From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-140.mta0.migadu.com [91.218.175.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0F67F1F94F for ; Sun, 20 Sep 2026 04:20:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.140 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789878055; cv=none; b=ihMybV88mGf3AWVlfQ6yyHLyfVBgrmO1mZpI8oat79kVnwM5GN2i6PQpXivobdyzJ8/+ei3yOcexui8kB0f0VKHl+fzLCG7YNS+sM9jSdo/SpZHq7s5CvQpzJ+EurWYQPQMfet1auEIiYrAmJgbj3awRgR6bADIMY8ErvY7U+TE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789878055; c=relaxed/simple; bh=i3fx8YLMM25rGFTkB/3MYE/SKkwCBHP2NgBwbXTXgc4=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=aFLui1rvfuZ0zYFizlyJyr+iFWlhD7BjLb8B9KIzrJJ5Dn8VilE2LXMRqVt69cSdpM4Vahr0DLSKC2/yNM0zKRyshPXCHyYqG0e6F+fhawwRqVi4TEtfHbdYhVv85W+PARGGQGnXhc3p/kKTsZIR9V35MTz2oX1zVnMFIljeJS4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=XX4Z0raJ; arc=none smtp.client-ip=91.218.175.140 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="XX4Z0raJ" X-Envelope-To: stable@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=i3fx8YLMM25rGFTkB/3MYE/SKkwCBHP2NgBwbXTXgc4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1789878051; v=1; x=1790482851; b=XX4Z0raJnBR5tTovOnOLRc6GH8am+962N6sA5Dop8weDpvh4uzRsOlEKxVJVfpeOeOE3lNE1 bs31NXyjYP/U25tO8omBGkvLKaICnX5T90JQVoNke2MZDOmXV4raeFz8Z2jFizL+7rrEWPvf4su 8/MuWbwu+TKKadNxbGkP/ue8= X-Envelope-To: stable@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 2345de5eea115e61; Sun, 20 Sep 2026 04:20:41 +0000 X-Mizu-Trace-ID: 2345de5eea115e61 X-Migadu-Flow: FLOW_OUT From: Hao Ge To: Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Hao Ge , stable@vger.kernel.org Subject: [PATCH] tracing: Fix crash in print_graph_function_event() Date: Sun, 20 Sep 2026 12:21:39 +0800 Message-Id: <20260920042139.70736-1-hao.ge@linux.dev> X-Mailer: git-send-email 2.25.1 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit tracing_set_tracer() assigns the flags of the newly selected tracer with: tr->current_trace_flags = t->flags ? : t->tracer->flags; If the new tracer defines neither static flags nor default flags, like hwlat, irqsoff or wakeup, add_tracer() has no per instance flags copy for it, and both operands are NULL, so tr->current_trace_flags ends up being NULL as well. The ring buffer is not cleared when switching tracers: it is reset inside tracer_init(), after current_trace_flags was already assigned. If the buffer still holds function_graph entries of the previous tracer, printing one of them dereferences the NULL pointer. This was hit by cat trace during an ftrace stress test: BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor read access in kernel mode RIP: 0010:print_graph_function_event+0xe/0x20 Call Trace: s_show+0x4d/0x110 seq_read_iter+0x2d4/0x490 seq_read+0x130/0x170 vfs_read+0xb5/0x370 ksys_read+0x6d/0xe0 do_syscall_64+0xbc/0x450 The snapshot buffer is not reset at all, so this can be reproduced without a race: echo function_graph > /sys/kernel/tracing/current_tracer echo 1 > /sys/kernel/tracing/snapshot echo hwlat > /sys/kernel/tracing/current_tracer cat /sys/kernel/tracing/snapshot Rather than teaching every user of tr->current_trace_flags about this corner case, make sure every tracer has a set of flags: register_tracer() assigns the empty no_tracer_flags to tracers that define none. The existing users already handle opts == NULL, so these tracers keep behaving as before. Fixes: e29aa918a928 ("tracing: Have function graph tracer define options per instance") Cc: stable@vger.kernel.org Signed-off-by: Hao Ge --- kernel/trace/trace.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index e4a490d3d08c..1bd757b65be4 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -99,6 +99,12 @@ struct tracers { struct tracer_flags *flags; }; +/* + * Tracers that do not define their own flags use these empty flags, + * as tr->current_trace_flags must never be NULL. + */ +static struct tracer_flags no_tracer_flags; + /* * To prevent the comm cache from being overwritten when no * tracing is active, only save the comm when a trace event @@ -1485,9 +1491,11 @@ int __init register_tracer(struct tracer *type) } } - /* store the tracer for __set_tracer_option */ + /* All tracers must have flags, use the empty flags if none are defined */ if (type->flags) type->flags->trace = type; + else if (!type->default_flags) + type->default_flags = &no_tracer_flags; ret = do_run_tracer_selftest(type); if (ret < 0) @@ -8096,9 +8104,6 @@ static int add_tracer(struct trace_array *tr, struct tracer *tracer) flags = tracer->flags; if (!flags) { - if (!tracer->default_flags) - return 0; - /* * If the tracer defines default flags, it means the flags are * per trace instance. -- 2.25.1