linux-kernel - Re: [PATCH 1/3] rcu: Return early if callback is not specified

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Message-ID: <aEtAtLu6IIv_0QHs@tardis.local>
Date: Thu, 12 Jun 2025 14:03:48 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Boqun Feng <boqun@...me.name>, "Paul E. McKenney" <paulmck@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	RCU <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 1/3] rcu: Return early if callback is not specified

On Thu, Jun 12, 2025 at 07:46:12PM +0200, Uladzislau Rezki wrote:
> On Thu, Jun 12, 2025 at 10:30:38AM -0700, Boqun Feng wrote:
> > 
> > 
> > On Tue, Jun 10, 2025, at 12:33 PM, Joel Fernandes wrote:
> > > On 6/10/2025 1:34 PM, Uladzislau Rezki (Sony) wrote:
> > >> Currently the call_rcu() API does not check whether a callback
> > >> pointer is NULL. If NULL is passed, rcu_core() will try to invoke
> > >> it, resulting in NULL pointer dereference and a kernel crash.
> > >> 
> > >> To prevent this and improve debuggability, this patch adds a check
> > >> for NULL and emits a kernel stack trace to help identify a faulty
> > >> caller.
> > >> 
> > >> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > >
> > > Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>
> > >
> > 
> > Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> > 
> Thank you for review, Boqun!
> 
> > > I will add this first one (only this one since we're discussing the others) to a
> > > new rcu/fixes-for-6.16 branch, but let me know if any objections.
> > >
> > 
> > Not sure it´s urgent enough given the current evidence.
> > 
> Let me clarify it a bit. My point is that, we get a kernel crash in a
> subsystem we are responsible for, i.e. no matter if there are faulty
> users of it(third party applications), the point is users can crash it.
> 

Fair enough.

> The kernel robot reports it and it is already a strong indication that
> the subsystem is not hardened against invalid inputs:
> 
> "BUG: unable to handle kernel NULL pointer dereference in rcu_core (3)"
> 
> so this in the rcu_core() which is part of RCU.
> 
> But, anyway Joel should decide. I shared my opinion :)
> 

Of course, my point is that the urgency is not high enough so we have to
put it in rcu/fixes, but it's a fix, and if Joel had the time to do
it, feel free. Joel's decision.

Regards,
Boqun

> --
> Uladzislau Rezki