Implement read_cgroup_file() helper to read from cgroup control files,
e.g. statistics.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 tools/testing/selftests/bpf/cgroup_helpers.c | 39 ++++++++++++++++++++
 tools/testing/selftests/bpf/cgroup_helpers.h |  2 +
 2 files changed, 41 insertions(+)

diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index e4535451322e..3ffd4b764f91 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -125,6 +125,45 @@ int enable_controllers(const char *relative_path, const char *controllers)
 	return __enable_controllers(cgroup_path, controllers);
 }
 
+static size_t __read_cgroup_file(const char *cgroup_path, const char *file,
+				 char *buf, size_t size)
+{
+	char file_path[PATH_MAX + 1];
+	size_t ret;
+	int fd;
+
+	snprintf(file_path, sizeof(file_path), "%s/%s", cgroup_path, file);
+	fd = open(file_path, O_RDONLY);
+	if (fd < 0) {
+		log_err("Opening %s", file_path);
+		return -1;
+	}
+
+	ret = read(fd, buf, size);
+	close(fd);
+	return ret;
+}
+
+/**
+ * read_cgroup_file() - Read to a cgroup file
+ * @relative_path: The cgroup path, relative to the workdir
+ * @file: The name of the file in cgroupfs to read to
+ * @buf: Buffer to read from the file
+ * @size: Size of the buffer
+ *
+ * Read to a file in the given cgroup's directory.
+ *
+ * If successful, the number of read bytes is returned.
+ */
+size_t read_cgroup_file(const char *relative_path, const char *file,
+			char *buf, size_t size)
+{
+	char cgroup_path[PATH_MAX - 24];
+
+	format_cgroup_path(cgroup_path, relative_path);
+	return __read_cgroup_file(cgroup_path, file, buf, size);
+}
+
 static int __write_cgroup_file(const char *cgroup_path, const char *file,
 			       const char *buf)
 {
diff --git a/tools/testing/selftests/bpf/cgroup_helpers.h b/tools/testing/selftests/bpf/cgroup_helpers.h
index 502845160d88..821cb76db1f7 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.h
+++ b/tools/testing/selftests/bpf/cgroup_helpers.h
@@ -11,6 +11,8 @@
 
 /* cgroupv2 related */
 int enable_controllers(const char *relative_path, const char *controllers);
+size_t read_cgroup_file(const char *relative_path, const char *file,
+			char *buf, size_t size);
 int write_cgroup_file(const char *relative_path, const char *file,
 		      const char *buf);
 int write_cgroup_file_parent(const char *relative_path, const char *file,
-- 
2.50.1