Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe_entry

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 1 Sep 2025 08:00:15 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> On Mon, Sep 01, 2025 at 05:06:55PM +0900, Masami Hiramatsu wrote:
> > On Fri, 29 Aug 2025 04:11:02 -0700
> > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > 
> > > 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@chinatelecom.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.
> > 
> > That is similar to the kprobes, which also allows accessing in
> > rcu critical section or under mutex.
> > 
> > > 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.
> > 
> > What about changing it alloing it with preempt disabled flag?
> 
> I am not sure that "it" that you are proposing changing.  ;-)

Sorry, Ii meant the rcu_dereference_check().

> 
> However, another option for the the above rcu_dereference_check() to
> become something like this:
> 
> 	rcu_dereference_check(p, lockdep_rht_mutex_is_held(ht) ||
> 				 rcu_read_lock_any_held());
> 
> This would be happy with any RCU reader, including rcu_read_lock(),
> preempt_disable(), local_irq_disable(), local_bh_disable(), and various
> handler contexts.  One downside is that this would *always* be happy in
> a kernel built with CONFIG_PREEMPT_{NONE,VOLUNTARY}=y.

Ah, indeed. This means that we lose the ability to explicitly check
whether the rcu pointer is in a critical section on that kernel.

> 
> If this is happening often enough, it would be easy for me to create an
> rcu_dereference_all_check() that allows all forms of vanilla RCU readers
> (but not, for example, SRCU readers), but with only two use cases,
> it is not clear to me that this is an overall win.

OK, I think this discussion is important for the patch from Menglong [1]

[1] https://lore.kernel.org/all/20250829021436.19982-1-dongml2@chinatelecom.cn/

because this does not make an rcu critical section while using `head`
but it works because fprobe_entry() runs under preempt_disable().

Is it better to use `guard(rcu)()` instead of rcu_read_lock() so that
it explicitly secure the `head` usage? I just wonder if there is any
downside to extend rcu_read_lock() area (still in the same
preempt_disable section).

Thank you,

> 
> Or am I missing a turn in here somewhere?
> 
> 							Thanx, Paul
> 
> > Thank you,
> > 
> > > 
> > > 							Thanx, Paul
> > > 
> > > > -- Steve
> > > > 
> > > > 
> > > > > 
> > > > > Reported-by: kernel test robot <oliver.sang@intel.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@chinatelecom.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)
> > > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>