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

> 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

[ ... ]

> +/**
> + * 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
          ^^^^^^^^^^^^^^^^^^^^^^^^

This isn't a bug, but should this say "Buffer to store data read from the
file" or "Buffer to read into"? The current wording suggests data is read
from the buffer, but the code reads from the file into the buffer. Compare
with write_cgroup_file() which says "Buffer to write to the file" - that
phrasing works for writes but the inverse doesn't quite work for reads.

> + * @size: Size of the buffer
> + *
> + * Read from a file in the given cgroup's directory.
> + *
> + * If successful, the number of read bytes is returned.
> + */

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21382620878

AI-authorship-score: low
AI-authorship-explanation: The code closely follows existing patterns in the file with a minor documentation error typical of copy-paste adaptation, suggesting human authorship.
issues-found: 1
issue-severity-score: low
issue-severity-explanation: Minor documentation error in test code where @buf description says the opposite of what the code does.