Revert "proc/wchan: use printk format instead of lookup_symbol_name()"

This reverts commit 152c432b.

When a kernel address couldn't be symbolized for /proc/$pid/wchan, it
would leak the raw value, a potential information exposure. This is a
regression compared to the safer pre-v5.12 behavior.
Reported-by: kernel test robot <oliver.sang@intel.com>
Reported-by: Vito Caputo <vcaputo@pengaru.com>
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20211008111626.090829198@infradead.org

fs/proc/base.c

@@ -67,6 +67,7 @@
 #include <linux/mm.h>
 #include <linux/swap.h>
 #include <linux/rcupdate.h>
+#include <linux/kallsyms.h>
 #include <linux/stacktrace.h>
 #include <linux/resource.h>
 #include <linux/module.h>
@@ -386,17 +387,19 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 			  struct pid *pid, struct task_struct *task)
 {
 	unsigned long wchan;
+	char symname[KSYM_NAME_LEN];
+
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+		goto print0;
 
-	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
 	wchan = get_wchan(task);
-	else
-		wchan = 0;
+	if (wchan && !lookup_symbol_name(wchan, symname)) {
+		seq_puts(m, symname);
+		return 0;
+	}
 
-	if (wchan)
-		seq_printf(m, "%ps", (void *) wchan);
-	else
+print0:
 	seq_putc(m, '0');
-
 	return 0;
 }
 #endif /* CONFIG_KALLSYMS */