Re: [PATCH] selftests: proc: Make sure wchan works when it exists

On Fri, Oct 08, 2021 at 04:55:04PM -0700, Kees Cook wrote:

This makes sure that wchan contains a sensible symbol when a process is blocked.

Specifically this calls the sleep() syscall, and expects the architecture to have called schedule() from a function that has "sleep" somewhere in its name.

This exposes internal kernel symbol to userspace. Why would want to test that?

Doing s/sleep/SLEEP/g doesn't change kernel but now the test is broken.

For example, on the architectures I tested (x86_64, arm64, arm, mips, and powerpc) this is "hrtimer_nanosleep":

+/*

• • Make sure that wchan returns a reasonable symbol when blocked.
• */

Test should be "contains C identifier" then?

+int main(void) +{

• char buf[64];
• pid_t child;
• int sync[2], fd;
• if (pipe(sync) < 0)

• perror_exit("pipe");
  

• child = fork();
• if (child < 0)

• perror_exit("fork");
  

• if (child == 0) {

• /* Child */
  

• if (close(sync[0]) < 0)
  

• 	perror_exit("child close sync[0]");
  

• if (close(sync[1]) < 0)
  

• 	perror_exit("child close sync[1]");
  

Redundant close().

• sleep(10);
  

• _exit(0);
  

• }
• /* Parent */
• if (close(sync[1]) < 0)

• perror_exit("parent close sync[1]");
  

Redundant close().

• if (read(sync[0], buf, 1) != 0)

• perror_exit("parent read sync[0]");
  

Racy if child is scheduled out after first close in the child.

• snprintf(buf, sizeof(buf), "/proc/%d/wchan", child);
• fd = open(buf, O_RDONLY);
• if (fd < 0) {

• if (errno == ENOENT)
  

• 	return 4;
  

• perror_exit(buf);
  

• }
• memset(buf, 0, sizeof(buf));
• if (read(fd, buf, sizeof(buf) - 1) < 1)

• perror_exit(buf);
  

• if (strstr(buf, "sleep") == NULL) {

• fprintf(stderr, "FAIL: did not find 'sleep' in wchan '%s'\n", buf);
  

• return 1;
  

• }
• printf("ok: found 'sleep' in wchan '%s'\n", buf);
• if (kill(child, SIGKILL) < 0)

• perror_exit("kill");
  

• if (waitpid(child, NULL, 0) != child) {

• fprintf(stderr, "waitpid: got the wrong child!?\n");
  

• return 1;
  

• }
• return 0;

+}