Re: [PATCH bpf-next v3 3/4] selftests/bpf: Add read_cgroup_file() to cgroup_helpers

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sat Aug 8, 2026 at 10:05 AM EDT, Jiayuan Chen wrote:
> 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>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

The "- 24" is questionable, but it's already all over the file. We can
clean it up separately.

> ---
>  tools/testing/selftests/bpf/cgroup_helpers.c | 67 ++++++++++++++++++++
>  tools/testing/selftests/bpf/cgroup_helpers.h |  4 ++
>  2 files changed, 71 insertions(+)
>
> 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);