Re: [PATCH v2 2/2] netconsole: allow selection of egress interface via MAC address

[Uday Shankar]

On Wed, Feb 05, 2025 at 11:07:45AM -0800, Breno Leitao wrote:
> > +	else if (is_valid_ether_addr(np->dev_mac))
> > +		ndev = dev_getbyhwaddr_rcu(net, ARPHRD_ETHER, np->dev_mac);
> 
> You do not have the RCU read lock here. You have the rtnl(), which is
> sufficient, but, CONFIG_PROVE_RCU_LIST will show something as:
> 
> 	WARNING: suspicious RCU usage
> 	6.13.0-09701-g6610c7be45bb-dirty #18 Not tainted
> 	-----------------------------
> 	net/core/dev.c:1143 RCU-list traversed in non-reader section!!
> 	other info that might help us debug this:
> 	rcu_scheduler_active = 2, debug_locks = 1
> 	1 lock held by swapper/0/1:
> 	 #0: ffffffff832795b8 (rtnl_mutex){+.+.}-{4:4}, at: netpoll_setup+0x48/0x540
> 	stack backtrace:
> 	CPU: 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.13.0-virtme-09701-g6610c7be45bb-dirty #18
> 	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> 	Call Trace:
> 	 <TASK>
> 	 dump_stack_lvl+0x9f/0xf0
> 	 lockdep_rcu_suspicious+0x11a/0x150
> 	 dev_getbyhwaddr_rcu+0xb6/0xc0
> 	 netpoll_setup+0x8a/0x540
> 	 ? netpoll_parse_options+0x2bd/0x310
> 
> This is not a problem per-se, since you have RTNL. We probably need to
> tell for_each_netdev_rcu() to not comply about "RCU-list traversed in
> non-reader section" if RTNL is held. Not sure why we didn't hit in the
> test infrastructure, tho:
> 
> 	https://patchwork.kernel.org/project/netdevbpf/patch/20250204-netconsole-v2-2-5ef5eb5f6056@xxxxxxxxxxxxxxx/

I don't think there is an automated test that will hit this path yet. I
guess you got this trace from your manual testing?

> 
> Anyway, no action item for you here. I am talking to Jakub on a way to
> solve it, and I should send a fix soon.

/**
 * list_for_each_entry_rcu	-	iterate over rcu list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 * @cond:	optional lockdep expression if called from non-RCU protection.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_entry_rcu(pos, head, member, cond...)		\
	for (__list_check_rcu(dummy, ## cond, 0),			\
	     pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
		&pos->member != (head);					\
		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

If we do something like

list_for_each_entry_rcu(..., lockdep_rtnl_is_held())
	...

I think that code will be okay with being called with either rcu or rtnl
held. Of course, we need to plumb it through the net-specific helpers.