selftests/bpf: Add read_cgroup_file() to cgroup_helpers

cgroup_helpers has write_cgroup_file()/write_cgroup_file_parent() but no
read counterpart. Add read_cgroup_file() and read_cgroup_file_parent() so
a forked child can read a cgroup file (e.g. memory.current) from the work
dir owned by the parent that set the environment up, without hand-building
the /mnt/... path.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260922101831.192102-3-jiayuan.chen@linux.dev

2 files changed, 71 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index 45cd0b479fe3..4183ff6150c2 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -188,6 +188,73 @@ int write_cgroup_file_parent(const char *relative_path, const char *file,
 	return __write_cgroup_file(cgroup_path, file, buf);
 }
 
+static int __read_cgroup_file(const char *cgroup_path, const char *file,
+			      char *buf, size_t len)
+{
+	char file_path[PATH_MAX + 1];
+	ssize_t got;
+	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;
+	}
+
+	got = read(fd, buf, len - 1);
+	if (got < 0) {
+		log_err("Reading %s", file_path);
+		close(fd);
+		return 1;
+	}
+	buf[got] = '\0';
+	close(fd);
+	return 0;
+}
+
+/**
+ * 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 into, NUL-terminated on success
+ * @len: Size of @buf
+ *
+ * Read from a file in the given cgroup's directory.
+ *
+ * If successful, 0 is returned.
+ */
+int read_cgroup_file(const char *relative_path, const char *file,
+		     char *buf, size_t len)
+{
+	char cgroup_path[PATH_MAX - 24];
+
+	format_cgroup_path(cgroup_path, relative_path);
+	return __read_cgroup_file(cgroup_path, file, buf, len);
+}
+
+/**
+ * read_cgroup_file_parent() - Read from a cgroup file in the parent process
+ *                             workdir
+ * @relative_path: The cgroup path, relative to the parent process workdir
+ * @file: The name of the file in cgroupfs to read from
+ * @buf: Buffer to read into, NUL-terminated on success
+ * @len: Size of @buf
+ *
+ * Read from a file in the given cgroup's directory under the parent process
+ * workdir.
+ *
+ * If successful, 0 is returned.
+ */
+int read_cgroup_file_parent(const char *relative_path, const char *file,
+			    char *buf, size_t len)
+{
+	char cgroup_path[PATH_MAX - 24];
+
+	format_parent_cgroup_path(cgroup_path, relative_path);
+	return __read_cgroup_file(cgroup_path, file, buf, len);
+}
+
 /**
  * setup_cgroup_environment() - Setup the cgroup environment
  *
diff --git a/tools/testing/selftests/bpf/cgroup_helpers.h b/tools/testing/selftests/bpf/cgroup_helpers.h
index 3857304be874..d42d2e13044e 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.h
+++ b/tools/testing/selftests/bpf/cgroup_helpers.h
@@ -15,6 +15,10 @@ 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,
 			     const char *buf);
+int read_cgroup_file(const char *relative_path, const char *file,
+		     char *buf, size_t len);
+int read_cgroup_file_parent(const char *relative_path, const char *file,
+			    char *buf, size_t len);
 int cgroup_setup_and_join(const char *relative_path);
 int get_root_cgroup(void);
 int create_and_get_cgroup(const char *relative_path);