linux-kernel - Re: [PATCHv5 bpf-next 5/9] ftrace: Add update_ftrace_direct

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Message-ID: <20251217204814.38756224@robin>
Date: Wed, 17 Dec 2025 20:48:14 -0500
From: Steven Rostedt <rostedt@...nel.org>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Florent Revest <revest@...gle.com>, Mark Rutland <mark.rutland@....com>,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Menglong Dong
 <menglong8.dong@...il.com>, Song Liu <song@...nel.org>
Subject: Re: [PATCHv5 bpf-next 5/9] ftrace: Add update_ftrace_direct_del
 function

On Mon, 15 Dec 2025 22:13:58 +0100
Jiri Olsa <jolsa@...nel.org> wrote:
> +/**
> + * hash_sub - substracts @b from @a 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_sub(struct ftrace_hash *a, struct ftrace_hash *b)
> +{
> +	struct ftrace_func_entry *entry, *del;
> +	struct ftrace_hash *sub;
> +	int size, i;
> +
> +	sub = alloc_and_copy_ftrace_hash(a->size_bits, a);
> +	if (!sub)
> +		goto error;

Again, this can be just return NULL;

> +
> +	size = 1 << b->size_bits;
> +	for (i = 0; i < size; i++) {

You can make this for (int i = 0; ...) too.

> +		hlist_for_each_entry(entry, &b->buckets[i], hlist) {
> +			del = __ftrace_lookup_ip(sub, entry->ip);
> +			if (WARN_ON_ONCE(!del))
> +				goto error;

And you can remove the error label here too:

			if (WARN_ON_ONCE(!del)) {
				free_ftrace_hash(sub);
				return NULL;
			}


> +			remove_hash_entry(sub, del);
> +			kfree(del);
> +		}
> +	}
> +	return sub;
> +
> + error:
> +	free_ftrace_hash(sub);
> +	return NULL;
> +}
> +
> +int update_ftrace_direct_del(struct ftrace_ops *ops, struct ftrace_hash *hash)
> +{
> +	struct ftrace_hash *old_direct_functions = NULL, *new_direct_functions;
> +	struct ftrace_hash *old_filter_hash, *new_filter_hash = NULL;
> +	struct ftrace_func_entry *del, *entry;

One variable per line.

> +	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;
> +
> +	mutex_lock(&direct_mutex);
> +
> +	old_filter_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
> +
> +	if (!hash_count(old_filter_hash))
> +		goto out_unlock;
> +
> +	/* Make sure requested entries are already registered. */
> +	size = 1 << hash->size_bits;
> +	for (i = 0; i < size; i++) {

	for (int i = 0; ...) 

-- Steve

> +		hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> +			del = __ftrace_lookup_ip(direct_functions, entry->ip);
> +			if (!del || del->direct != entry->direct)
> +				goto out_unlock;
> +		}
> +	}
> +
> +	err = -ENOMEM;
> +	new_filter_hash = hash_sub(old_filter_hash, hash);
> +	if (!new_filter_hash)
> +		goto out_unlock;
> +
> +	new_direct_functions = hash_sub(direct_functions, hash);
> +	if (!new_direct_functions)
> +		goto out_unlock;
> +
> +	/* If there's nothing left, we need to unregister the ops. */
> +	if (ftrace_hash_empty(new_filter_hash)) {
> +		err = unregister_ftrace_function(ops);
> +		if (!err) {
> +			/* cleanup for possible another register call */
> +			ops->func = NULL;
> +			ops->trampoline = 0;
> +			ftrace_free_filter(ops);
> +			ops->func_hash->filter_hash = NULL;
> +		}
> +	} 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) {
> +		/* free the new_direct_functions */
> +		old_direct_functions = new_direct_functions;
> +	} else {
> +		rcu_assign_pointer(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 */
>  
>  /**