param: fix lots of bugs with writing charp params from sysfs, by leaking mem.

e180a6b7 "param: fix charp parameters set via sysfs" fixed the case
where charp parameters written via sysfs were freed, leaving drivers
accessing random memory.

Unfortunately, storing a flag in the kparam struct was a bad idea: it's
rodata so setting it causes an oops on some archs.  But that's not all:

1) module_param_array() on charp doesn't work reliably, since we use an
   uninitialized temporary struct kernel_param.
2) there's a fundamental race if a module uses this parameter and then
   it's changed: they will still access the old, freed, memory.

The simplest fix (ie. for 2.6.32) is to never free the memory.  This
prevents all these problems, at cost of a memory leak.  In practice, there
are only 18 places where a charp is writable via sysfs, and all are
root-only writable.
Reported-by: Takashi Iwai <tiwai@suse.de>
Cc: Sitsofe Wheeler <sitsofe@yahoo.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

include/linux/moduleparam.h

@@ -37,7 +37,6 @@ typedef int (*param_set_fn)(const char *val, struct kernel_param *kp);
 typedef int (*param_get_fn)(char *buffer, struct kernel_param *kp);
 
 /* Flag bits for kernel_param.flags */
-#define KPARAM_KMALLOCED	1
 #define KPARAM_ISBOOL		2
 
 struct kernel_param {

kernel/params.c

@@ -218,13 +218,9 @@ int param_set_charp(const char *val, struct kernel_param *kp)
 		return -ENOSPC;
 	}
 
-	if (kp->flags & KPARAM_KMALLOCED)
-		kfree(*(char **)kp->arg);
-
 	/* This is a hack.  We can't need to strdup in early boot, and we
 	 * don't need to; this mangled commandline is preserved. */
 	if (slab_is_available()) {
-		kp->flags |= KPARAM_KMALLOCED;
 		*(char **)kp->arg = kstrdup(val, GFP_KERNEL);
 		if (!kp->arg)
 			return -ENOMEM;
@@ -605,11 +601,7 @@ void module_param_sysfs_remove(struct module *mod)
 
 void destroy_params(const struct kernel_param *params, unsigned num)
 {
-	unsigned int i;
-
-	for (i = 0; i < num; i++)
-		if (params[i].flags & KPARAM_KMALLOCED)
-			kfree(*(char **)params[i].arg);
+	/* FIXME: This should free kmalloced charp parameters.  It doesn't. */
 }
 
 static void __init kernel_add_sysfs_param(const char *name,