[PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Breno Leitao]

The function __netpoll_send_skb() is being invoked without holding the
RCU read lock. This oversight triggers a warning message when
CONFIG_PROVE_RCU_LIST is enabled:

	net/core/netpoll.c:330 suspicious rcu_dereference_check() usage!

	 netpoll_send_skb
	 netpoll_send_udp
	 write_ext_msg
	 console_flush_all
	 console_unlock
	 vprintk_emit

To prevent npinfo from disappearing unexpectedly, ensure that
__netpoll_send_skb() is protected with the RCU read lock.

Fixes: 2899656b494dcd1 ("netpoll: take rcu_read_lock_bh() in netpoll_send_skb_on_dev()")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 net/core/netpoll.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 62b4041aae1ae..cac389105e2d1 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -326,6 +326,7 @@ static netdev_tx_t __netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
 
 	lockdep_assert_irqs_disabled();
 
+	guard(rcu)();
 	dev = np->dev;
 	npinfo = rcu_dereference_bh(dev->npinfo);
 

---
base-commit: 7eb172143d5508b4da468ed59ee857c6e5e01da6
change-id: 20250303-netpoll_rcu_v2-fed72eb0cb83

Best regards,
-- 
Breno Leitao <leitao@debian.org>

Re: [PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Jakub Kicinski]

On Mon, 03 Mar 2025 03:44:12 -0800 Breno Leitao wrote:
> +	guard(rcu)();

Scoped guards if you have to.
Preferably just lock/unlock like a normal person..

Re: [PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Breno Leitao]

Hello Jakub,

On Tue, Mar 04, 2025 at 05:47:32PM -0800, Jakub Kicinski wrote:
> On Mon, 03 Mar 2025 03:44:12 -0800 Breno Leitao wrote:
> > +	guard(rcu)();
> 
> Scoped guards if you have to.
> Preferably just lock/unlock like a normal person..

Sure, I thought that we would be moving to scoped guards all across the
board, at least that was my reading for a similar patch I sent a while
ago:

	https://lore.kernel.org/all/20250224123016.GA17456@noisy.programming.kicks-ass.net/

Anyway, in which case should I use scoped guard instead of just being
like a normal person?

Thanks for the review,
--breno

Re: [PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Jakub Kicinski]

On Wed, 5 Mar 2025 01:09:49 -0800 Breno Leitao wrote:
> On Tue, Mar 04, 2025 at 05:47:32PM -0800, Jakub Kicinski wrote:
> > On Mon, 03 Mar 2025 03:44:12 -0800 Breno Leitao wrote:  
> > > +	guard(rcu)();  
> > 
> > Scoped guards if you have to.
> > Preferably just lock/unlock like a normal person..  
> 
> Sure, I thought that we would be moving to scoped guards all across the
> board, at least that was my reading for a similar patch I sent a while
> ago:
> 
> 	https://lore.kernel.org/all/20250224123016.GA17456@noisy.programming.kicks-ass.net/
> 
> Anyway, in which case should I use scoped guard instead 

We are certainly not moving to guards in networking. Too C++-sy.
Just lock / unlock please, correctly around the variable you actually
intend to protect.

Quoting documentation:

  Using device-managed and cleanup.h constructs
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  Netdev remains skeptical about promises of all "auto-cleanup" APIs,
  including even ``devm_`` helpers, historically. They are not the preferred
  style of implementation, merely an acceptable one.
  
  Use of ``guard()`` is discouraged within any function longer than 20 lines,
  ``scoped_guard()`` is considered more readable. Using normal lock/unlock is
  still (weakly) preferred.
  
  Low level cleanup constructs (such as ``__free()``) can be used when building
  APIs and helpers, especially scoped iterators. However, direct use of
  ``__free()`` within networking core and drivers is discouraged.
  Similar guidance applies to declaring variables mid-function.
  
See: https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#using-device-managed-and-cleanup-h-constructs

Re: [PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Andrew Lunn]

On Wed, Mar 05, 2025 at 01:09:49AM -0800, Breno Leitao wrote:
> Hello Jakub,
> 
> On Tue, Mar 04, 2025 at 05:47:32PM -0800, Jakub Kicinski wrote:
> > On Mon, 03 Mar 2025 03:44:12 -0800 Breno Leitao wrote:
> > > +	guard(rcu)();
> > 
> > Scoped guards if you have to.
> > Preferably just lock/unlock like a normal person..
> 
> Sure, I thought that we would be moving to scoped guards all across the
> board, at least that was my reading for a similar patch I sent a while
> ago:
> 
> 	https://lore.kernel.org/all/20250224123016.GA17456@noisy.programming.kicks-ass.net/
> 
> Anyway, in which case should I use scoped guard instead of just being
> like a normal person?

https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html

  Section 1.6.5: Using device-managed and cleanup.h constructs

  Netdev remains skeptical about promises of all “auto-cleanup” APIs,
  including even devm_ helpers, historically. They are not the
  preferred style of implementation, merely an acceptable one.

  Use of guard() is discouraged within any function longer than 20
  lines, scoped_guard() is considered more readable. Using normal
  lock/unlock is still (weakly) preferred.

  Low level cleanup constructs (such as __free()) can be used when
  building APIs and helpers, especially scoped iterators. However,
  direct use of __free() within networking core and drivers is
  discouraged. Similar guidance applies to declaring variables
  mid-function.

So you need to spend time to find out what each subsystems view is on
various APIs.

	Andrew

Re: [PATCH net] netpoll: guard __netpoll_send_skb() with RCU read lock

[Breno Leitao]

On Wed, Mar 05, 2025 at 05:09:14PM +0100, Andrew Lunn wrote:
> On Wed, Mar 05, 2025 at 01:09:49AM -0800, Breno Leitao wrote:
> > Hello Jakub,
> > 
> > On Tue, Mar 04, 2025 at 05:47:32PM -0800, Jakub Kicinski wrote:
> > > On Mon, 03 Mar 2025 03:44:12 -0800 Breno Leitao wrote:
> > > > +	guard(rcu)();
> > > 
> > > Scoped guards if you have to.
> > > Preferably just lock/unlock like a normal person..
> > 
> > Sure, I thought that we would be moving to scoped guards all across the
> > board, at least that was my reading for a similar patch I sent a while
> > ago:
> > 
> > 	https://lore.kernel.org/all/20250224123016.GA17456@noisy.programming.kicks-ass.net/
> > 
> > Anyway, in which case should I use scoped guard instead of just being
> > like a normal person?
> 
> https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html
> 
>   Section 1.6.5: Using device-managed and cleanup.h constructs
> 
>   Netdev remains skeptical about promises of all “auto-cleanup” APIs,
>   including even devm_ helpers, historically. They are not the
>   preferred style of implementation, merely an acceptable one.
> 
>   Use of guard() is discouraged within any function longer than 20
>   lines, scoped_guard() is considered more readable. Using normal
>   lock/unlock is still (weakly) preferred.
> 
>   Low level cleanup constructs (such as __free()) can be used when
>   building APIs and helpers, especially scoped iterators. However,
>   direct use of __free() within networking core and drivers is
>   discouraged. Similar guidance applies to declaring variables
>   mid-function.
> 
> So you need to spend time to find out what each subsystems view is on
> various APIs.

That is clear. thanks for the heads-up!

--breno