Re: [PATCH bpf-next v3 6/6] resolve_btfids: change in-place update with raw binary output

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-12-05 at 14:35 -0800, Ihor Solodrai wrote:
> Currently resolve_btfids updates .BTF_ids section of an ELF file
> in-place, based on the contents of provided BTF, usually within the
> same input file, and optionally a BTF base.
> 
> Change resolve_btfids behavior to enable BTF transformations as part
> of its main operation. To achieve this, in-place ELF write in
> resolve_btfids is replaced with generation of the following binaries:
>   * ${1}.BTF with .BTF section data
>   * ${1}.BTF_ids with .BTF_ids section data if it existed in ${1}
>   * ${1}.BTF.base with .BTF.base section data for out-of-tree modules
> 
> The execution of resolve_btfids and consumption of its output is
> orchestrated by scripts/gen-btf.sh introduced in this patch.
> 
> The motivation for emitting binary data is that it allows simplifying
> resolve_btfids implementation by delegating ELF update to the $OBJCOPY
> tool [1], which is already widely used across the codebase.
> 
> There are two distinct paths for BTF generation and resolve_btfids
> application in the kernel build: for vmlinux and for kernel modules.
> 
> For the vmlinux binary a .BTF section is added in a roundabout way to
> ensure correct linking. The patch doesn't change this approach, only
> the implementation is a little different.
> 
> Before this patch it worked as follows:
> 
>   * pahole consumed .tmp_vmlinux1 [2] and added .BTF section with
>     llvm-objcopy [3] to it
>   * then everything except the .BTF section was stripped from .tmp_vmlinux1
>     into a .tmp_vmlinux1.bpf.o object [2], later linked into vmlinux
>   * resolve_btfids was executed later on vmlinux.unstripped [4],
>     updating it in-place
> 
> After this patch gen-btf.sh implements the following:
> 
>   * pahole consumes .tmp_vmlinux1 and produces a *detached* file with
>     raw BTF data
>   * resolve_btfids consumes .tmp_vmlinux1 and detached BTF to produce
>     (potentially modified) .BTF, and .BTF_ids sections data
>   * a .tmp_vmlinux1.bpf.o object is then produced with objcopy copying
>     BTF output of resolve_btfids
>   * .BTF_ids data gets embedded into vmlinux.unstripped in
>     link-vmlinux.sh by objcopy --update-section
> 
> For kernel modules, creating a special .bpf.o file is not necessary,
> and so embedding of sections data produced by resolve_btfids is
> straightforward with objcopy.
> 
> With this patch an ELF file becomes effectively read-only within
> resolve_btfids, which allows deleting elf_update() call and satellite
> code (like compressed_section_fix [5]).
> 
> Endianness handling of .BTF_ids data is also changed. Previously the
> "flags" part of the section was bswapped in sets_patch() [6], and then
> Elf_Type was modified before elf_update() to signal to libelf that
> bswap may be necessary. With this patch we explicitly bswap entire
> data buffer on load and on dump.
> 
> [1] https://lore.kernel.org/bpf/131b4190-9c49-4f79-a99d-c00fac97fa44@linux.dev/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n110
> [3] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/btf_encoder.c?h=v1.31#n1803
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n284
> [5] https://lore.kernel.org/bpf/20200819092342.259004-1-jolsa@kernel.org/
> [6] https://lore.kernel.org/bpf/cover.1707223196.git.vmalik@redhat.com/
> 
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> @@ -552,6 +528,13 @@ static int symbols_collect(struct object *obj)
>  	return 0;
>  }
>  
> +static inline bool is_envvar_set(const char *var_name)
> +{
> +	const char *value = getenv(var_name);
> +
> +	return value && value[0] != '\0';
> +}
> +

This is a leftover, not used anywhere.

[...]

> @@ -860,23 +913,34 @@ int main(int argc, const char **argv)
>  	 */
>  	if (obj.efile.idlist_shndx == -1 ||
>  	    obj.efile.symbols_shndx == -1) {
> -		pr_debug("Cannot find .BTF_ids or symbols sections, nothing to do\n");
> -		err = 0;
> -		goto out;
> +		pr_debug("Cannot find .BTF_ids or symbols sections, skip symbols resolution\n");
> +		goto dump_btf;
>  	}
>  
>  	if (symbols_collect(&obj))
>  		goto out;
>  
> -	if (load_btf(&obj))
> -		goto out;
> -
>  	if (symbols_resolve(&obj))
>  		goto out;
>  
>  	if (symbols_patch(&obj))
>  		goto out;
>  
> +	err = make_out_path(out_path, obj.path, BTF_IDS_SECTION);
> +	if (err || dump_raw_btf_ids(&obj, out_path))
> +		goto out;
> +
> +dump_btf:
> +	err = make_out_path(out_path, obj.path, BTF_ELF_SEC);
> +	if (err || dump_raw_btf(obj.btf, out_path))

Nit: 'err' is not set if dump_raw_btf() errors out.
     Maybe use:

     	   err = make_out_path(out_path, obj.path, BTF_ELF_SEC);
     	   err = err ?: dump_raw_btf(obj.btf, out_path);
	   if (err)
	      goto out;
     ?

> +		goto out;
> +
> +	if (obj.base_btf && obj.distill_base) {
> +		err = make_out_path(out_path, obj.path, BTF_BASE_ELF_SEC);
> +		if (err || dump_raw_btf(obj.base_btf, out_path))
> +			goto out;
> +	}
> +
>  	if (!(fatal_warnings && warnings))
>  		err = 0;
>  out:

[...]