ftrace - fix dynamic ftrace memory leak

The ftrace dynamic function update allocates a record to store the
instruction pointers that are being modified. If the modified
instruction pointer fails to update, then the record is marked as
failed and nothing more is done.

Worse, if the modification fails, but the record ip function is still
called, it will allocate a new record and try again. In just a matter
of time, will this cause a serious memory leak and crash the system.

This patch plugs this memory leak. When a record fails, it is
included back into the pool of records to be used. Now a record may
fail over and over again, but the number of allocated records will
not increase.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/ftrace.h

@@ -43,9 +43,10 @@ extern void mcount(void);
 # define FTRACE_HASHSIZE	(1<<FTRACE_HASHBITS)
 
 enum {
-	FTRACE_FL_FAILED	= (1 << 0),
-	FTRACE_FL_FILTER	= (1 << 1),
-	FTRACE_FL_ENABLED	= (1 << 2),
+	FTRACE_FL_FREE		= (1 << 0),
+	FTRACE_FL_FAILED	= (1 << 1),
+	FTRACE_FL_FILTER	= (1 << 2),
+	FTRACE_FL_ENABLED	= (1 << 3),
 };
 
 struct dyn_ftrace {

kernel/trace/ftrace.c

@@ -188,6 +188,8 @@ static int ftraced_suspend;
 
 static int ftrace_record_suspend;
 
+static struct dyn_ftrace *ftrace_free_records;
+
 static inline int
 notrace ftrace_ip_in_hash(unsigned long ip, unsigned long key)
 {
@@ -211,8 +213,35 @@ ftrace_add_hash(struct dyn_ftrace *node, unsigned long key)
 	hlist_add_head(&node->node, &ftrace_hash[key]);
 }
 
+static notrace void ftrace_free_rec(struct dyn_ftrace *rec)
+{
+	/* no locking, only called from kstop_machine */
+
+	rec->ip = (unsigned long)ftrace_free_records;
+	ftrace_free_records = rec;
+	rec->flags |= FTRACE_FL_FREE;
+}
+
 static notrace struct dyn_ftrace *ftrace_alloc_dyn_node(unsigned long ip)
 {
+	struct dyn_ftrace *rec;
+
+	/* First check for freed records */
+	if (ftrace_free_records) {
+		rec = ftrace_free_records;
+
+		/* todo, disable tracing altogether on this warning */
+		if (unlikely(!(rec->flags & FTRACE_FL_FREE))) {
+			WARN_ON_ONCE(1);
+			ftrace_free_records = NULL;
+			return NULL;
+		}
+
+		ftrace_free_records = (void *)rec->ip;
+		memset(rec, 0, sizeof(*rec));
+		return rec;
+	}
+
 	if (ftrace_pages->index == ENTRIES_PER_PAGE) {
 		if (!ftrace_pages->next)
 			return NULL;
@@ -356,8 +385,16 @@ __ftrace_replace_code(struct dyn_ftrace *rec,
 	}
 
 	failed = ftrace_modify_code(ip, old, new);
-	if (failed)
+	if (failed) {
+		unsigned long key;
+		/* It is possible that the function hasn't been converted yet */
+		key = hash_long(ip, FTRACE_HASHBITS);
+		if (!ftrace_ip_in_hash(ip, key)) {
 			rec->flags |= FTRACE_FL_FAILED;
+			ftrace_free_rec(rec);
+		}
+
+	}
 }
 
 static void notrace ftrace_replace_code(int enable)
@@ -407,8 +444,10 @@ ftrace_code_disable(struct dyn_ftrace *rec)
 	call = ftrace_call_replace(ip, MCOUNT_ADDR);
 
 	failed = ftrace_modify_code(ip, call, nop);
-	if (failed)
+	if (failed) {
 		rec->flags |= FTRACE_FL_FAILED;
+		ftrace_free_rec(rec);
+	}
 }
 
 static int notrace __ftrace_modify_code(void *data)