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 <[email protected]>
Reviewed-by: Emil Tsalapatis <[email protected]> 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);
