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Message-Id: <20250905143336.85682a1443ed998137bee0b7@kernel.org>
Date: Fri, 5 Sep 2025 14:33:36 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tracing: fprobe: fix suspicious rcu usage in
fprobe_entry
On Thu, 4 Sep 2025 14:27:29 +0800
Menglong Dong <menglong8.dong@...il.com> 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
>
> As we discussed in [1], fix this by using guard(rcu)() in fprobe_entry()
> to protect the rhltable_lookup() and rhl_for_each_entry_rcu() with
> rcu_read_lock and suppress this warning.
>
> Link: https://lore.kernel.org/all/20250829021436.19982-1-dongml2@chinatelecom.cn/ [1]
> 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>
Looks good to me. Let me pick this to probes/for-next.
Thanks!
> ---
> v2:
> - use guard(rcu)() instead of rcu_read_lock()
> ---
> kernel/trace/fprobe.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index fb127fa95f21..6a205903b1ed 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -269,6 +269,7 @@ static int fprobe_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops,
> if (WARN_ON_ONCE(!fregs))
> return 0;
>
> + guard(rcu)();
> head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
> reserved_words = 0;
> rhl_for_each_entry_rcu(node, pos, head, hlist) {
> --
> 2.51.0
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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