[PATCH RFC] x86/lld: fix symbol map generation

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The symbol map generation (and thus the debug info attached to Xen) is
partially broken when using LLVM LD.  That's due to LLD converting
almost all symbols from global to local in the last linking step, and
thus confusing tools/symbols into adding a file prefix to all text
symbols, the results looks like:

Xen call trace:
   [<ffff82d040449fe8>] R xxhash64.c#__start_xen+0x3938/0x39c0
   [<ffff82d040203734>] F __high_start+0x94/0xa0

In order to workaround this create a list of global symbols prior to
the linking step, and use objcopy to convert the symbols in the final
binary back to global before processing with tools/symbols.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
I haven't found a way to prevent LLD from converting the symbols, so
I've come up with this rather crappy workaround.

Not applied to EFI, partially because I don't have an environment with
LLD capable of generating the EFI binary.

Obtaining the global symbol list could likely be a target on itself,
if it is to be shared between the ELF and the EFI binary generation.
---
 xen/arch/x86/Makefile | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
index 177a2ff742..f3817827bc 100644
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -134,24 +134,34 @@ $(TARGET): $(TARGET)-syms $(efi-y) $(obj)/boot/mkelf32
 CFLAGS-$(XEN_BUILD_EFI) += -DXEN_BUILD_EFI
 
 $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
+       # Dump global text symbols before the linking step
+       $(NM) -pa --format=bsd $< | awk '{ if($$2 == "T") print $$3}' \
+           > $(@D)/.$(@F).global-syms
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds -N $< $(build_id_linker) \
-           $(objtree)/common/symbols-dummy.o -o $(@D)/.$(@F).0
+           $(objtree)/common/symbols-dummy.o -o $(@D)/.$(@F).0.tmp
+       # LLVM LD has converted global symbols into local ones as part of the
+       # linking step, convert those back to global before using tools/symbols.
+       $(OBJCOPY) --globalize-symbols=$(@D)/.$(@F).global-syms \
+           $(@D)/.$(@F).0.tmp $(@D)/.$(@F).0
        $(NM) -pa --format=sysv $(@D)/.$(@F).0 \
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
                >$(@D)/.$(@F).0.S
        $(MAKE) $(build)=$(@D) $(@D)/.$(@F).0.o
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds -N $< $(build_id_linker) \
-           $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1
+           $(@D)/.$(@F).0.o -o $(@D)/.$(@F).1.tmp
+       $(OBJCOPY) --globalize-symbols=$(@D)/.$(@F).global-syms \
+           $(@D)/.$(@F).1.tmp $(@D)/.$(@F).1
        $(NM) -pa --format=sysv $(@D)/.$(@F).1 \
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort 
$(syms-warn-dup-y) \
                >$(@D)/.$(@F).1.S
        $(MAKE) $(build)=$(@D) $(@D)/.$(@F).1.o
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds -N $< $(build_id_linker) \
-           $(orphan-handling-y) $(@D)/.$(@F).1.o -o $@
+           $(orphan-handling-y) $(@D)/.$(@F).1.o -o $@.tmp
+       $(OBJCOPY) --globalize-symbols=$(@D)/.$(@F).global-syms $@.tmp $@
        $(NM) -pa --format=sysv $(@D)/$(@F) \
                | $(objtree)/tools/symbols --all-symbols --xensyms --sysv 
--sort \
                >$(@D)/$(@F).map
-       rm -f $(@D)/.$(@F).[0-9]* $(@D)/..$(@F).[0-9]*
+       rm -f $(@D)/.$(@F).[0-9]* $(@D)/..$(@F).[0-9]* $(@D)/.$(@F).global-syms
 ifeq ($(CONFIG_XEN_IBT),y)
        $(SHELL) $(srctree)/tools/check-endbr.sh $@
 endif
@@ -266,6 +276,7 @@ $(obj)/xen.lds $(obj)/efi.lds: $(src)/xen.lds.S FORCE
 clean-files := \
     include/asm/asm-macros.* \
     $(objtree)/.xen-syms.[0-9]* \
+    $(objtree)/.xen-syms.global-syms \
     $(objtree)/.xen.elf32 \
     $(objtree)/.xen.efi.[0-9]* \
     efi/*.efi
-- 
2.35.1