linux-kernel - Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe

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Message-ID: <d1da3939-62e6-4ad1-afcc-5710ce3f6cbd@paulmck-laptop>
Date: Fri, 29 Aug 2025 04:11:02 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Menglong Dong <dongml2@...natelecom.cn>, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	kernel test robot <oliver.sang@...el.com>, tgraf@...g.ch,
	herbert@...dor.apana.org.au, linux-crypto@...r.kernel.org
Subject: Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe_entry

On Thu, Aug 28, 2025 at 10:23:57PM -0400, Steven Rostedt wrote:
> On Fri, 29 Aug 2025 10:14:36 +0800
> Menglong Dong <dongml2@...natelecom.cn> wrote:
> 
> > rcu_read_lock() is not needed in fprobe_entry, but rcu_dereference_check()
> > is used in rhltable_lookup(), which causes suspicious RCU usage warning:
> > 
> >   WARNING: suspicious RCU usage
> >   6.17.0-rc1-00001-gdfe0d675df82 #1 Tainted: G S
> >   -----------------------------
> >   include/linux/rhashtable.h:602 suspicious rcu_dereference_check() usage!
> >   ......
> >   stack backtrace:
> >   CPU: 1 UID: 0 PID: 4652 Comm: ftracetest Tainted: G S
> >   Tainted: [S]=CPU_OUT_OF_SPEC, [I]=FIRMWARE_WORKAROUND
> >   Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.1.1 10/07/2015
> >   Call Trace:
> >    <TASK>
> >    dump_stack_lvl+0x7c/0x90
> >    lockdep_rcu_suspicious+0x14f/0x1c0
> >    __rhashtable_lookup+0x1e0/0x260
> >    ? __pfx_kernel_clone+0x10/0x10
> >    fprobe_entry+0x9a/0x450
> >    ? __lock_acquire+0x6b0/0xca0
> >    ? find_held_lock+0x2b/0x80
> >    ? __pfx_fprobe_entry+0x10/0x10
> >    ? __pfx_kernel_clone+0x10/0x10
> >    ? lock_acquire+0x14c/0x2d0
> >    ? __might_fault+0x74/0xc0
> >    function_graph_enter_regs+0x2a0/0x550
> >    ? __do_sys_clone+0xb5/0x100
> >    ? __pfx_function_graph_enter_regs+0x10/0x10
> >    ? _copy_to_user+0x58/0x70
> >    ? __pfx_kernel_clone+0x10/0x10
> >    ? __x64_sys_rt_sigprocmask+0x114/0x180
> >    ? __pfx___x64_sys_rt_sigprocmask+0x10/0x10
> >    ? __pfx_kernel_clone+0x10/0x10
> >    ftrace_graph_func+0x87/0xb0
> > 
> > Fix this by using rcu_read_lock() for rhltable_lookup(). Alternatively, we
> > can use rcu_lock_acquire(&rcu_lock_map) here to obtain better performance.
> > However, it's not a common usage :/
> 
> So this is needed even though it's called under preempt_disable().
> 
> Paul, do we need to add an rcu_read_lock() because the code in rht
> (rhashtable) requires RCU read lock?
> 
> I thought that rcu_read_lock() and preempt_disable() have been merged?

Yes, preempt_disable() does indeed start an RCU read-side critical section,
just as surely as rcu_read_lock() does.

However, this is a lockdep check inside of __rhashtable_lookup():

	rht_dereference_rcu(ht->tbl, ht)

Which is defined as:

	rcu_dereference_check(p, lockdep_rht_mutex_is_held(ht));

This is explicitly telling lockdep that rcu_read_lock() is OK and
holding ht->mutex is OK, but nothing else is.

So an alternative way to fix this is to declare it to be a false positive,
and then avoid that false positive by adding a check that preemption
is disabled.  Adding the rhashtable maintainers for their perspective.

							Thanx, Paul

> -- Steve
> 
> 
> > 
> > Reported-by: kernel test robot <oliver.sang@...el.com>
> > Closes: https://lore.kernel.org/oe-lkp/202508281655.54c87330-lkp@intel.com
> > Fixes: dfe0d675df82 ("tracing: fprobe: use rhltable for fprobe_ip_table")
> > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > ---
> >  kernel/trace/fprobe.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index fb127fa95f21..fece0f849c1c 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -269,7 +269,9 @@ static int fprobe_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops,
> >  	if (WARN_ON_ONCE(!fregs))
> >  		return 0;
> >  
> > +	rcu_read_lock();
> >  	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
> > +	rcu_read_unlock();
> >  	reserved_words = 0;
> >  	rhl_for_each_entry_rcu(node, pos, head, hlist) {
> >  		if (node->addr != func)
>