libbpf: Allow specification of "kprobe/function+offset"

kprobes can be placed on most instructions in a function, not
just entry, and ftrace and bpftrace support the function+offset
notification for probe placement.  Adding parsing of func_name
into func+offset to bpf_program__attach_kprobe() allows the
user to specify

SEC("kprobe/bpf_fentry_test5+0x6")

...for example, and the offset can be passed to perf_event_open_probe()
to support kprobe attachment.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210714094400.396467-8-jolsa@kernel.org

tools/lib/bpf/libbpf.c

@@ -10348,6 +10348,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
 
 struct bpf_program_attach_kprobe_opts {
 	bool retprobe;
+	unsigned long offset;
 };
 
 static struct bpf_link*
@@ -10360,7 +10361,7 @@ bpf_program__attach_kprobe_opts(struct bpf_program *prog,
 	int pfd, err;
 
 	pfd = perf_event_open_probe(false /* uprobe */, opts->retprobe, func_name,
-				    0 /* offset */, -1 /* pid */);
+				    opts->offset, -1 /* pid */);
 	if (pfd < 0) {
 		pr_warn("prog '%s': failed to create %s '%s' perf event: %s\n",
 			prog->name, opts->retprobe ? "kretprobe" : "kprobe", func_name,
@@ -10394,12 +10395,31 @@ static struct bpf_link *attach_kprobe(const struct bpf_sec_def *sec,
 				      struct bpf_program *prog)
 {
 	struct bpf_program_attach_kprobe_opts opts;
+	unsigned long offset = 0;
+	struct bpf_link *link;
 	const char *func_name;
+	char *func;
+	int n, err;
 
 	func_name = prog->sec_name + sec->len;
 	opts.retprobe = strcmp(sec->sec, "kretprobe/") == 0;
 
-	return bpf_program__attach_kprobe_opts(prog, func_name, &opts);
+	n = sscanf(func_name, "%m[a-zA-Z0-9_.]+%lx", &func, &offset);
+	if (n < 1) {
+		err = -EINVAL;
+		pr_warn("kprobe name is invalid: %s\n", func_name);
+		return libbpf_err_ptr(err);
+	}
+	if (opts.retprobe && offset != 0) {
+		err = -EINVAL;
+		pr_warn("kretprobes do not support offset specification\n");
+		return libbpf_err_ptr(err);
+	}
+
+	opts.offset = offset;
+	link = bpf_program__attach_kprobe_opts(prog, func, &opts);
+	free(func);
+	return link;
 }
 
 struct bpf_link *bpf_program__attach_uprobe(struct bpf_program *prog,