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@kernel.org>
Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260106-fix-gen-btf-sh-lto-v2-1-01d3e1c241c4@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/scripts/gen-btf.sh b/scripts/gen-btf.sh
index d6457661b9b67fda059dbd1a49c15493035636bc..be21ccee3487cfb4184312f6259eda3d1ec443cd 100755 (executable)
--- 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}