proc: test /proc/self/wchan

This patch starts testing /proc. Many more tests to come (I promise).

Read from /proc/self/wchan should always return "0" as current is in
TASK_RUNNING state while reading /proc/self/wchan.

Link: http://lkml.kernel.org/r/20180226212006.GA742@avx2
Signed-off-by: Alexey Dobriyan <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/Makefile

···
 TARGETS += net
 TARGETS += nsfs
 TARGETS += powerpc
+TARGETS += proc
 TARGETS += pstore
 TARGETS += ptrace
 TARGETS += seccomp

tools/testing/selftests/proc/.gitignore

···
+/proc-self-wchan

tools/testing/selftests/proc/Makefile

···
+CFLAGS += -Wall -O2
+
+TEST_GEN_PROGS :=
+TEST_GEN_PROGS += proc-self-wchan
+
+include ../lib.mk

tools/testing/selftests/proc/config

···
+CONFIG_PROC_FS=y

tools/testing/selftests/proc/proc-self-wchan.c

···
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <unistd.h>
+
+int main(void)
+{
+	char buf[64];
+	int fd;
+
+	fd = open("/proc/self/wchan", O_RDONLY);
+	if (fd == -1) {
+		if (errno == ENOENT)
+			return 2;
+		return 1;
+	}
+
+	buf[0] = '\0';
+	if (read(fd, buf, sizeof(buf)) != 1)
+		return 1;
+	if (buf[0] != '0')
+		return 1;
+	return 0;
+}