[PATCH v2 17/23] bpf: selftests: introduce read_cgroup_file() helper

Roman Gushchin
2025-10-27T23:22:48Z

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 20cede4db3ce..8fb02fe4c4aa 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -126,6 +126,45 @@ 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 3857304be874..9f9bb6b5d992 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,
--
2.51.0