On Tue, Nov 25, 2025 at 03:14:27PM +0100, Jan Beulich wrote:
> In particular when linking with lld, which converts hidden symbols to
> local ones, the ELF symbol table can change in unhelpful ways between the
> first two linking passes, resulting in the .rodata contributions to change
> between the 2nd and 3rd pass. That, however, renders our embedded symbol
> table pretty much unusable; the recently introduced self-test may then
> also fail. (Another difference between compiling a C file and assembling
> the generated ones is that - with -fdata-sections in use - the .rodata
> contributions move between passes 1 and 2, when we'd prefer them not to.)
>
> Make tools/symbols capable of producing an "empty" assembly file, such
> that unwanted differences between passes 1 and 2 go away when then using
> the corresponding objects in place of common/symbols-dummy.o.
>
> Reported-by: Andrew Cooper <[email protected]>
> Reported-by: Roger Pau Monné <[email protected]>
> Signed-off-by: Jan Beulich <[email protected]>
LGTM, not sure whether you want to extend to other arches in this
same patch, or simply guard the build of symbols-dummy.o for non-x86
arches.
> ---
> May want mirroring to other arch-es.
>
> --- a/xen/arch/x86/Makefile
> +++ b/xen/arch/x86/Makefile
> @@ -134,8 +134,10 @@ $(TARGET): $(TARGET)-syms $(efi-y) $(obj
> CFLAGS-$(XEN_BUILD_EFI) += -DXEN_BUILD_EFI
>
> $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
> + $(objtree)/tools/symbols $(all_symbols) --empty > $(dot-target).0.S
> + $(MAKE) $(build)=$(@D) $(dot-target).0.o
> $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> - $(objtree)/common/symbols-dummy.o -o $(dot-target).0
It would be good if we could now stop building symbols-dummy.o as part
of extra-y. Maybe you could guard it with ifneq ($(CONFIG_X86),y) in
the Makefile?
Or otherwise remove this from all arches thus removing
common/symbols-dummy.c.
> + $(dot-target).0.o -o $(dot-target).0
> $(NM) -pa --format=sysv $(dot-target).0 \
> | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
> > $(dot-target).1.S
> @@ -207,9 +209,11 @@ $(TARGET).efi: $(objtree)/prelink.o $(no
> ifeq ($(CONFIG_DEBUG_INFO),y)
> $(if $(filter --strip-debug,$(EFI_LDFLAGS)),echo,:) "Will strip debug
> info from $(@F)"
> endif
> + $(objtree)/tools/symbols $(all_symbols) --empty > $(dot-target).0s.S
> + $(MAKE) $(build)=$(@D) .$(@F).0s.o
> $(foreach base, $(VIRT_BASE) $(ALT_BASE), \
> $(LD) $(call EFI_LDFLAGS,$(base)) -T $(obj)/efi.lds $<
> $(relocs-dummy) \
> - $(objtree)/common/symbols-dummy.o $(note_file_option) \
> + $(dot-target).0s.o $(note_file_option) \
> -o $(dot-target).$(base).0 &&) :
> $(MKRELOC) $(foreach base,$(VIRT_BASE)
> $(ALT_BASE),$(dot-target).$(base).0) \
> > $(dot-target).1r.S
> --- a/xen/tools/symbols.c
> +++ b/xen/tools/symbols.c
> @@ -672,7 +672,10 @@ int main(int argc, char **argv)
> warn_dup = true;
> else if (strcmp(argv[i], "--error-dup") == 0)
> warn_dup = error_dup = true;
> - else if (strcmp(argv[i], "--xensyms") == 0)
> + else if (strcmp(argv[i], "--empty") == 0) {
> + write_src();
> + return 0;
Oh, that was easier than I was expecting for symbols to generate a
working empty assembly file.
Thanks, Roger.
