proc: test that /proc/*/task doesn't contain "0"

add selftest to check for the errant "0" in /proc/*/task

Link: https://lkml.kernel.org/r/YV88AnVzHxPafQ9o@localhost.localdomain
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

Author: Alexey Dobriyan

tools/testing/selftests/proc/.gitignore

@@ -11,6 +11,7 @@
 /proc-self-syscall
 /proc-self-wchan
 /proc-subset-pid
+/proc-tid0
 /proc-uptime-001
 /proc-uptime-002
 /read

tools/testing/selftests/proc/Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -Wall -O2 -Wno-unused-function
 CFLAGS += -D_GNU_SOURCE
+LDFLAGS += -pthread
 
 TEST_GEN_PROGS :=
 TEST_GEN_PROGS += fd-001-lookup
@@ -13,6 +14,7 @@ TEST_GEN_PROGS += proc-self-map-files-002
 TEST_GEN_PROGS += proc-self-syscall
 TEST_GEN_PROGS += proc-self-wchan
 TEST_GEN_PROGS += proc-subset-pid
+TEST_GEN_PROGS += proc-tid0
 TEST_GEN_PROGS += proc-uptime-001
 TEST_GEN_PROGS += proc-uptime-002
 TEST_GEN_PROGS += read

tools/testing/selftests/proc/proc-tid0.c

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2021 Alexey Dobriyan <adobriyan@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+// Test that /proc/*/task never contains "0".
+#include <sys/types.h>
+#include <dirent.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <pthread.h>
+
+static pid_t pid = -1;
+
+static void atexit_hook(void)
+{
+	if (pid > 0) {
+		kill(pid, SIGKILL);
+	}
+}
+
+static void *f(void *_)
+{
+	return NULL;
+}
+
+static void sigalrm(int _)
+{
+	exit(0);
+}
+
+int main(void)
+{
+	pid = fork();
+	if (pid == 0) {
+		/* child */
+		while (1) {
+			pthread_t pth;
+			pthread_create(&pth, NULL, f, NULL);
+			pthread_join(pth, NULL);
+		}
+	} else if (pid > 0) {
+		/* parent */
+		atexit(atexit_hook);
+
+		char buf[64];
+		snprintf(buf, sizeof(buf), "/proc/%u/task", pid);
+
+		signal(SIGALRM, sigalrm);
+		alarm(1);
+
+		while (1) {
+			DIR *d = opendir(buf);
+			struct dirent *de;
+			while ((de = readdir(d))) {
+				if (strcmp(de->d_name, "0") == 0) {
+					exit(1);
+				}
+			}
+			closedir(d);
+		}
+
+		return 0;
+	} else {
+		perror("fork");
+		return 1;
+	}
+}