From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-166.mta0.migadu.com [91.218.175.166]) (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 6A0EB4AB3C8 for ; Thu, 15 Oct 2026 08:02:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.166 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792051377; cv=none; b=o//rWmhKfmYwFepJQjsX2bokxYFuAB38GLh+kExokzQq1C74fKxgdmBQY1T/Od6c7kVC5745N8FU8EcKWhwsJTjtBMsxqcVseQuf1/14vGhdAQu8FZQ+zNT7FXsa4Q9jj923a0TDlAKLI+wS3C6u1AtLD8QzVL6KFHt4GWW2LyQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792051377; c=relaxed/simple; bh=bd2dyGvu6eqSl5noAvuLcU4EENcETPUCuEO/37wxsPE=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=FHXVTroxWGsPza7KdZHzE0bqp0WtW9pY6AboNEXdqrtMKdQQiBEAUfHqNof68myGfnOSwK7moz9q/ka7Iqsc7l705yYIZUo7ZDJpVf85v/7lD5JpCMGBxXy5hnZbToeyDHLPvoqvLjI+om8Bs2a1WELIfck6i8ZLZWEAHwzLOH4= 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=s4TQKPIa; arc=none smtp.client-ip=91.218.175.166 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="s4TQKPIa" X-Envelope-To: linux-trace-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=bd2dyGvu6eqSl5noAvuLcU4EENcETPUCuEO/37wxsPE=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1792051373; v=1; x=1792656173; b=s4TQKPIaX6o7Og5RhXIatda8f0FON9GOxqxvg8Y2XCnxXkNOOrzAlAxUvzrylsFkvxl8lhbh 78VhE0mrCFesU1XNQiGF1zVAULDkLJeKXZQVjh7bwKgd/PE17sstcDmTwxIVTgdpBzjampvRu71 4YJ+3Q2f1SGfKS7xMolTHKpc= X-Envelope-To: linux-trace-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 1c4fe5bd21f346df; Thu, 15 Oct 2026 08:02:53 +0000 X-Mizu-Trace-ID: 1c4fe5bd21f346df 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 v2] tracing: Fix crash in print_graph_function_event() Date: Thu, 15 Oct 2026 16:03:51 +0800 Message-Id: <20261015080351.121316-1-hao.ge@linux.dev> X-Mailer: git-send-email 2.25.1 Precedence: bulk X-Mailing-List: linux-trace-kernel@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 it is never NULL: when the newly selected tracer has no flags at all, tracing_set_tracer() falls back to a static empty no_tracer_flags. run_tracer_selftest() can hit the same case, so the same fallback is added there too. 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 Suggested-by: Steven Rostedt Signed-off-by: Hao Ge --- Changes in v2: - Rework per Steve's suggestion. v1 could trip the WARN_ON_ONCE() in add_tracer() at boot, when an instance was created before the tracer registers. v1 Link: https://lore.kernel.org/all/20260920042139.70736-1-hao.ge@linux.dev/ --- kernel/trace/trace.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index e4a490d3d08c..fba9521d2511 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 @@ -1319,6 +1325,8 @@ static int run_tracer_selftest(struct tracer *type) tr->current_trace = type; tr->current_trace_flags = type->flags ? : type->default_flags; + if (!tr->current_trace_flags) + tr->current_trace_flags = &no_tracer_flags; #ifdef CONFIG_TRACER_MAX_TRACE if (tracer_uses_snapshot(type)) { @@ -5090,6 +5098,8 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf) } tr->current_trace_flags = t->flags ? : t->tracer->flags; + if (!tr->current_trace_flags) + tr->current_trace_flags = &no_tracer_flags; if (trace->init) { ret = tracer_init(trace, tr); -- 2.25.1