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

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Message-ID: <55a1fc6b-5129-4017-af17-e0476626b1b1@linux.dev>
Date: Tue, 6 Jan 2026 14:51:43 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Nathan Chancellor <nathan@...nel.org>, 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>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next v2] scripts/gen-btf.sh: Ensure initial object in
 gen_btf_o is ELF with correct endianness

On 1/6/26 2:44 PM, Nathan Chancellor wrote:
> 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} -

Acked-by: Ihor Solodrai <ihor.solodrai@...ux.dev>

Thank you!

>  	${OBJCOPY} --add-section .BTF=${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>
>