linux-kernel - Re: [RFC PATCH v7 3/7] tools/resolve_btfids: Add --btf

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Message-ID: <7c04a6c3010ce41fc7ad0a6b26c94f43dde82593.camel@gmail.com>
Date: Thu, 20 Nov 2025 16:18:55 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Donglin Peng <dolinux.peng@...il.com>, ast@...nel.org, 
	andrii.nakryiko@...il.com
Cc: zhangxiaoqin@...omi.com, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org,  Donglin Peng <pengdonglin@...omi.com>, Alan Maguire
 <alan.maguire@...cle.com>, Song Liu <song@...nel.org>
Subject: Re: [RFC PATCH v7 3/7] tools/resolve_btfids: Add --btf_sort option
 for BTF name sorting

On Wed, 2025-11-19 at 11:15 +0800, Donglin Peng wrote:

[...]

> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index db76335dd917..d5eb4ee70e88 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -27,6 +27,7 @@ pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes
>  
>  ifneq ($(KBUILD_EXTMOD),)
>  module-pahole-flags-$(call test-ge, $(pahole-ver), 128) += --btf_features=distilled_base
> +module-resolve_btfid-flags-y = --distilled_base
                                  ^^^^^^^^^^^^^^^^
This flag should be guarded by pahole version as well.  However, I'd
suggest not adding this flag at all, but instead modify resolve_btfids
to check for BTF_BASE_ELF_SEC section existence and acting accordingly.

>  endif
>  
>  endif
> @@ -35,3 +36,4 @@ pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
>  
>  export PAHOLE_FLAGS := $(pahole-flags-y)
>  export MODULE_PAHOLE_FLAGS := $(module-pahole-flags-y)
> +export MODULE_RESOLVE_BTFID_FLAGS := $(module-resolve_btfid-flags-y)
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index 542ba462ed3e..4481dda2f485 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -40,6 +40,7 @@ quiet_cmd_btf_ko = BTF [M] $@
>  		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
>  	else								\
>  		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) $(MODULE_PAHOLE_FLAGS) --btf_base $(objtree)/vmlinux $@; \
> +		$(RESOLVE_BTFIDS) -b $(objtree)/vmlinux $(MODULE_RESOLVE_BTFID_FLAGS) --btf_sort $@;	\
                                                                              ^^^^^^^^^^
                                             Agree with Ihor (and off-list discussion with Alexei),
                                             this flag appears redundant. Are there situations when
                                             one would like to avoid sorting kernel/module BTF?

>  		$(RESOLVE_BTFIDS) -b $(objtree)/vmlinux $@;		\
>  	fi;

[...]

> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index d47191c6e55e..dc0badd6f375 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c

[...]

> +static int update_btf_section(const char *path, const struct btf *btf,
> +				  const char *btf_secname)
> +{
> +	GElf_Shdr shdr_mem, *shdr;
> +	Elf_Data *btf_data = NULL;
> +	Elf_Scn *scn = NULL;
> +	Elf *elf = NULL;
> +	const void *raw_btf_data;
> +	uint32_t raw_btf_size;
> +	int fd, err = -1;
> +	size_t strndx;
> +
> +	fd = open(path, O_RDWR);
> +	if (fd < 0) {
> +		pr_err("FAILED to open %s\n", path);
> +		return -1;
> +	}
> +
> +	if (elf_version(EV_CURRENT) == EV_NONE) {
> +		pr_err("FAILED to set libelf version");
> +		goto out;
> +	}
> +
> +	elf = elf_begin(fd, ELF_C_RDWR, NULL);
> +	if (elf == NULL) {
> +		pr_err("FAILED to update ELF file");
> +		goto out;
> +	}
> +
> +	elf_flagelf(elf, ELF_C_SET, ELF_F_LAYOUT);
> +
> +	elf_getshdrstrndx(elf, &strndx);
> +	while ((scn = elf_nextscn(elf, scn)) != NULL) {
> +		char *secname;
> +
> +		shdr = gelf_getshdr(scn, &shdr_mem);
> +		if (shdr == NULL)
> +			continue;
> +		secname = elf_strptr(elf, strndx, shdr->sh_name);
> +		if (strcmp(secname, btf_secname) == 0) {
> +			btf_data = elf_getdata(scn, btf_data);
> +			break;
> +		}
> +	}
> +
> +	raw_btf_data = btf__raw_data(btf, &raw_btf_size);

`btf__raw_data()` can return NULL.

> +
> +	if (btf_data) {
> +		if (raw_btf_size != btf_data->d_size) {
> +			pr_err("FAILED: size mismatch");
> +			goto out;
> +		}
> +
> +		btf_data->d_buf = (void *)raw_btf_data;
> +		btf_data->d_type = ELF_T_WORD;
> +		elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
> +
> +		if (elf_update(elf, ELF_C_WRITE) >= 0)
> +			err = 0;
> +	}
> +
> +out:
> +	if (fd != -1)
> +		close(fd);
> +	if (elf)
> +		elf_end(elf);
> +	return err;
> +}

[...]