ftrace: Add update_ftrace_direct_add function

Adding update_ftrace_direct_add function that adds all entries
(ip -> addr) provided in hash argument to direct ftrace ops
and updates its attachments.

The difference to current register_ftrace_direct is
 - hash argument that allows to register multiple ip -> direct
   entries at once
 - we can call update_ftrace_direct_add multiple times on the
   same ftrace_ops object, becase after first registration with
   register_ftrace_function_nolock, it uses ftrace_update_ops to
   update the ftrace_ops object

This change will allow us to have simple ftrace_ops for all bpf
direct interface users in following changes.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/bpf/20251230145010.103439-5-jolsa@kernel.org

include/linux/ftrace.h

@@ -543,6 +543,8 @@ int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr,
 int modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr);
 int modify_ftrace_direct_nolock(struct ftrace_ops *ops, unsigned long addr);
 
+int update_ftrace_direct_add(struct ftrace_ops *ops, struct ftrace_hash *hash);
+
 void ftrace_stub_direct_tramp(void);
 
 #else
@@ -569,6 +571,11 @@ static inline int modify_ftrace_direct_nolock(struct ftrace_ops *ops, unsigned l
 	return -ENODEV;
 }
 
+static inline int update_ftrace_direct_add(struct ftrace_ops *ops, struct ftrace_hash *hash)
+{
+	return -ENODEV;
+}
+
 /*
  * This must be implemented by the architecture.
  * It is the way the ftrace direct_ops helper, when called

kernel/trace/ftrace.c

@@ -6278,6 +6278,146 @@ int modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 	return err;
 }
 EXPORT_SYMBOL_GPL(modify_ftrace_direct);
+
+static unsigned long hash_count(struct ftrace_hash *hash)
+{
+	return hash ? hash->count : 0;
+}
+
+/**
+ * hash_add - adds two struct ftrace_hash and returns the result
+ * @a: struct ftrace_hash object
+ * @b: struct ftrace_hash object
+ *
+ * Returns struct ftrace_hash object on success, NULL on error.
+ */
+static struct ftrace_hash *hash_add(struct ftrace_hash *a, struct ftrace_hash *b)
+{
+	struct ftrace_func_entry *entry;
+	struct ftrace_hash *add;
+	int size;
+
+	size = hash_count(a) + hash_count(b);
+	if (size > 32)
+		size = 32;
+
+	add = alloc_and_copy_ftrace_hash(fls(size), a);
+	if (!add)
+		return NULL;
+
+	size = 1 << b->size_bits;
+	for (int i = 0; i < size; i++) {
+		hlist_for_each_entry(entry, &b->buckets[i], hlist) {
+			if (add_ftrace_hash_entry_direct(add, entry->ip, entry->direct) == NULL) {
+				free_ftrace_hash(add);
+				return NULL;
+			}
+		}
+	}
+	return add;
+}
+
+/**
+ * update_ftrace_direct_add - Updates @ops by adding direct
+ * callers provided in @hash
+ * @ops: The address of the struct ftrace_ops object
+ * @hash: The address of the struct ftrace_hash object
+ *
+ * This is used to add custom direct callers (ip -> addr) to @ops,
+ * specified in @hash. The @ops will be either registered or updated.
+ *
+ * Returns: zero on success. Non zero on error, which includes:
+ *  -EINVAL - The @hash is empty
+ */
+int update_ftrace_direct_add(struct ftrace_ops *ops, struct ftrace_hash *hash)
+{
+	struct ftrace_hash *old_direct_functions = NULL;
+	struct ftrace_hash *new_direct_functions;
+	struct ftrace_hash *old_filter_hash;
+	struct ftrace_hash *new_filter_hash = NULL;
+	struct ftrace_func_entry *entry;
+	int err = -EINVAL;
+	int size;
+	bool reg;
+
+	if (!hash_count(hash))
+		return -EINVAL;
+
+	mutex_lock(&direct_mutex);
+
+	/* Make sure requested entries are not already registered. */
+	size = 1 << hash->size_bits;
+	for (int i = 0; i < size; i++) {
+		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
+			if (__ftrace_lookup_ip(direct_functions, entry->ip))
+				goto out_unlock;
+		}
+	}
+
+	old_filter_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
+
+	/* If there's nothing in filter_hash we need to register the ops. */
+	reg = hash_count(old_filter_hash) == 0;
+	if (reg) {
+		if (ops->func || ops->trampoline)
+			goto out_unlock;
+		if (ops->flags & FTRACE_OPS_FL_ENABLED)
+			goto out_unlock;
+	}
+
+	err = -ENOMEM;
+	new_filter_hash = hash_add(old_filter_hash, hash);
+	if (!new_filter_hash)
+		goto out_unlock;
+
+	new_direct_functions = hash_add(direct_functions, hash);
+	if (!new_direct_functions)
+		goto out_unlock;
+
+	old_direct_functions = direct_functions;
+	rcu_assign_pointer(direct_functions, new_direct_functions);
+
+	if (reg) {
+		ops->func = call_direct_funcs;
+		ops->flags |= MULTI_FLAGS;
+		ops->trampoline = FTRACE_REGS_ADDR;
+		ops->local_hash.filter_hash = new_filter_hash;
+
+		err = register_ftrace_function_nolock(ops);
+		if (err) {
+			/* restore old filter on error */
+			ops->local_hash.filter_hash = old_filter_hash;
+
+			/* cleanup for possible another register call */
+			ops->func = NULL;
+			ops->trampoline = 0;
+		} else {
+			new_filter_hash = old_filter_hash;
+		}
+	} else {
+		err = ftrace_update_ops(ops, new_filter_hash, EMPTY_HASH);
+		/*
+		 * new_filter_hash is dup-ed, so we need to release it anyway,
+		 * old_filter_hash either stays on error or is already released
+		 */
+	}
+
+	if (err) {
+		/* reset direct_functions and free the new one */
+		rcu_assign_pointer(direct_functions, old_direct_functions);
+		old_direct_functions = new_direct_functions;
+	}
+
+ out_unlock:
+	mutex_unlock(&direct_mutex);
+
+	if (old_direct_functions && old_direct_functions != EMPTY_HASH)
+		call_rcu_tasks(&old_direct_functions->rcu, register_ftrace_direct_cb);
+	free_ftrace_hash(new_filter_hash);
+
+	return err;
+}
+
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
 
 /**