On Mon, Feb 12, 2024 at 6:16 PM Masahiro Yamada masahiroy@kernel.org wrote:
On Tue, Feb 13, 2024 at 11:06 AM Nathan Chancellor nathan@kernel.org wrote:
Commit 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF") changed the ELF type of .btf.vmlinux.bin.o to ET_REL via dd, which works fine for little endian platforms:
00000000 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 |.ELF............| -00000010 03 00 b7 00 01 00 00 00 00 00 00 80 00 80 ff ff |................| +00000010 01 00 b7 00 01 00 00 00 00 00 00 80 00 80 ff ff |................|
However, for big endian platforms, it changes the wrong byte, resulting in an invalid ELF file type, which ld.lld rejects:
00000000 7f 45 4c 46 02 02 01 00 00 00 00 00 00 00 00 00 |.ELF............| -00000010 00 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................| +00000010 01 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
Type: <unknown>: 103
ld.lld: error: .btf.vmlinux.bin.o: unknown file type
Fix this by updating the entire 16-bit e_type field rather than just a single byte, so that everything works correctly for all platforms and linkers.
00000000 7f 45 4c 46 02 02 01 00 00 00 00 00 00 00 00 00 |.ELF............| -00000010 00 03 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................| +00000010 00 01 00 16 00 00 00 01 00 00 00 00 00 10 00 00 |................|
Type: REL (Relocatable file)
While in the area, update the comment to mention that binutils 2.35+ matches LLD's behavior of rejecting an ET_EXEC input, which occurred after the comment was added.
Cc: stable@vger.kernel.org Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF") Link: https://github.com/llvm/llvm-project/pull/75643 Suggested-by: Masahiro Yamada masahiroy@kernel.org Signed-off-by: Nathan Chancellor nathan@kernel.org
Thanks for updating the comment.
Reviewed-by: Fangrui Song maskray@google.com
Thanks.
I will wait for a few days until the reviewers come back to give Reviewed-by again.
Changes in v2:
- Rather than change the seek value for dd, update the entire e_type field (Masahiro). Due to this change, I did not carry forward the tags of v1.
- Slightly update commit message to remove mention of ET_EXEC, which does not match the dump (Masahiro).
- Update comment to mention binutils 2.35+ has the same behavior as LLD (Fangrui).
- Link to v1: https://lore.kernel.org/r/20240208-fix-elf-type-btf-vmlinux-bin-o-big-endian...
scripts/link-vmlinux.sh | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index a432b171be82..7862a8101747 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -135,8 +135,13 @@ gen_btf() ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \ --strip-all ${1} ${2} 2>/dev/null # Change e_type to ET_REL so that it can be used to link final vmlinux.
# Unlike GNU ld, lld does not allow an ET_EXEC input.printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
# GNU ld 2.35+ and lld do not allow an ET_EXEC input.if is_enabled CONFIG_CPU_BIG_ENDIAN; thenet_rel='\0\1'elseet_rel='\1\0'fiprintf "${et_rel}" | dd of=${2} conv=notrunc bs=1 seek=16 status=none}
# Create ${2} .S file with all symbols from the ${1} object file
base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478 change-id: 20240208-fix-elf-type-btf-vmlinux-bin-o-big-endian-dbc55a1e1296
Best regards,
Nathan Chancellor nathan@kernel.org
-- Best Regards Masahiro Yamada