linux-kernel - Re: [PATCH bpf-next] scripts/gen-btf.sh: Disable LTO when generating initial .o file

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Message-ID: <ff8187bd-0bae-4b49-8844-6c975a2e79c6@linux.dev>
Date: Mon, 5 Jan 2026 14:01:36 -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] scripts/gen-btf.sh: Disable LTO when generating
 initial .o file

On 1/5/26 1:12 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.
> 
> Fixes: 600605853f87 ("scripts/gen-btf.sh: Fix .btf.o generation when compiling for RISCV")
> Signed-off-by: Nathan Chancellor <nathan@...nel.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..08b46b91c04b 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_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}

Hi Nathan, thank you for the patch.

I'm starting to think it wasn't a good idea to do

	echo "" | ${CC} ...

here, given the number of associated bugs.

Before gen-btf.sh was introduced, the .btf.o binary was generated with this [1]:

	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
		--strip-all ${1} "${btf_data}" 2>/dev/null

I changed to ${CC} on the assumption it's a quicker operation than
stripping entire vmlinux. But maybe it's not worth it and we should
change back to --strip-all? wdyt?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n110

> 
> ---
> base-commit: a069190b590e108223cd841a1c2d0bfb92230ecc
> change-id: 20260105-fix-gen-btf-sh-lto-007fe4908070
> 
> Best regards,
> --  
> Nathan Chancellor <nathan@...nel.org>
>