llvm-objcopy: error: unknown argument '--change-section-address'

[nathanchance]

After #779 was resolved for x86_64, I get the following error on 5.6-rc1 with CONFIG_DEBUG_INFO_BTF:

llvm-objcopy: error: unknown argument '--change-section-address'
llvm-objcopy: error: '.btf.vmlinux.bin': No such file or directory
Failed to generate BTF for vmlinux
Try to disable CONFIG_DEBUG_INFO_BTF

This was added in df786c9. Without that commit, the build finishes successfully.

cc @rupprecht

[nickdesaulniers]

cc @fomichev @MaskRay

[nickdesaulniers]

Filed https://llvm.org/pr45217, too.

[nickdesaulniers]

@fomichev has a patch internally I've tested, will be sent upstream shortly.

[nickdesaulniers]

submitted: https://lore.kernel.org/bpf/20200316222518.191601-1-sdf@google.com/T/#u

[MaskRay]

defconfig + CONFIG_DEBUG_INFO_BTF=y. On Debian, it requires apt install dwarves for pahole.

make CC=clang LD=ld.lld O=/tmp/out/x86_64

+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
+ objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o    
objcopy: architecture x86_64 unknown                                       
+ echo 'Failed to generate BTF for vmlinux'

The -B value is incorrect. It should be i386:x86_64. GNU objcopt>=2.34 (https://sourceware.org/bugzilla/show_bug.cgi?id=24968) does not need to specify -B.

A generic --change-section-address is difficult to implement in llvm-objcopy. It can disturb PT_LOAD segments. Currently, only --only-keep-debug can do such sophisticated rewriting of program headers. The semantics of --change-section-vma (ok, this refers to sh_addr) and --change-section-lma (Elf*_Shdr does not record LMA. What does this mean) are not very clear, either.