From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 78082230BD5; Fri, 31 Jul 2026 00:27:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785457625; cv=none; b=meNdwDB66uSsL25MgCHlpNiqehOmnSXnMe6lH/9PbjMibwGkkAuPQ8FwELngYGyHtdyfcDcRapF4gel3ltMUH+AfEH0mDEPsL8i7lTK60Gprxak/H3mSY9y98sGhVSmzsaC9FYPq2kCpj41oDczPaiHm3VsztjeF2Bg5ayvqh50= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785457625; c=relaxed/simple; bh=GsAeSXPb/i0tQwsqsCLE+eQ0ByBWcYcY6nJsp270jLE=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=d1mawDMGJu/mtlj07L9wi5c2z4uEel5ZIfbmMjU/RZFlrwbbAxhkfMaMH3aXTNuzWNy3ib1VfPlq5X00tyJArHJZ58GNYcceQwgHy31uPlHE86rXohdnOyeL6ilNfMafQr4oOL9bdvlg50GjB4ByfKWXEMmYAZHkyV415puxG+s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SeKocfuk; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SeKocfuk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 806471F000E9; Fri, 31 Jul 2026 00:27:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785457624; bh=gl1PAht9zZ/yejPkp7VZqU/MURfNgRRuRRye82hpQOI=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=SeKocfukZKCk6+kvVnAs5b9RkfWRqOQ1kvNeHvel/Jv9eQydDwSrG5QxuB5bia8Xz oJ2Af33/xG//+xbdyYWsyau7G/E4oqF1k5aCy5zv8UZu3Oh/gLiiKL4uaw9i+ktj/b Ut1S5RZeAlqiiBJc4Qd1mrAlcS/HGiwpwJD4AcFR0QhFl0JcVseedxnixHNKUJU6nf x02FaAZyf0XGHrfrEsxymCsi3SRS+RdoQzqz7YwXjTQH3Zbm3BNp6LG6UeAsZ1WMWp kg/fezIasZtItT9koTn24s4SFKN2mfpkt0lY+OE/9cMTIeOqTCEwh9wvt7D//7GWpo 3q8/G/2bmShPA== Date: Fri, 31 Jul 2026 09:26:59 +0900 From: Masami Hiramatsu (Google) To: Michael Wu Cc: Steven Rostedt , linux-trace-kernel@vger.kernel.org, Mathieu Desnoyers , Namhyung Kim , linux-kernel@vger.kernel.org Subject: Re: [PATCH V2] tracing: Fix race between update_event_fields and event_define_fields Message-Id: <20260731092659.af05c03f86e827d3ab45672f@kernel.org> In-Reply-To: <08216429-43ba-efc2-f6b1-459210d07b02@allwinnertech.com> References: <8147bb04-b890-f18a-9cab-48b712f54856@allwinnertech.com> <20260730173157.e756d8a021d827b7aa9686cf@kernel.org> <9613182c-6299-3c68-5afb-2bf8d9f90f03@allwinnertech.com> <08216429-43ba-efc2-f6b1-459210d07b02@allwinnertech.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Thu, 30 Jul 2026 19:13:28 +0800 Michael Wu wrote: > Hi Masami, > Thanks for the review. I've rebased the patch. The new version is below. As Steve said, please send a clean patch mail (as a new thread) instead of embedding it in the mail. Embedding patch is just for testing, not for merging. Thank you, > > --- > From ec00b31c58b56f4cbbe6dd67c9f88a1d87d8d5ac Mon Sep 17 00:00:00 2001 > From: Michael Wu > Date: Wed, 29 Jul 2026 17:34:09 +0800 > Subject: [PATCH v2] tracing: Fix race between update_event_fields and > event_define_fields > > event_define_fields() (pri=1 MODULE_STATE_COMING notifier, locked by > event_mutex) populates class->fields via list_add(), while > update_event_fields() (called from the pri=0 notifier path via > trace_event_update_all) traverses class->fields protected only by > trace_event_sem. These are two different locks guarding the same > data structure, so during cross-module loading a reader on one CPU can > observe partially initialized list nodes being concurrently added by a > writer on another CPU. > > On arm64 with weak memory ordering, __list_add() writes to two > different cache lines: > > next->prev = new; // (1) ordinary store > new->next = next; // (2) ordinary store — > field's cache line > new->prev = prev; // (3) ordinary store > WRITE_ONCE(prev->next, new); // (4) release store — head's > cache line > > The store buffer can drain (2) and (4) independently since they target > different cache lines. A remote CPU may observe (4) before (2): it > sees prev->next pointing to the new node, but the new node's link.next > is still zero (kmem_cache_alloc zero-initialized via KMEM_CACHE with > SLAB_PANIC). Since offsetof(struct ftrace_event_field, link) == 0, > list_for_each_entry() derives field == NULL from link.next == 0 and > crashes at field->type (offset 0x18): > > Unable to handle kernel access ... at virtual address 0000000000000018 > pc : update_event_fields+0xf8/0x368 > Call trace: > update_event_fields+0xf8/0x368 > trace_event_update_all+0x7c/0x2b4 > trace_module_notify+0x4c/0x1dc > notifier_call_chain+0x84/0x168 > blocking_notifier_call_chain_robust+0x64/0xd4 > load_module+0x10c8/0x123c > __arm64_sys_finit_module+0x230/0x31c > > Fix by holding event_mutex in trace_event_update_all() before > down_write(&trace_event_sem). This serializes the reader against > event_define_fields() which takes event_mutex for the entire > write-side critical section. The lock ordering event_mutex -> > trace_event_sem is already established at three other sites > (trace_remove_event_call, event_trace_add_tracer, > event_trace_del_tracer), so this introduces no ordering conflict. > > An alternative of placing mutex_lock inside update_event_fields() was > considered but rejected — it would invert the established lock > ordering (trace_event_sem -> event_mutex) and risk ABBA deadlock. > > Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event > types as well") > Cc: stable@vger.kernel.org > Signed-off-by: Michael Wu > --- > kernel/trace/trace_events.