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 <adobriyan@gmail.com>
Cc: Chunguang Wu <fullspring2018@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/tools/testing/selftests/proc/proc-self-isnt-kthread.c b/tools/testing/selftests/proc/proc-self-isnt-kthread.c
new file mode 100644
index 000000000000..e01f4e0a91b4
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-self-isnt-kthread.c
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2024 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 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;
+}