param: use free hook for charp (fix leak of charp parameters)

Instead of using a "I kmalloced this" flag, we keep track of the kmalloced
strings and use that list to check if we need to kfree (in practice, the
list is very short).

This means that kparams can be const again, and plugs a leak.  This
is important for drivers/usb/gadget/nokia.c which gets modprobe/rmmod'ed
frequently on the N9000.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Cc: Artem Bityutskiy <dedekind1@gmail.com>
Tested-by: Phil Carmody <ext-phil.2.carmody@nokia.com>

kernel/params.c

@@ -31,6 +31,45 @@
 #define DEBUGP(fmt, a...)
 #endif
 
+/* This just allows us to keep track of which parameters are kmalloced. */
+struct kmalloced_param {
+	struct list_head list;
+	char val[];
+};
+static DEFINE_MUTEX(param_lock);
+static LIST_HEAD(kmalloced_params);
+
+static void *kmalloc_parameter(unsigned int size)
+{
+	struct kmalloced_param *p;
+
+	p = kmalloc(sizeof(*p) + size, GFP_KERNEL);
+	if (!p)
+		return NULL;
+
+	mutex_lock(&param_lock);
+	list_add(&p->list, &kmalloced_params);
+	mutex_unlock(&param_lock);
+
+	return p->val;
+}
+
+/* Does nothing if parameter wasn't kmalloced above. */
+static void maybe_kfree_parameter(void *param)
+{
+	struct kmalloced_param *p;
+
+	mutex_lock(&param_lock);
+	list_for_each_entry(p, &kmalloced_params, list) {
+		if (p->val == param) {
+			list_del(&p->list);
+			kfree(p);
+			break;
+		}
+	}
+	mutex_unlock(&param_lock);
+}
+
 static inline char dash2underscore(char c)
 {
 	if (c == '-')
@@ -219,12 +258,15 @@ int param_set_charp(const char *val, const struct kernel_param *kp)
 		return -ENOSPC;
 	}
 
-	/* This is a hack.  We can't need to strdup in early boot, and we
+	maybe_kfree_parameter(*(char **)kp->arg);
+
+	/* This is a hack.  We can't kmalloc in early boot, and we
 	 * don't need to; this mangled commandline is preserved. */
 	if (slab_is_available()) {
-		*(char **)kp->arg = kstrdup(val, GFP_KERNEL);
+		*(char **)kp->arg = kmalloc_parameter(strlen(val)+1);
 		if (!*(char **)kp->arg)
 			return -ENOMEM;
+		strcpy(*(char **)kp->arg, val);
 	} else
 		*(const char **)kp->arg = val;
 
@@ -238,9 +280,15 @@ int param_get_charp(char *buffer, const struct kernel_param *kp)
 }
 EXPORT_SYMBOL(param_get_charp);
 
+static void param_free_charp(void *arg)
+{
+	maybe_kfree_parameter(*((char **)arg));
+}
+
 struct kernel_param_ops param_ops_charp = {
 	.set = param_set_charp,
 	.get = param_get_charp,
+	.free = param_free_charp,
 };
 EXPORT_SYMBOL(param_ops_charp);