[PATCH bpf-next v3 11/17] bpf: selftests: introduce read_cgroup_file() helper

[Roman Gushchin <roman.gushchin@linux.dev>]

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 | 45 ++++++++++++++++++++
 tools/testing/selftests/bpf/cgroup_helpers.h |  3 ++
 2 files changed, 48 insertions(+)

diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index 20cede4db3ce..fc5f22409ce5 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -126,6 +126,51 @@ int enable_controllers(const char *relative_path, const char *controllers)
 	return __enable_controllers(cgroup_path, controllers);
 }
 
+static ssize_t __read_cgroup_file(const char *cgroup_path, const char *file,
+				 char *buf, size_t size)
+{
+	char file_path[PATH_MAX + 1];
+	ssize_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);
+	if (ret < 0) {
+		close(fd);
+		log_err("Reading %s", file_path);
+		return -1;
+	}
+
+	close(fd);
+	return ret;
+}
+
+/**
+ * read_cgroup_file() - Read from a cgroup file
+ * @relative_path: The cgroup path, relative to the workdir
+ * @file: The name of the file in cgroupfs to read from
+ * @buf: Buffer to read from the file
+ * @size: Size of the buffer
+ *
+ * Read from a file in the given cgroup's directory.
+ *
+ * If successful, the number of read bytes is returned.
+ */
+ssize_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 3857304be874..66a08b64838b 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.h
+++ b/tools/testing/selftests/bpf/cgroup_helpers.h
@@ -4,6 +4,7 @@
 
 #include <errno.h>
 #include <string.h>
+#include <sys/types.h>
 
 #define clean_errno() (errno == 0 ? "None" : strerror(errno))
 #define log_err(MSG, ...) fprintf(stderr, "(%s:%d: errno: %s) " MSG "\n", \
@@ -11,6 +12,8 @@
 
 /* cgroupv2 related */
 int enable_controllers(const char *relative_path, const char *controllers);
+ssize_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,
-- 
2.52.0