Re: [PATCHv5 bpf-next 6/9] ftrace: Add update_ftrace_direct_mod function

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 15 Dec 2025 22:13:59 +0100
Jiri Olsa <jolsa@kernel.org> wrote:

> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 48dc0de5f2ce..95a38fb18ed7 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -6489,6 +6489,78 @@ int update_ftrace_direct_del(struct ftrace_ops *ops, struct ftrace_hash *hash)
>  	return err;
>  }
>  

Kerneldoc needed.

> +int update_ftrace_direct_mod(struct ftrace_ops *ops, struct ftrace_hash *hash, bool do_direct_lock)
> +{
> +	struct ftrace_func_entry *entry, *tmp;
> +	static struct ftrace_ops tmp_ops = {
> +		.func		= ftrace_stub,
> +		.flags		= FTRACE_OPS_FL_STUB,
> +	};
> +	struct ftrace_hash *orig_hash;
> +	unsigned long size, i;
> +	int err = -EINVAL;
> +
> +	if (!hash_count(hash))
> +		return -EINVAL;
> +	if (check_direct_multi(ops))
> +		return -EINVAL;
> +	if (!(ops->flags & FTRACE_OPS_FL_ENABLED))
> +		return -EINVAL;
> +	if (direct_functions == EMPTY_HASH)
> +		return -EINVAL;
> +
> +	if (do_direct_lock)
> +		mutex_lock(&direct_mutex);

This optional taking of the direct_mutex lock needs some serious rationale
and documentation.

> +
> +	orig_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
> +	if (!orig_hash)
> +		goto unlock;
> +
> +	/* Enable the tmp_ops to have the same functions as the direct ops */
> +	ftrace_ops_init(&tmp_ops);
> +	tmp_ops.func_hash = ops->func_hash;
> +
> +	err = register_ftrace_function_nolock(&tmp_ops);
> +	if (err)
> +		goto unlock;
> +
> +	/*
> +	 * Call __ftrace_hash_update_ipmodify() here, so that we can call
> +	 * ops->ops_func for the ops. This is needed because the above
> +	 * register_ftrace_function_nolock() worked on tmp_ops.
> +	 */
> +	err = __ftrace_hash_update_ipmodify(ops, orig_hash, orig_hash, true);
> +	if (err)
> +		goto out;
> +
> +	/*
> +	 * Now the ftrace_ops_list_func() is called to do the direct callers.
> +	 * We can safely change the direct functions attached to each entry.
> +	 */
> +	mutex_lock(&ftrace_lock);

I'm going to need some time staring at this code. It looks like it may be
relying on some internals here.

-- Steve


> +
> +	size = 1 << hash->size_bits;
> +	for (i = 0; i < size; i++) {
> +		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> +			tmp = __ftrace_lookup_ip(direct_functions, entry->ip);
> +			if (!tmp)
> +				continue;
> +			tmp->direct = entry->direct;
> +		}
> +	}
> +
> +	mutex_unlock(&ftrace_lock);
> +
> +out:
> +	/* Removing the tmp_ops will add the updated direct callers to the functions */
> +	unregister_ftrace_function(&tmp_ops);
> +
> +unlock:
> +	if (do_direct_lock)
> +		mutex_unlock(&direct_mutex);
> +	return err;
> +}
> +
>  #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>  
>  /**