proc: test "Kthread:" field

/proc/${pid}/status got Kthread field recently.

Test that userspace program is not reported as kernel thread.

Test that kernel thread is reported as kernel thread.
Use kthreadd with pid 2 for this.

Link: https://lkml.kernel.org/r/818c4c41-8668-4566-97a9-7254abf819ee@p183
Signed-off-by: Alexey Dobriyan <[e-mail]>
Cc: Chunguang Wu <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/proc/.gitignore

···
 /fd-001-lookup
 /fd-002-posix-eq
 /fd-003-kthread
+/proc-2-is-kthread
 /proc-fsconfig-hidepid
 /proc-loadavg-001
 /proc-multiple-procfs
···
 /proc-pid-vm
 /proc-self-map-files-001
 /proc-self-map-files-002
+/proc-self-isnt-kthread
 /proc-self-syscall
 /proc-self-wchan
 /proc-subset-pid

tools/testing/selftests/proc/Makefile

···
 TEST_GEN_PROGS += fd-001-lookup
 TEST_GEN_PROGS += fd-002-posix-eq
 TEST_GEN_PROGS += fd-003-kthread
+TEST_GEN_PROGS += proc-2-is-kthread
 TEST_GEN_PROGS += proc-loadavg-001
 TEST_GEN_PROGS += proc-empty-vm
 TEST_GEN_PROGS += proc-pid-vm
 TEST_GEN_PROGS += proc-self-map-files-001
 TEST_GEN_PROGS += proc-self-map-files-002
+TEST_GEN_PROGS += proc-self-isnt-kthread
 TEST_GEN_PROGS += proc-self-syscall
 TEST_GEN_PROGS += proc-self-wchan
 TEST_GEN_PROGS += proc-subset-pid

tools/testing/selftests/proc/proc-2-is-kthread.c

···
+/*
+ * Copyright (c) 2024 Alexey Dobriyan <[e-mail]>
+ *
+ * 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 kernel thread is reported as such. */
+#undef NDEBUG
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <unistd.h>
+
+int main(void)
+{
+	/*
+	 * The following solutions don't really work:
+	 *
+	 * 1) jit kernel module which creates kernel thread:
+	 * test becomes arch-specific,
+	 * problems with mandatory module signing,
+	 * problems with lockdown mode,
+	 * doesn't work with CONFIG_MODULES=n at all,
+	 * kthread creation API is formally unstable internal kernel API,
+	 * need a mechanism to report test kernel thread's PID back,
+	 *
+	 * 2) ksoftirqd/0 and kswapd0 look like stable enough kernel threads,
+	 * but their PIDs are unstable.
+	 *
+	 * Check against kthreadd which always seem to exist under pid 2.
+	 */
+	int fd = open("/proc/2/status", O_RDONLY);
+	assert(fd >= 0);
+
+	char buf[4096];
+	ssize_t rv = read(fd, buf, sizeof(buf));
+	assert(0 <= rv && rv < sizeof(buf));
+	buf[rv] = '\0';
+
+	assert(strstr(buf, "Kthread:\t1\n"));
+
+	return 0;
+}

tools/testing/selftests/proc/proc-self-isnt-kthread.c

···
+/*
+ * Copyright (c) 2024 Alexey Dobriyan <[e-mail]>
+ *
+ * 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 userspace program is not kernel thread. */
+#undef NDEBUG
+#include <assert.h>
+#include <fcntl.h>
+#include <string.h>
+#include <unistd.h>
+
+int main(void)
+{
+	int fd = open("/proc/self/status", O_RDONLY);
+	assert(fd >= 0);
+
+	char buf[4096];
+	ssize_t rv = read(fd, buf, sizeof(buf));
+	assert(0 <= rv && rv < sizeof(buf));
+	buf[rv] = '\0';
+
+	/* This test is very much not kernel thread. */
+	assert(strstr(buf, "Kthread:\t0\n"));
+
+	return 0;
+}