Re: [PATCH bpf-next v2 1/3] bpftool: Skip non-autoload programs when generating light skeletons

[Quentin Monnet <qmo@kernel.org>]

On 09/09/2026 05:24, Thiébaud Weksteen wrote:
> When generating a light skeleton (bpftool gen skeleton -L),
> bpf_object__load() skips loading programs marked as non-autoload (e.g.
> SEC("?...")), so the generated loader program only records and populates
> file descriptors for autoloaded programs.
> 
> Previously, bpftool emitted struct bpf_prog_desc fields, link fields,
> and attach/detach/destroy functions for all programs in the BPF object,
> causing the loader program to store subsequent program FDs into
> incorrect skeleton struct fields when non-autoload programs were
> present.
> 
> Furthermore, bpf_object__load() can update a program's autoload status
> during preparation (e.g. for struct_ops programs when resolving kernel
> BTF members or adjusting autoload based on map autocreate settings).
> Move bpf_object__gen_loader() and bpf_object__load() out of gen_trace()
> into do_skeleton() before counting programs and emitting struct fields so
> that struct field declarations and attach/detach/destroy functions all
> observe the final post-load autoload state.
> 
> Skip programs with !bpf_program__autoload(prog) when counting programs
> and generating progs/links struct fields as well as attach, detach, and
> destroy functions for light skeletons.
> 
> Fixes: d510296d331a ("bpftool: Use syscall/loader program in "prog load" and "gen skeleton" command.")
> Signed-off-by: Thiébaud Weksteen <tweek@google.com>
> ---
> Changes since v1:
> - Move bpf_object__gen_loader() and bpf_object__load() out of gen_trace()
> 
>  .../bpf/bpftool/Documentation/bpftool-gen.rst |  4 +-
>  tools/bpf/bpftool/gen.c                       | 64 ++++++++++++-------
>  2 files changed, 44 insertions(+), 24 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/Documentation/bpftool-gen.rst b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> index d0a36f442db7..1cdecf3e4fa5 100644
> --- a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> +++ b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
> @@ -184,7 +184,9 @@ OPTIONS
>  -L, --use-loader
>      For skeletons, generate a "light" skeleton (also known as "loader"
>      skeleton). A light skeleton contains a loader eBPF program. It does not use
> -    the majority of the libbpf infrastructure, and does not need libelf.
> +    the majority of the libbpf infrastructure, and does not need libelf. BPF
> +    programs marked as non-autoload (e.g., via **SEC("?...")**) are skipped and
> +    not included in the generated skeleton.
>  
>  -S, --sign
>      For skeletons, generate a signed skeleton. This option must be used with
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index a50540ef6521..e9a1a018f270 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c

[...]

> @@ -712,19 +721,6 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  	char ident[256];
>  	int err = 0;
>  
> -	if (sign_progs)
> -		opts.gen_hash = true;
> -
> -	err = bpf_object__gen_loader(obj, &opts);
> -	if (err)
> -		return err;
> -
> -	err = bpf_object__load(obj);
> -	if (err) {
> -		p_err("failed to load object file");
> -		goto out;
> -	}
> -
>  	/* If there was no error during load then gen_loader_opts
>  	 * are populated with the loader program.
>  	 */


This comment could be updated (or moved to do_skeleton()).


> @@ -752,7 +748,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  				goto cleanup;				    \n\
>  			skel->ctx.sz = (char *)&skel->links - (char *)skel; \n\
>  		",
> -		obj_name, opts.data_sz);
> +		obj_name, opts->data_sz);
>  	bpf_object__for_each_map(map, obj) {
>  		const void *mmap_data = NULL;
>  		size_t mmap_size = 0;
> @@ -795,22 +791,22 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
>  			static const char opts_data[] __attribute__((__aligned__(8))) = \"\\\n\
>  		",
>  		obj_name);
> -	print_hex(opts.data, opts.data_sz);
> +	print_hex(opts->data, opts->data_sz);
>  	codegen("\
>  		\n\
>  		\";							    \n\
>  			static const char opts_insn[] __attribute__((__aligned__(8))) = \"\\\n\
>  		");
> -	print_hex(opts.insns, opts.insns_sz);
> +	print_hex(opts->insns, opts->insns_sz);
>  	codegen("\
>  		\n\
>  		\";\n");
>  
>  	if (sign_progs) {
> -		sopts.insns = opts.insns;
> -		sopts.insns_sz = opts.insns_sz;
> -		sopts.data = opts.data;
> -		sopts.data_sz = opts.data_sz;
> +		sopts.insns = opts->insns;
> +		sopts.insns_sz = opts->insns_sz;
> +		sopts.data = opts->data;
> +		sopts.data_sz = opts->data_sz;
>  		sopts.excl_prog_hash = prog_sha;
>  		sopts.excl_prog_hash_sz = sizeof(prog_sha);
>  		sopts.signature = sig_buf;
> @@ -1250,6 +1246,7 @@ static int do_skeleton(int argc, char **argv)
>  	char header_guard[MAX_OBJ_NAME_LEN + sizeof("__SKEL_H__")];
>  	size_t map_cnt = 0, prog_cnt = 0, attach_map_cnt = 0, file_sz, mmap_sz;
>  	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
> +	DECLARE_LIBBPF_OPTS(gen_loader_opts, gen_opts);
>  	char obj_name[MAX_OBJ_NAME_LEN] = "", *obj_data;
>  	struct bpf_object *obj = NULL;
>  	const char *file;
> @@ -1326,6 +1323,21 @@ static int do_skeleton(int argc, char **argv)
>  		goto out_obj;
>  	}
>  
> +	if (use_loader) {
> +		if (sign_progs)
> +			gen_opts.gen_hash = true;
> +
> +		err = bpf_object__gen_loader(obj, &gen_opts);
> +		if (err)
> +			goto out;
> +
> +		err = bpf_object__load(obj);
> +		if (err) {
> +			p_err("failed to load object file");
> +			goto out;
> +		}
> +	}


Could you please add a comment above this block to explain why it's at
the current location (to avoid autoload flags update), to avoid people
moving it by accident, please?

Looks good otherwise, thank you!