linux-kernel - Re: [PATCHv3 bpf-next 4/8] ftrace: Add update_ftrace_direct

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Message-Id: <35e436ca1a638c9e30456c2a3645c23a20b2483160e3bb34b7c73fe42776f4df@mail.kernel.org>
Date: Thu, 20 Nov 2025 22:23:46 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: jolsa@...nel.org,rostedt@...nel.org,revest@...gle.com,mark.rutland@....com
Cc: bpf@...r.kernel.org,linux-kernel@...r.kernel.org,linux-trace-kernel@...r.kernel.org,linux-arm-kernel@...ts.infradead.org,ast@...nel.org,daniel@...earbox.net,andrii@...nel.org,menglong8.dong@...il.com,song@...nel.org,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCHv3 bpf-next 4/8] ftrace: Add update_ftrace_direct_del function

> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 9cf4cd56d..c571deeff 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -551,6 +551,7 @@ 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);
> +int update_ftrace_direct_del(struct ftrace_ops *ops, struct ftrace_hash *hash);
>
>  void ftrace_stub_direct_tramp(void);
>
> @@ -583,6 +584,11 @@ static inline int update_ftrace_direct_add(struct ftrace_ops *ops, struct ftrace
>  	return -ENODEV;
>  }
>
> +static inline int update_ftrace_direct_del(struct ftrace_ops *ops, struct ftrace_hash *hash)
> +{
> +	return -ENODEV;
> +}
> +

[ ... ]

> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 850ff55ff..cc730a8fd 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c

[ ... ]

> +/**
> + * 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);
                                           ^^^^^^^^^^^^

Can hash_sub() handle a NULL hash pointer in parameter a?  I notice the
parallel function hash_add() safely handles NULL via hash_count() and
alloc_and_copy_ftrace_hash() which checks ftrace_hash_empty().

However, hash_sub() directly dereferences a->size_bits before passing a
to alloc_and_copy_ftrace_hash().  This will crash if a is NULL.

This matters because update_ftrace_direct_del() calls hash_sub() with
old_filter_hash which can be NULL:

    old_filter_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
    ...
    new_filter_hash = hash_sub(old_filter_hash, hash);

The existing update_ftrace_direct_add() has the same defensive check and
passes the potentially-NULL old_filter_hash to hash_add(), which handles
it correctly.

> +	if (!sub)
> +		goto error;
> +
> +	size = 1 << b->size_bits;
> +	for (i = 0; i < size; i++) {
> +		hlist_for_each_entry(entry, &b->buckets[i], hlist) {
> +			del = __ftrace_lookup_ip(sub, entry->ip);
> +			if (WARN_ON_ONCE(!del))
> +				goto error;
> +			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)
> +{

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19552032346