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: 9a94f277c4fb ("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

1 files changed, 21 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/test_subprogs_unused.c b/tools/testing/selftests/bpf/progs/test_subprogs_unused.c
new file mode 100644
index 000000000000..75d975f8cf90
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_subprogs_unused.c
@@ -0,0 +1,21 @@
+#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;
+}