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c > index 956692856fa8..9632788da5af 100644 > --- a/kernel/trace/trace_events.c > +++ b/kernel/trace/trace_events.c > @@ -3566,6 +3566,7 @@ void trace_event_update_all(struct trace_eval_map > **map, int len) > int last_i; > int i; > > + mutex_lock(&event_mutex); > down_write(&trace_event_sem); > list_for_each_entry_safe(call, p, &ftrace_events, list) { > /* events are usually grouped together with systems */ > @@ -3604,6 +3605,7 @@ void trace_event_update_all(struct trace_eval_map > **map, int len) > cond_resched(); > } > up_write(&trace_event_sem); > + mutex_unlock(&event_mutex); > } > > static bool event_in_systems(struct trace_event_call *call, > -- > 2.29.0 > > On 7/30/2026 6:54 PM, Michael Wu wrote: > > event_define_fields() (pri=1 MODULE_STATE_COMING notifier, locked by > > event_mutex) populates class->fields via list_add(), while > > update_event_fields() (called from the pri=0 notifier path via > > trace_event_update_all) traverses class->fields protected only by > > trace_event_sem.  These are two different locks guarding the same > > data structure, so during cross-module loading a reader on one CPU can > > observe partially initialized list nodes being concurrently added by a > > writer on another CPU. > > > > On arm64 with weak memory ordering, __list_add() writes to two > > different cache lines: > > > >         next->prev = new;                // (1) ordinary store > >         new->next = next;                // (2) ordinary store — > > field's cache line > >         new->prev = prev;                // (3) ordinary store > >         WRITE_ONCE(prev->next, new);    // (4) release store — head's > > cache line > > > > The store buffer can drain (2) and (4) independently since they target > > different cache lines.  A remote CPU may observe (4) before (2): it > > sees prev->next pointing to the new node, but the new node's link.next > > is still zero (kmem_cache_alloc zero-initialized via KMEM_CACHE with > > SLAB_PANIC).  Since offsetof(struct ftrace_event_field, link) == 0, > > list_for_each_entry() derives field == NULL from link.next == 0 and > > crashes at field->type (offset 0x18): > > > >   Unable to handle kernel access ... at virtual address 0000000000000018 > >   pc : update_event_fields+0xf8/0x368 > >   Call trace: > >    update_event_fields+0xf8/0x368 > >    trace_event_update_all+0x7c/0x2b4 > >    trace_module_notify+0x4c/0x1dc > >    notifier_call_chain+0x84/0x168 > >    blocking_notifier_call_chain_robust+0x64/0xd4 > >    load_module+0x10c8/0x123c > >    __arm64_sys_finit_module+0x230/0x31c > > > > Fix by holding event_mutex in trace_event_update_all() before > > down_write(&trace_event_sem).  This serializes the reader against > > event_define_fields() which takes event_mutex for the entire > > write-side critical section.  The lock ordering event_mutex -> > > trace_event_sem is already established at three other sites > > (trace_remove_event_call, event_trace_add_tracer, > > event_trace_del_tracer), so this introduces no ordering conflict. > > > > An alternative of placing mutex_lock inside update_event_fields() was > > considered but rejected — it would invert the established lock > > ordering (trace_event_sem -> event_mutex) and risk ABBA deadlock. > > > > Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event > > types as well") > > Cc: stable@vger.kernel.org > > --- > >  kernel/trace/trace_events.c | 2 ++ > >  1 file changed, 2 insertions(+) > > > > diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c > > index 956692856fa8..9632788da5af 100644 > > --- a/kernel/trace/trace_events.c > > +++ b/kernel/trace/trace_events.c > > @@ -3566,6 +3566,7 @@ void trace_event_update_all(struct trace_eval_map > > **map, int len) > >      int last_i; > >      int i; > > > > +    mutex_lock(&event_mutex); > >      down_write(&trace_event_sem); > >      list_for_each_entry_safe(call, p, &ftrace_events, list) { > >          /* events are usually grouped together with systems */ > > @@ -3604,6 +3605,7 @@ void trace_event_update_all(struct trace_eval_map > > **map, int len) > >          cond_resched(); > >      } > >      up_write(&trace_event_sem); > > +    mutex_unlock(&event_mutex); > >  } > > > >  static bool event_in_systems(struct trace_event_call *call, > > > -- > Regards, > Michael Wu -- Masami Hiramatsu (Google)