From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out28-4.mail.aliyun.com (out28-4.mail.aliyun.com [115.124.28.4]) (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 A8D9B3A960F; Thu, 30 Jul 2026 10:54:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.28.4 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785408860; cv=none; b=Y5GT0einIveEm+/IYbOtGtwSpMSSL8N3OwBCpkE5+j8xMlZZ0+PeWRaWOiWp3DwYvgTP5Z+fcs1MoRV9TyF3QiqqnrXnHHUna+qU1FhFj0owVMM8iVIG7g/69oyHFO7EvS8uVD7zog/twE4jzWiT1GExHz7LH6cOonUEAofbAUo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785408860; c=relaxed/simple; bh=xpqaXO+hiYGSYMYnml/pYbToKxrlDqajpDni92l2RYw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=t3O2/glR3Hx5IihG42f1gxOFytimZGZkD8lcFAkVh0vAt9P9/yXIWHZ/ICpknplaEia5LjMItn91UwSnFjUn0Ke/W/YdhK6A1kBtmNmN5QQPWo0LtV6nQFNIdClZuRfjZ8YGEgssTyLHg5UAXuImSLS0XiEsXvmI9R+4Sp4Hlq0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=allwinnertech.com; spf=pass smtp.mailfrom=allwinnertech.com; arc=none smtp.client-ip=115.124.28.4 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=allwinnertech.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=allwinnertech.com X-Alimail-AntiSpam:AC=CONTINUE;BC=0.07183099|-1;CH=green;DM=|CONTINUE|false|;DS=CONTINUE|ham_alarm|0.436116-0.21577-0.348115;FP=13256731553195148666|0|0|0|0|-1|-1|-1;HT=maildocker-contentspam011083013073;MF=michael@allwinnertech.com;NM=1;PH=DS;RN=6;RT=6;SR=0;TI=SMTPD_---.iZT.u3r_1785408845; Received: from 192.168.208.183(mailfrom:michael@allwinnertech.com fp:SMTPD_---.iZT.u3r_1785408845 cluster:ay29) by smtp.aliyun-inc.com; Thu, 30 Jul 2026 18:54:06 +0800 Message-ID: <9613182c-6299-3c68-5afb-2bf8d9f90f03@allwinnertech.com> Date: Thu, 30 Jul 2026 18:54:05 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [PATCH] tracing: Fix race between update_event_fields and event_define_fields Content-Language: en-US To: "Masami Hiramatsu (Google)" Cc: Steven Rostedt , linux-trace-kernel@vger.kernel.org, Mathieu Desnoyers , Namhyung Kim , linux-kernel@vger.kernel.org References: <8147bb04-b890-f18a-9cab-48b712f54856@allwinnertech.com> <20260730173157.e756d8a021d827b7aa9686cf@kernel.org> From: Michael Wu In-Reply-To: <20260730173157.e756d8a021d827b7aa9686cf@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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, -- 2.29.0 On 7/30/2026 4:31 PM, Masami Hiramatsu (Google) wrote: > Hi Michael, > > On Thu, 30 Jul 2026 13:37:59 +0800 > Michael Wu wrote: > >> Kernel: Linux version 6.18.21 >> Platform: Android > > We will accept the patch which is for the upstream kernel here. > Can you rebase the patch on top of trace/fixes branch of > https://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git > or linus tree? > >> >> 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. > > I confirmed this is still valid on upstream kernel. > I have some comments, see below; > >> >> 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. >> > > Also add: > > Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well") > Cc: stable@vger.kernel.org > > Thank you, > >> 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 >> >> >> -- >> Regards, >> Michael Wu > > -- Regards, Michael Wu