Re: [PATCH for-4.22?] x86/EFI: arrange for non-discardable .reloc in xen.efi

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Jun 16, 2026 at 06:17:54PM +0200, Jan Beulich wrote:
> Loaders respecting IMAGE_SCN_MEM_DISCARDABLE would not load such sections,
> yet we need to access it ourselves when switching out of "physical mode".
> Leverage behavior new to GNU ld 2.46: Any contribution to .reloc which
> doesn't have the discardable flag set (which cannot even be expressed in
> ELF) will yield the output section also non-discardable.
> 
> Since for intermediate binaries we don't care about section attributes,
> link in the new object only on the final linking pass.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

This seems to be enough to indeed make xen.efi .reloc non-discardable
with binutils 2.46.

Acked-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

> ---
> With a new enough linker, $(dot-target).2r.o could be leveraged instead:
> $(dot-target).2r.S is going to be empty (or if it's not, the .reloc
> section there would already have the wanted property), so instead of
> adding the new relocs-empty.S we could also insert the section directive
> there. Overall that seemed more convoluted to me, though.
> 
> --- a/xen/arch/x86/Makefile
> +++ b/xen/arch/x86/Makefile
> @@ -196,7 +196,8 @@ note_file_option ?= $(note_file)
>  
>  extra-$(XEN_BUILD_PE) += efi.lds
>  ifeq ($(XEN_BUILD_PE),y)
> -$(TARGET).efi: $(objtree)/prelink.o $(note_file) $(obj)/efi.lds 
> $(obj)/efi/relocs-dummy.o $(obj)/efi/mkreloc
> +$(TARGET).efi: $(obj)/efi/relocs-dummy.o $(obj)/efi/relocs-empty.o 
> $(obj)/efi/mkreloc
> +$(TARGET).efi: $(objtree)/prelink.o $(note_file) $(obj)/efi.lds
>  ifeq ($(CONFIG_DEBUG_INFO),y)
>       $(if $(filter --strip-debug,$(EFI_LDFLAGS)),echo,:) "Will strip debug 
> info from $(@F)"
>  endif
> @@ -227,7 +228,7 @@ endif
>       $(MAKE) $(build)=$(@D) .$(@F).2r.o .$(@F).2s.o
>       $(call compare-symbol-tables, $(dot-target).1r.o, $(dot-target).2r.o)
>       $(call compare-symbol-tables, $(dot-target).1s.o, $(dot-target).2s.o)
> -     $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T $(obj)/efi.lds $< \
> +     $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T $(obj)/efi.lds $< 
> $(obj)/efi/relocs-empty.o \
>             $(dot-target).2r.o $(dot-target).2s.o $(orphan-handling-y) \
>             $(note_file_option) -o $@
>       $(NM) -pa --format=sysv $@ \
> @@ -247,7 +248,7 @@ $(TARGET).efi: FORCE
>  endif
>  
>  # These should already have been rebuilt when building the prerequisite of 
> "prelink.o"
> -$(obj)/efi/buildid.o $(obj)/efi/relocs-dummy.o: ;
> +$(obj)/efi/buildid.o $(obj)/efi/relocs-dummy.o $(obj)/efi/relocs-empty.o: ;
>  
>  .PHONY: include
>  include: $(objtree)/arch/x86/include/asm/asm-macros.h
> --- a/xen/arch/x86/efi/Makefile
> +++ b/xen/arch/x86/efi/Makefile
> @@ -17,5 +17,5 @@ obj-y := common-stub.o stub.o
>  obj-$(XEN_BUILD_EFI) := $(filter-out %.init.o,$(EFIOBJ-y))
>  obj-bin-$(XEN_BUILD_EFI) := $(filter %.init.o,$(EFIOBJ-y))
>  obj-bin-y += mbi2.init.o
> -extra-$(XEN_BUILD_EFI) += buildid.o relocs-dummy.o
> +extra-$(XEN_BUILD_EFI) += buildid.o relocs-dummy.o relocs-empty.o
>  nocov-$(XEN_BUILD_EFI) += stub.o
> --- /dev/null
> +++ b/xen/arch/x86/efi/relocs-empty.S
> @@ -0,0 +1,8 @@
> +/*
> + * Empty .reloc section, simply to indicate to GNU ld that the output .reloc
> + * section in xen.efi should not be marked IMAGE_SCN_MEM_DISCARDABLE.  This
> + * requires GNU ld 2.46 or newer to actually be understood in the intended 
> way.
> + */
> +
> +     .section .reloc, "a", @progbits
> +     .balign 4

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

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