libbpf-tools: fix fentry_try_attach bug

the bpf verifier would complain on program exit without initing R0 but
the buggy implementation makes up one BPF_EXIT instruction only, which
would be rejected by the verifier, and what's worse is that it does not
log any err about the try-to-load failure, which makes all tools use this
api would silently fall back to kprobe unexpectedly.

this patch fixs it and also prints verbose message about the result of the try.

Signed-off-by: Ze Gao <[email protected]>

Author: Ze Gao

libbpf-tools/trace_helpers.c

@@ -993,16 +993,25 @@ bool is_kernel_module(const char *name)
 
 static bool fentry_try_attach(int id)
 {
-	struct bpf_insn insns[] = { { .code = BPF_JMP | BPF_EXIT } };
-	LIBBPF_OPTS(bpf_prog_load_opts, opts);
 	int prog_fd, attach_fd;
-
-	opts.expected_attach_type = BPF_TRACE_FENTRY;
-	opts.attach_btf_id = id,
-
-	prog_fd = bpf_prog_load(BPF_PROG_TYPE_TRACING, "test", NULL, insns, 1, &opts);
-	if (prog_fd < 0)
+	char error[4096];
+	struct bpf_insn insns[] = {
+		{ .code = BPF_ALU64 | BPF_MOV | BPF_K, .dst_reg = BPF_REG_0, .imm = 0 },
+		{ .code = BPF_JMP | BPF_EXIT },
+	};
+	LIBBPF_OPTS(bpf_prog_load_opts, opts,
+			.expected_attach_type = BPF_TRACE_FENTRY,
+			.attach_btf_id = id,
+			.log_buf = error,
+			.log_size = sizeof(error),
+	);
+
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_TRACING, "test", "GPL", insns,
+			sizeof(insns) / sizeof(struct bpf_insn), &opts);
+	if (prog_fd < 0) {
+		fprintf(stderr, "failed to try attaching to fentry: %s\n", error);
 		return false;
+	}
 
 	attach_fd = bpf_raw_tracepoint_open(NULL, prog_fd);
 	if (attach_fd >= 0)