procfs: do not list TID 0 in /proc/<pid>/task

If a task exits concurrently, task_pid_nr_ns may return 0.

Link: https://lkml.kernel.org/r/8735pn5dx7.fsf@oldenburg.str.redhat.com
Signed-off-by: Florian Weimer <fweimer@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

Author: fweimer-rh

fs/proc/base.c

@@ -3800,6 +3800,9 @@ static int proc_task_readdir(struct file *file, struct dir_context *ctx)
 		char name[10 + 1];
 		unsigned int len;
 		tid = task_pid_nr_ns(task, ns);
+		if (!tid)
+			/* The task has just exited. */
+			continue;
 		len = snprintf(name, sizeof(name), "%u", tid);
 		if (!proc_fill_cache(file, ctx, name, len,
 				proc_task_instantiate, task, NULL)) {