>> at offset 0x34
>>>     readelf: xen/xen: Warning:  type: 0x4, namesize: 0x00554e47, descsize: 
>>> 0xc0010001, alignment: 4
>>>
>>> Grub doesn't care, but launching such xen with kexec doesn't work.
>>>
>>> Initially found when booting Xen via Heads: 
>>> https://openqa.qubes-os.org/tests/60151#step/install_startup/11
>>>
>>> Andy says:
>>>> yeah, I've seen the same on XTF
>>>> binutil's recent elf notes for CET compatibility use an unsigned long
>>>> so they're not compatible when we build as 64bit and then re-package as 32
>>>> I think we need to strip all elf notes in mkelf32
>>
>> Instead of complicating mkelf32 (we want to retain at least the build-id
>> note, after all, and for PVH_GUEST builds also .note.Xen) why don't we
>> discard the unwanted/unneeded notes then from the linker script, just
>> like we already do for xen.efi?
> 
> Ok, this seems to help:
> 
> ---8<----
> diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
> index 8930e14fc40e..f0831bd677e7 100644
> --- a/xen/arch/x86/xen.lds.S
> +++ b/xen/arch/x86/xen.lds.S
> @@ -192,13 +192,6 @@ SECTIONS
>  #endif
>  #endif
>  
> -#ifndef EFI
> -  /* Retain these just for the purpose of possible analysis tools. */
> -  DECL_SECTION(.note) {
> -       *(.note.*)
> -  } PHDR(note) PHDR(text)
> -#endif
> -
>    _erodata = .;
>  
>    . = ALIGN(SECTION_ALIGN);
> ---8<---
> 
> The comment suggests some notes could be useful, but given they are
> broken anyway and nobody complained so far, maybe not really?

Well, much depends on those unspecified analysis tools.

Jan