Re: xen (multiboot) binary gets corrupted ELF notes

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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