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

["Thiébaud Weksteen" <tweek@google.com>]

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
@@ -583,6 +583,9 @@ static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 	bpf_object__for_each_program(prog, obj) {
 		const char *tp_name;
 
+		if (!bpf_program__autoload(prog))
+			continue;
+
 		codegen("\
 			\n\
 			\n\
@@ -629,6 +632,8 @@ static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 		", obj_name);
 
 	bpf_object__for_each_program(prog, obj) {
+		if (!bpf_program__autoload(prog))
+			continue;
 		codegen("\
 			\n\
 				ret = ret < 0 ? ret : %1$s__%2$s__attach(skel);   \n\
@@ -646,6 +651,8 @@ static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 		", obj_name);
 
 	bpf_object__for_each_program(prog, obj) {
+		if (!bpf_program__autoload(prog))
+			continue;
 		codegen("\
 			\n\
 				skel_closenz(skel->links.%1$s_fd);	    \n\
@@ -676,6 +683,8 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 		obj_name);
 
 	bpf_object__for_each_program(prog, obj) {
+		if (!bpf_program__autoload(prog))
+			continue;
 		codegen("\
 			\n\
 				skel_closenz(skel->progs.%1$s.prog_fd);	    \n\
@@ -701,9 +710,9 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 		obj_name);
 }
 
-static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *header_guard)
+static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *header_guard,
+		     const struct gen_loader_opts *opts)
 {
-	DECLARE_LIBBPF_OPTS(gen_loader_opts, opts);
 	struct bpf_load_and_run_opts sopts = {};
 	char sig_buf[MAX_SIG_SIZE];
 	__u8 prog_sha[SHA256_DIGEST_LENGTH];
@@ -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.
 	 */
@@ -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;
+		}
+	}
+
 	bpf_object__for_each_map(map, obj) {
 		if (!get_map_ident(map, ident, sizeof(ident))) {
 			p_err("ignoring unrecognized internal map '%s'...",
@@ -1339,6 +1351,8 @@ static int do_skeleton(int argc, char **argv)
 		map_cnt++;
 	}
 	bpf_object__for_each_program(prog, obj) {
+		if (use_loader && !bpf_program__autoload(prog))
+			continue;
 		prog_cnt++;
 	}
 
@@ -1402,6 +1416,8 @@ static int do_skeleton(int argc, char **argv)
 	if (prog_cnt) {
 		printf("\tstruct {\n");
 		bpf_object__for_each_program(prog, obj) {
+			if (use_loader && !bpf_program__autoload(prog))
+				continue;
 			if (use_loader)
 				printf("\t\tstruct bpf_prog_desc %s;\n",
 				       bpf_program__name(prog));
@@ -1415,6 +1431,8 @@ static int do_skeleton(int argc, char **argv)
 	if (prog_cnt + attach_map_cnt) {
 		printf("\tstruct {\n");
 		bpf_object__for_each_program(prog, obj) {
+			if (use_loader && !bpf_program__autoload(prog))
+				continue;
 			if (use_loader)
 				printf("\t\tint %s_fd;\n",
 				       bpf_program__name(prog));
@@ -1451,7 +1469,7 @@ static int do_skeleton(int argc, char **argv)
 			goto out;
 	}
 	if (use_loader) {
-		err = gen_trace(obj, obj_name, header_guard);
+		err = gen_trace(obj, obj_name, header_guard, &gen_opts);
 		goto out;
 	}
 
-- 
2.55.0.979.g7e5102b832-goog