[PATCH] kbuild: Fix changing ELF file type for output of gen_btf for big endian

Nathan Chancellor
2024-02-08T20:21:32Z

Commit 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
changed the ELF type of .btf.vmlinux.bin.o from ET_EXEC 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 using a different seek value for dd when targeting big
endian, so that the correct byte gets changed and everything works
correctly for all 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)

Cc: stable@vger.kernel.org
Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
Link: https://github.com/llvm/llvm-project/pull/75643
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
scripts/link-vmlinux.sh | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index a432b171be82..8a9f48b3cb32 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -135,8 +135,15 @@ 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
+ # Unlike GNU ld, lld does not allow an ET_EXEC input. Make sure the correct
+ # byte gets changed with big endian platforms, otherwise e_type may be an
+ # invalid value.
+ if is_enabled CONFIG_CPU_BIG_ENDIAN; then
+ seek=17
+ else
+ seek=16
+ fi
+ printf '\1' | dd of=${2} conv=notrunc bs=1 seek=${seek} 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>