CONFIG_DEBUG_INFO_BTF does not work with llvm-objdump + llvm-objcopy

[nathanchance]

I came across a patch on the netdev mailing list regarding GNU objcopy and I wanted to make sure that llvm-obcopy would not regress. As it turns out, llvm-objcopy does not work at all on either x86_64 or aarch64 (I have not tested anything else).

x86_64 failure (defconfig + CONFIG_DEBUG_INFO + CONFIG_DEBUG_INFO_BTF):

$ make -j$(nproc) -s CC=clang LD=ld.lld OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump O=out.x86_64 distclean defconfig vmlinux
...
llvm-objcopy: error: invalid output format: 'ELF64-x86-64'

AArch64 failure (defconfig + CONFIG_DEBUG_INFO_BTF):

$ make -j$(nproc) -s ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu- LD=ld.lld NM=llvm-nm OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump O=out.aarch64 distclean defconfig vmlinux
...
llvm-objcopy: error: invalid output format: 'ELF64-aarch64-little'

The relevant snippet of code from scripts/link-vmlinux.sh:

# generate .BTF typeinfo from DWARF debuginfo
# ${1} - vmlinux image
# ${2} - file to dump raw BTF data into
gen_btf()
{
	local pahole_ver
	local bin_arch

	if ! [ -x "$(command -v ${PAHOLE})" ]; then
		info "BTF" "${1}: pahole (${PAHOLE}) is not available"
		return 1
	fi

	pahole_ver=$(${PAHOLE} --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/')
	if [ "${pahole_ver}" -lt "113" ]; then
		info "BTF" "${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.13"
		return 1
	fi

	info "BTF" ${2}
	vmlinux_link ${1}
	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}

	# dump .BTF section into raw binary file to link with final vmlinux
	bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \
		cut -d, -f1 | cut -d' ' -f2)
	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
		awk '{print $4}')
	${OBJCOPY} --dump-section .BTF=.btf.vmlinux.bin ${1} 2>/dev/null
	${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
		--rename-section .data=.BTF .btf.vmlinux.bin ${2}
}

They are using the output from ${OBJDUMP} to set the -O flag on ${OBJCOPY}. With the GNU tools, these appear to be mapped 1:1 but that does not appear to be the case for LLVM.

For x86_64: llvm-objdump's file format is ELF64-x86-64 but llvm-objcopy expects an -O value of elf64-x86-64.

For AArch64 : llvm-objdump's file format is ELF64-aarch64-little but llvm-objcopy expects an -O value of elf64-littleaarch64.

I am not sure how to reconcile this.

NOTE: To reproduce this, you will need pahole 1.13 or newer. I just did this:

git clone git://git.kernel.org/pub/scm/devel/pahole/pahole.git
mkdir pahole/build
cd pahole/build
cmake -D__LIB=lib ..
make -j$(nproc)

and just added that build folder to my PATH to avoid actually installing it.

[tpimh]

Maybe this should be fixed on LLVM side? Probably the easiest way to fix it is to change llvm-objdump output to match the expected -O value for llvm-objcopy.

[nathanchance]

cc @rupprecht, I thought I did that initially but I forgot.

[rupprecht]

https://reviews.llvm.org/D74433 is one possible solution. There are other ways that might make this work, so we'll see which one sticks.

[rupprecht]

Also @MaskRay fyi

[MaskRay]

I vote for updating all the llvm::object::ObjectFile::getFileFormatName() derivatives to use lower case.

Please make comments if you agree so: https://reviews.llvm.org/D74433

[rupprecht]

Should be fixed as of 60a8a504f16dbbc5f2a6887ecb668ef4cb834949, lemme know if not.

[nathanchance]

x86_64 is fixed (although running into a different issue that I will post here shortly).

AArch64 still fails because llvm-objdump outputs elf64-aarch64-little but llvm-objcopy expects elf64-littleaarch64.

[nickdesaulniers]

@rupprecht thoughts on aarch64?

[rupprecht]

I agree they should be compatible. For ELF it was fortunately just a casing issue, but the aarch64 difference is annoying. The two choices are:

• Modify llvm-objdump to print a GNU objdump compatible name
• Change llvm-objcopy to allow either bfd or llvm style names

I'm not sure which one I'd prefer, because they each have their pros/cons -- on one hand, we don't need to inherit so much legacy bfd naming. On the other hand, we do want llvm-objdump to produce GNU objdump compatible output. Either way, though, we should have llvm-objdump and llvm-objcopy be on the same page.

[MaskRay]

Jordan asked me to finish this:/

https://reviews.llvm.org/D76046 will update ARM and AArch64 to match BFD names.

[MaskRay]

https://reviews.llvm.org/D76046 made llvm-objdump's "file format" line match ARM/AArch64/RISCV and big-endian PPC32/PPC64.

[nathanchance]

With the commit causing #871 reverted, BTF can be successfully generated for aarch64 with LLVM at llvm/llvm-project@0616e99.