Tools

Compile Kernel with WLLVM

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make CC=wllvm defconfig
make CC=wllvm -j$(nproc)

Extract Bitcode

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extract-bc vmlinux

Then vmlinux.bc is the bitcode.

Cross-compile

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BINUTILS_TARGET_PREFIX=aarch64-linux-gnu make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTCC=clang CC=wllvm defconfig
BINUTILS_TARGET_PREFIX=aarch64-linux-gnu make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- HOSTCC=clang CC=wllvm -j$(nproc)

Then, extract bitcode from vmlinux.

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extract-bc vmlinux

Problems

symbol multiply defined during bitcode extraction

When extracting bitcode from vmlinux (arm64), an error occurs:

error: Linking globals named ‘sort’: symbol multiply defined!

Get all .bc file names by

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extract-bc -m vmlinux

and it generates vmlinux.llvm.manifest, which is something like:

/home/xx/dev/linux-5.3.6-arm-llvm/init/.main.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.version.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.do_mounts.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.do_mounts_initrd.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.initramfs.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.calibrate.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/init/.init_task.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/arch/arm64/kernel/.debug-monitors.o.bc
/home/xx/dev/linux-5.3.6-arm-llvm/arch/arm64/kernel/.irq.o.bc

And then use a script to find all code that defines function sort:

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#!/bin/sh
cat vmlinux.llvm.manifest |
    while read line;
    do
        llvm-dis-9 "$line" -o .llvm.tmp.ll;
        cat .llvm.tmp.ll | grep -n '@sort(' | grep define | sed "s|^|${line}: |";
    done

We find that sort has been defined in both lib/.sort.o.bc and drivers/firmware/efi/libstub/.lib-sort.o.bc.

Read drivers/firmware/efi/libstub/Makefile and find that it first compiles all c files in lib/ to lib-*.o by

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$(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
	$(call if_changed_rule,cc_o_c)

and then use objcopy to add __efistub_ prefix to them.

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$(obj)/%.stub.o: $(obj)/%.o FORCE
	$(call if_changed,stubcopy)

#
# Strip debug sections and some other sections that may legally contain
# absolute relocations, so that we can inspect the remaining sections for
# such relocations. If none are found, regenerate the output object, but
# this time, use objcopy and leave all sections in place.
#
quiet_cmd_stubcopy = STUBCPY $@
      cmd_stubcopy =							\
	$(STRIP) --strip-debug -o $@ $<;				\
	if $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y); then		\
		echo "$@: absolute symbol references not allowed in the EFI stub" >&2; \
		/bin/false;						\
	fi;								\
	$(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@

However, it doesn’t change the corresponding .bc, so error occurs.

If we don’t care about efi, simply remove drivers/firmware/efi/libstub/.*.o.bc in vmlinux.llvm.manifest and then

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llvm-link -o vmlinux.bc `cat vmlinux.llvm.manifest`

can solve this problem.