libbpf: Don't attempt to load unused subprog as an entry-point BPF program

If BPF code contains unused BPF subprogram and there are no other subprogram
calls (which can realistically happen in real-world applications given
sufficiently smart Clang code optimizations), libbpf will erroneously assume
that subprograms are entry-point programs and will attempt to load them with
UNSPEC program type.

Fix by not relying on subcall instructions and rather detect it based on the
structure of BPF object's sections.

Fixes: 9a94f277 ("tools: libbpf: restore the ability to load programs from .text section")
Reported-by: Dmitrii Banshchikov <dbanschikov@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201107000251.256821-1-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -560,8 +560,6 @@ bpf_object__init_prog(struct bpf_object *obj, struct bpf_program *prog,
 		      const char *name, size_t sec_idx, const char *sec_name,
 		      size_t sec_off, void *insn_data, size_t insn_data_sz)
 {
-	int i;
-
 	if (insn_data_sz == 0 || insn_data_sz % BPF_INSN_SZ || sec_off % BPF_INSN_SZ) {
 		pr_warn("sec '%s': corrupted program '%s', offset %zu, size %zu\n",
 			sec_name, name, sec_off, insn_data_sz);
@@ -600,13 +598,6 @@ bpf_object__init_prog(struct bpf_object *obj, struct bpf_program *prog,
 		goto errout;
 	memcpy(prog->insns, insn_data, insn_data_sz);
 
-	for (i = 0; i < prog->insns_cnt; i++) {
-		if (insn_is_subprog_call(&prog->insns[i])) {
-			obj->has_subcalls = true;
-			break;
-		}
-	}
-
 	return 0;
 errout:
 	pr_warn("sec '%s': failed to allocate memory for prog '%s'\n", sec_name, name);
@@ -3280,7 +3271,19 @@ bpf_object__find_program_by_title(const struct bpf_object *obj,
 static bool prog_is_subprog(const struct bpf_object *obj,
 			    const struct bpf_program *prog)
 {
-	return prog->sec_idx == obj->efile.text_shndx && obj->has_subcalls;
+	/* For legacy reasons, libbpf supports an entry-point BPF programs
+	 * without SEC() attribute, i.e., those in the .text section. But if
+	 * there are 2 or more such programs in the .text section, they all
+	 * must be subprograms called from entry-point BPF programs in
+	 * designated SEC()'tions, otherwise there is no way to distinguish
+	 * which of those programs should be loaded vs which are a subprogram.
+	 * Similarly, if there is a function/program in .text and at least one
+	 * other BPF program with custom SEC() attribute, then we just assume
+	 * .text programs are subprograms (even if they are not called from
+	 * other programs), because libbpf never explicitly supported mixing
+	 * SEC()-designated BPF programs and .text entry-point BPF programs.
+	 */
+	return prog->sec_idx == obj->efile.text_shndx && obj->nr_programs > 1;
 }
 
 struct bpf_program *

tools/testing/selftests/bpf/prog_tests/subprogs.c

@@ -3,12 +3,14 @@
 #include <test_progs.h>
 #include <time.h>
 #include "test_subprogs.skel.h"
+#include "test_subprogs_unused.skel.h"
 
 static int duration;
 
 void test_subprogs(void)
 {
 	struct test_subprogs *skel;
+	struct test_subprogs_unused *skel2;
 	int err;
 
 	skel = test_subprogs__open_and_load();
@@ -26,6 +28,10 @@ void test_subprogs(void)
 	CHECK(skel->bss->res3 != 19, "res3", "got %d, exp %d\n", skel->bss->res3, 19);
 	CHECK(skel->bss->res4 != 36, "res4", "got %d, exp %d\n", skel->bss->res4, 36);
 
+	skel2 = test_subprogs_unused__open_and_load();
+	ASSERT_OK_PTR(skel2, "unused_progs_skel");
+	test_subprogs_unused__destroy(skel2);
+
 cleanup:
 	test_subprogs__destroy(skel);
 }

tools/testing/selftests/bpf/progs/test_subprogs_unused.c

+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+
+const char LICENSE[] SEC("license") = "GPL";
+
+__attribute__((maybe_unused)) __noinline int unused1(int x)
+{
+	return x + 1;
+}
+
+static __attribute__((maybe_unused)) __noinline int unused2(int x)
+{
+	return x + 2;
+}
+
+SEC("raw_tp/sys_enter")
+int main_prog(void *ctx)
+{
+	return 0;
+}