From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 09A9B2EDD7C; Mon, 1 Sep 2025 08:06:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756714020; cv=none; b=svzsJMiYyF/HJPRQ8lXaXV3S7/Nk1sf7JAXlYtDfG3f6gw3BjTQjUziJ81dlzowuZwfr5bFpSypooQ0D1f905KG427ckmqpn+hZJQVD4iWA59DDIou8ZWajpLhTFqx6F8I7ycrBYK2fyFF0T4DLwLERpWn/bv4m8SV9VPlgxWYI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756714020; c=relaxed/simple; bh=VLpNS+bUWt279KK8jGyWrJFiwF8KwIr7Y32Y2kKXe9I=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=sOwIXvcqWNLwEN2tNtyntwQQ3MtdTjDhbfSkMb+QuGzbSxsEe2oqy8EvV55fi9yqjBJPwD1YGAx9MoX1kikDThNVvYv3ieeBXKWfdA99x9AubjRf/4GUF8IIURkg4iajO8TQjH2t32W1kQ/c8DMpHT7ISInjEy6VsAMcDafjRbM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dGt8xmJO; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dGt8xmJO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2C78DC4CEF0; Mon, 1 Sep 2025 08:06:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756714019; bh=VLpNS+bUWt279KK8jGyWrJFiwF8KwIr7Y32Y2kKXe9I=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=dGt8xmJOTDfXL69BWi4P5wTdsZoYcC3mBi8hYyPJW074UCn9Tm0f4Rcm6dswzScl6 ZZcSNn9v7JdNMB+7opPjArafXDpgDaWucvRE3mv9BfUnFDneaptU5GWAiji+IIxIrd /oq2sDDR4/T9k8qj8Uvi1fgLJ2oVEtRiEGQXrknA04Fnx19SDyg2pBkUpqMlLiYi9N ULzqxUsEyr7dGL5G0Xw8zoxQtaFdu7qh5nkdps/GqMuElJ2EQqRR2Y6bzdJMJmv+cF Vw6FrugYoSfUjlAHNQhoDEo0l/Lu0xXhIHmVkvvk3Umv1NKGEtzkySA+V3J4cjCG91 K+HBB/uBBd3nw== Date: Mon, 1 Sep 2025 17:06:55 +0900 From: Masami Hiramatsu (Google) To: paulmck@kernel.org Cc: Steven Rostedt , Menglong Dong , mhiramat@kernel.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, kernel test robot , tgraf@suug.ch, herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org Subject: Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe_entry Message-Id: <20250901170655.0757884ad7c2afb63ced3230@kernel.org> In-Reply-To: References: <20250829021436.19982-1-dongml2@chinatelecom.cn> <20250828222357.55fab4c2@batman.local.home> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 29 Aug 2025 04:11:02 -0700 "Paul E. McKenney" 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 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: > > > > > > 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? Thank you, > > Thanx, Paul > > > -- Steve > > > > > > > > > > Reported-by: kernel test robot > > > 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 > > > --- > > > 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)