bpf: add a new API libbpf_num_possible_cpus()

Adding a new API libbpf_num_possible_cpus() that helps user with
per-CPU map operations.
Signed-off-by: Hechao Li <hechaol@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -3827,3 +3827,60 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
 					     desc->array_offset, addr);
 	}
 }
+
+int libbpf_num_possible_cpus(void)
+{
+	static const char *fcpu = "/sys/devices/system/cpu/possible";
+	int len = 0, n = 0, il = 0, ir = 0;
+	unsigned int start = 0, end = 0;
+	static int cpus;
+	char buf[128];
+	int error = 0;
+	int fd = -1;
+
+	if (cpus > 0)
+		return cpus;
+
+	fd = open(fcpu, O_RDONLY);
+	if (fd < 0) {
+		error = errno;
+		pr_warning("Failed to open file %s: %s\n",
+			   fcpu, strerror(error));
+		return -error;
+	}
+	len = read(fd, buf, sizeof(buf));
+	close(fd);
+	if (len <= 0) {
+		error = len ? errno : EINVAL;
+		pr_warning("Failed to read # of possible cpus from %s: %s\n",
+			   fcpu, strerror(error));
+		return -error;
+	}
+	if (len == sizeof(buf)) {
+		pr_warning("File %s size overflow\n", fcpu);
+		return -EOVERFLOW;
+	}
+	buf[len] = '\0';
+
+	for (ir = 0, cpus = 0; ir <= len; ir++) {
+		/* Each sub string separated by ',' has format \d+-\d+ or \d+ */
+		if (buf[ir] == ',' || buf[ir] == '\0') {
+			buf[ir] = '\0';
+			n = sscanf(&buf[il], "%u-%u", &start, &end);
+			if (n <= 0) {
+				pr_warning("Failed to get # CPUs from %s\n",
+					   &buf[il]);
+				return -EINVAL;
+			} else if (n == 1) {
+				end = start;
+			}
+			cpus += end - start + 1;
+			il = ir + 1;
+		}
+	}
+	if (cpus <= 0) {
+		pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
+		return -EINVAL;
+	}
+	return cpus;
+}

tools/lib/bpf/libbpf.h

@@ -454,6 +454,22 @@ bpf_program__bpil_addr_to_offs(struct bpf_prog_info_linear *info_linear);
 LIBBPF_API void
 bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear);
 
+/*
+ * A helper function to get the number of possible CPUs before looking up
+ * per-CPU maps. Negative errno is returned on failure.
+ *
+ * Example usage:
+ *
+ *     int ncpus = libbpf_num_possible_cpus();
+ *     if (ncpus < 0) {
+ *          // error handling
+ *     }
+ *     long values[ncpus];
+ *     bpf_map_lookup_elem(per_cpu_map_fd, key, values);
+ *
+ */
+LIBBPF_API int libbpf_num_possible_cpus(void);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif

tools/lib/bpf/libbpf.map

@@ -172,4 +172,5 @@ LIBBPF_0.0.4 {
 		btf_dump__new;
 		btf__parse_elf;
 		bpf_object__load_xattr;
+		libbpf_num_possible_cpus;
 } LIBBPF_0.0.3;