linux-kernel - Re: [PATCH v2 2/7] tracing: fprobe: Cleanup fprobe hash when module unloading

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Message-Id: <20250408085241.cfd5f87e8197fa6f938e87b0@kernel.org>
Date: Tue, 8 Apr 2025 08:52:41 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] tracing: fprobe: Cleanup fprobe hash when module
 unloading

On Tue,  1 Apr 2025 00:35:44 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Cleanup fprobe address hash table on module unloading because the
> target symbols will be disappeared when unloading module and not
> sure the same symbol is mapped on the same address.
> 
> Note that this is at least disables the fprobes if a part of target
> symbols on the unloaded modules. Unlike kprobes, fprobe does not
> re-enable the probe point by itself. To do that, the caller should
> take care register/unregister fprobe when loading/unloading modules.
> This simplifies the fprobe state managememt related to the module
> loading/unloading.
> 

I think this is a kind of fix for commit 4346ba160409 ("fprobe: Rewrite
 fprobe on function-graph tracer") which introduced the fprobe_hlist_node.

Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")

Thanks,

> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>  kernel/trace/fprobe.c |  103 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 101 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index cb86f90d4b1e..95c6e3473a76 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -89,8 +89,11 @@ static bool delete_fprobe_node(struct fprobe_hlist_node *node)
>  {
>  	lockdep_assert_held(&fprobe_mutex);
>  
> -	WRITE_ONCE(node->fp, NULL);
> -	hlist_del_rcu(&node->hlist);
> +	/* Avoid double deleting */
> +	if (READ_ONCE(node->fp) != NULL) {
> +		WRITE_ONCE(node->fp, NULL);
> +		hlist_del_rcu(&node->hlist);
> +	}
>  	return !!find_first_fprobe_node(node->addr);
>  }
>  
> @@ -411,6 +414,102 @@ static void fprobe_graph_remove_ips(unsigned long *addrs, int num)
>  		ftrace_set_filter_ips(&fprobe_graph_ops.ops, addrs, num, 1, 0);
>  }
>  
> +#ifdef CONFIG_MODULES
> +
> +#define FPROBE_IPS_BATCH_INIT 8
> +/* instruction pointer address list */
> +struct fprobe_addr_list {
> +	int index;
> +	int size;
> +	unsigned long *addrs;
> +};
> +
> +static int fprobe_addr_list_add(struct fprobe_addr_list *alist, unsigned long addr)
> +{
> +	unsigned long *addrs;
> +
> +	if (alist->index >= alist->size)
> +		return -ENOMEM;
> +
> +	alist->addrs[alist->index++] = addr;
> +	if (alist->index < alist->size)
> +		return 0;
> +
> +	/* Expand the address list */
> +	addrs = kcalloc(alist->size * 2, sizeof(*addrs), GFP_KERNEL);
> +	if (!addrs)
> +		return -ENOMEM;
> +
> +	memcpy(addrs, alist->addrs, alist->size * sizeof(*addrs));
> +	alist->size *= 2;
> +	kfree(alist->addrs);
> +	alist->addrs = addrs;
> +
> +	return 0;
> +}
> +
> +static void fprobe_remove_node_in_module(struct module *mod, struct hlist_head *head,
> +					struct fprobe_addr_list *alist)
> +{
> +	struct fprobe_hlist_node *node;
> +	int ret = 0;
> +
> +	hlist_for_each_entry_rcu(node, head, hlist) {
> +		if (!within_module(node->addr, mod))
> +			continue;
> +		if (delete_fprobe_node(node))
> +			continue;
> +		/*
> +		 * If failed to update alist, just continue to update hlist.
> +		 * Therefore, at list user handler will not hit anymore.
> +		 */
> +		if (!ret)
> +			ret = fprobe_addr_list_add(alist, node->addr);
> +	}
> +}
> +
> +/* Handle module unloading to manage fprobe_ip_table. */
> +static int fprobe_module_callback(struct notifier_block *nb,
> +				  unsigned long val, void *data)
> +{
> +	struct fprobe_addr_list alist = {.size = FPROBE_IPS_BATCH_INIT};
> +	struct module *mod = data;
> +	int i;
> +
> +	if (val != MODULE_STATE_GOING)
> +		return NOTIFY_DONE;
> +
> +	alist.addrs = kcalloc(alist.size, sizeof(*alist.addrs), GFP_KERNEL);
> +	/* If failed to alloc memory, we can not remove ips from hash. */
> +	if (!alist.addrs)
> +		return NOTIFY_DONE;
> +
> +	mutex_lock(&fprobe_mutex);
> +	for (i = 0; i < FPROBE_IP_TABLE_SIZE; i++)
> +		fprobe_remove_node_in_module(mod, &fprobe_ip_table[i], &alist);
> +
> +	if (alist.index < alist.size && alist.index > 0)
> +		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
> +				      alist.addrs, alist.index, 1, 0);
> +	mutex_unlock(&fprobe_mutex);
> +
> +	kfree(alist.addrs);
> +
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block fprobe_module_nb = {
> +	.notifier_call = fprobe_module_callback,
> +	.priority = 0,
> +};
> +
> +static int __init init_fprobe_module(void)
> +{
> +	return register_module_notifier(&fprobe_module_nb);
> +}
> +early_initcall(init_fprobe_module);
> +#endif
> +
>  static int symbols_cmp(const void *a, const void *b)
>  {
>  	const char **str_a = (const char **) a;
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>