ftrace: Zero out ftrace hashes when a module is removed

When a ftrace filter has a module function, and that module is removed, the
filter still has its address as being enabled. This can cause interesting
side effects. Nothing dangerous, but unwanted functions can be traced
because of it.

 # cd /sys/kernel/tracing
 # echo ':mod:snd_seq' > set_ftrace_filter
 # cat set_ftrace_filter
snd_use_lock_sync_helper [snd_seq]
check_event_type_and_length [snd_seq]
snd_seq_ioctl_pversion [snd_seq]
snd_seq_ioctl_client_id [snd_seq]
snd_seq_ioctl_get_queue_tempo [snd_seq]
update_timestamp_of_queue [snd_seq]
snd_seq_ioctl_get_queue_status [snd_seq]
snd_seq_set_queue_tempo [snd_seq]
snd_seq_ioctl_set_queue_tempo [snd_seq]
snd_seq_ioctl_get_queue_timer [snd_seq]
seq_free_client1 [snd_seq]
[..]
 # rmmod snd_seq
 # cat set_ftrace_filter

 # modprobe kvm
 # cat set_ftrace_filter
kvm_set_cr4 [kvm]
kvm_emulate_hypercall [kvm]
kvm_set_dr [kvm]

This is because removing the snd_seq module after it was being filtered,
left the address of the snd_seq functions in the hash. When the kvm module
was loaded, some of its functions were loaded at the same address as the
snd_seq module. This would enable them to be filtered and traced.

Now we don't want to clear the hash completely. That would cause removing a
module where only its functions are filtered, to cause the tracing to enable
all functions, as an empty filter means to trace all functions. Instead,
just set the hash ip address to zero. Then it will never match any function.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -5690,10 +5690,51 @@ static int referenced_filters(struct dyn_ftrace *rec)
 	return cnt;
 }
 
+static void
+clear_mod_from_hash(struct ftrace_page *pg, struct ftrace_hash *hash)
+{
+	struct ftrace_func_entry *entry;
+	struct dyn_ftrace *rec;
+	int i;
+
+	if (ftrace_hash_empty(hash))
+		return;
+
+	for (i = 0; i < pg->index; i++) {
+		rec = &pg->records[i];
+		entry = __ftrace_lookup_ip(hash, rec->ip);
+		/*
+		 * Do not allow this rec to match again.
+		 * Yeah, it may waste some memory, but will be removed
+		 * if/when the hash is modified again.
+		 */
+		if (entry)
+			entry->ip = 0;
+	}
+}
+
+/* Clear any records from hashs */
+static void clear_mod_from_hashes(struct ftrace_page *pg)
+{
+	struct trace_array *tr;
+
+	mutex_lock(&trace_types_lock);
+	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
+		if (!tr->ops || !tr->ops->func_hash)
+			continue;
+		mutex_lock(&tr->ops->func_hash->regex_lock);
+		clear_mod_from_hash(pg, tr->ops->func_hash->filter_hash);
+		clear_mod_from_hash(pg, tr->ops->func_hash->notrace_hash);
+		mutex_unlock(&tr->ops->func_hash->regex_lock);
+	}
+	mutex_unlock(&trace_types_lock);
+}
+
 void ftrace_release_mod(struct module *mod)
 {
 	struct dyn_ftrace *rec;
 	struct ftrace_page **last_pg;
+	struct ftrace_page *tmp_page = NULL;
 	struct ftrace_page *pg;
 	int order;
 
@@ -5723,14 +5764,25 @@ void ftrace_release_mod(struct module *mod)
 
 			ftrace_update_tot_cnt -= pg->index;
 			*last_pg = pg->next;
-			order = get_count_order(pg->size / ENTRIES_PER_PAGE);
-			free_pages((unsigned long)pg->records, order);
-			kfree(pg);
+
+			pg->next = tmp_page;
+			tmp_page = pg;
 		} else
 			last_pg = &pg->next;
 	}
  out_unlock:
 	mutex_unlock(&ftrace_lock);
+
+	for (pg = tmp_page; pg; pg = tmp_page) {
+
+		/* Needs to be called outside of ftrace_lock */
+		clear_mod_from_hashes(pg);
+
+		order = get_count_order(pg->size / ENTRIES_PER_PAGE);
+		free_pages((unsigned long)pg->records, order);
+		tmp_page = pg->next;
+		kfree(pg);
+	}
 }
 
 void ftrace_module_enable(struct module *mod)