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

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

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.

Yeah, that kind of trick may not good. 

> However, it's not a common usage :/
> 
> 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>

OK, I agree this fixes the problem. Let me pick it.

> ---
>  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)
> -- 
> 2.51.0
> 


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