Subject: Re: [PATCH] net: procfs: Fix RCU stall and soft lockup in ptype_seq_next()

From: Jakub Kicinski <[email-masked]>
Date: Fri, 30 Jan 2026 19:18:45 -0800

On Wed, 28 Jan 2026 15:03:59 +0800 [email-masked] wrote:
> The root cause is in ptype_seq_next(): when iterating over packet
> types, it's possible that a packet type entry (pt) has been removed,
> its dev set to NULL, and pt->af_packet_net is not initialized.
> In that case, the function may return the same 'nxt' pointer indefinitely.
> This results in an infinite loop under RCU read-side critical section,
> causing an RCU stall and eventually a soft lockup.
> 
> Fix the issue by properly handling the case where 'nxt' points to
> an empty list, ensuring forward progress in the iterator.

> @@ -247,7 +247,7 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>  
>  	if (pt->af_packet_net) {
>  net_ptype_all:
> -		if (nxt != &net->ptype_all && nxt != &net->ptype_specific)
> +		if (!list_empty(nxt) && nxt != &net->ptype_all && nxt != &net->ptype_specific)
>  			goto found;
>  
>  		if (nxt == &net->ptype_all) {
> @@ -267,6 +267,9 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>  			return NULL;
>  		nxt = ptype_base[hash].next;
>  	}
> +
> +	if (list_empty(nxt))
> +		return NULL;
>  found:
>  	return list_entry(nxt, struct packet_type, list);
>  }

I'm not sure this fix works, TBH, we're dealing with an RCU list here.
The elements are not deleted with list_del_init(), so they won't
look "empty".

If the pt entries are under RCU protection I think the issue is that
af_packet is clearing pt->dev before waiting for the grace period to
expire.

Willem, is there a reason for that or just convenience?
-- 
pw-bot: cr