[RFC][PATCH 4/7] kprobe: Dont kfree() from breakpoint context

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

Breakpoint context might not be a safe context for kfree(), push it
out to RCU.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/kprobes.h |    2 +-
 kernel/kprobes.c        |   27 ++++++---------------------
 2 files changed, 7 insertions(+), 22 deletions(-)

--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -159,6 +159,7 @@ struct kretprobe_instance {
 	union {
 		struct llist_node llist;
 		struct hlist_node hlist;
+		struct rcu_head rcu;
 	};
 	struct kretprobe *rp;
 	kprobe_opcode_t *ret_addr;
@@ -398,7 +399,6 @@ int register_kretprobes(struct kretprobe
 void unregister_kretprobes(struct kretprobe **rps, int num);
 
 void kprobe_flush_task(struct task_struct *tk);
-void recycle_rp_inst(struct kretprobe_instance *ri, struct hlist_head *head);
 
 int disable_kprobe(struct kprobe *kp);
 int enable_kprobe(struct kprobe *kp);
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1214,8 +1214,7 @@ void kprobes_inc_nmissed_count(struct kp
 }
 NOKPROBE_SYMBOL(kprobes_inc_nmissed_count);
 
-void recycle_rp_inst(struct kretprobe_instance *ri,
-		     struct hlist_head *head)
+static void recycle_rp_inst(struct kretprobe_instance *ri)
 {
 	struct kretprobe *rp = ri->rp;
 
@@ -1226,9 +1225,9 @@ void recycle_rp_inst(struct kretprobe_in
 		raw_spin_lock(&rp->lock);
 		hlist_add_head(&ri->hlist, &rp->free_instances);
 		raw_spin_unlock(&rp->lock);
-	} else
-		/* Unregistering */
-		hlist_add_head(&ri->hlist, head);
+	} else {
+		kfree_rcu(ri, rcu);
+	}
 }
 NOKPROBE_SYMBOL(recycle_rp_inst);
 
@@ -1261,15 +1260,12 @@ void kprobe_busy_end(void)
 void kprobe_flush_task(struct task_struct *tk)
 {
 	struct kretprobe_instance *ri;
-	struct hlist_head empty_rp;
 	struct llist_node *node;
-	struct hlist_node *tmp;
 
 	/* Early boot, not yet initialized. */
 	if (unlikely(!kprobes_initialized))
 		return;
 
-	INIT_HLIST_HEAD(&empty_rp);
 
 	kprobe_busy_begin();
 
@@ -1280,12 +1276,7 @@ void kprobe_flush_task(struct task_struc
 		ri = container_of(node, struct kretprobe_instance, llist);
 		node = node->next;
 
-		recycle_rp_inst(ri, &empty_rp);
-	}
-
-	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
-		hlist_del(&ri->hlist);
-		kfree(ri);
+		recycle_rp_inst(ri);
 	}
 
 	kprobe_busy_end();
@@ -1937,7 +1928,6 @@ unsigned long __kretprobe_trampoline_han
 	unsigned long orig_ret_address = 0;
 	struct llist_node *first, *node;
 	struct hlist_head empty_rp;
-	struct hlist_node *tmp;
 
 	INIT_HLIST_HEAD(&empty_rp);
 
@@ -1980,16 +1970,11 @@ unsigned long __kretprobe_trampoline_han
 			__this_cpu_write(current_kprobe, &kprobe_busy);
 		}
 
-		recycle_rp_inst(ri, &empty_rp);
+		recycle_rp_inst(ri);
 
 		first = node;
 	}
 
-	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
-		hlist_del(&ri->hlist);
-		kfree(ri);
-	}
-
 	return orig_ret_address;
 }
 NOKPROBE_SYMBOL(__kretprobe_trampoline_handler)