linux-kernel - [PATCH bpf-next v2] scripts/gen-btf.sh: Ensure initial object in gen_btf

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Message-Id: <20260106-fix-gen-btf-sh-lto-v2-1-01d3e1c241c4@kernel.org>
Date: Tue, 06 Jan 2026 15:44:20 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Alexei Starovoitov <ast@...nel.org>, 
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Eduard Zingerman <eddyz87@...il.com>, 
 Yonghong Song <yonghong.song@...ux.dev>, 
 Ihor Solodrai <ihor.solodrai@...ux.dev>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH bpf-next v2] scripts/gen-btf.sh: Ensure initial object in
 gen_btf_o is ELF with correct endianness

After commit 600605853f87 ("scripts/gen-btf.sh: Fix .btf.o generation
when compiling for RISCV"), there is an error from llvm-objcopy when
CONFIG_LTO_CLANG is enabled:

  llvm-objcopy: error: '.tmp_vmlinux1.btf.o': The file was not recognized as a valid object file
  Failed to generate BTF for vmlinux

KBUILD_CFLAGS includes CC_FLAGS_LTO, which makes clang emit an LLVM IR
object, rather than an ELF one as expected by llvm-objcopy.

Most areas of the kernel deal with this by filtering out CC_FLAGS_LTO
from KBUILD_CFLAGS for the particular object or directory but this is
not so easy to do in bash. Just include '-fno-lto' after KBUILD_CFLAGS
to ensure an ELF object is consistently created as the initial .o file.

Additionally, while there is no reported or discovered bug yet, the
absence of KBUILD_CPPFLAGS from this command could result in incorrect
endianness because KBUILD_CPPFLAGS typically contains '-mbig-endian' and
'-mlittle-endian' so that biendian toolchains can be used. Include it in
this ${CC} command to hopefully limit necessary changes to this command
for the foreseeable future.

Fixes: 600605853f87 ("scripts/gen-btf.sh: Fix .btf.o generation when compiling for RISCV")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
Changes in v2:
- Conservatively include KBUILD_CPPFLAGS as well, as it contains
  endianness flags for some targets.
- Link to v1: https://patch.msgid.link/20260105-fix-gen-btf-sh-lto-v1-1-18052ea055a9@kernel.org
---
 scripts/gen-btf.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/gen-btf.sh b/scripts/gen-btf.sh
index d6457661b9b6..be21ccee3487 100755
--- a/scripts/gen-btf.sh
+++ b/scripts/gen-btf.sh
@@ -87,7 +87,7 @@ gen_btf_o()
 	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
 	# deletes all symbols including __start_BTF and __stop_BTF, which will
 	# be redefined in the linker script.
-	echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CFLAGS} -c -x c -o ${btf_data} -
+	echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} -fno-lto -c -x c -o ${btf_data} -
 	${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \
 		--set-section-flags .BTF=alloc,readonly ${btf_data}
 	${OBJCOPY} --only-section=.BTF --strip-all ${btf_data}

---
base-commit: a069190b590e108223cd841a1c2d0bfb92230ecc
change-id: 20260105-fix-gen-btf-sh-lto-007fe4908070

Best regards,
--  
Nathan Chancellor <nathan@...nel.org>