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 BB9A64DDB5D; Fri, 16 Oct 2026 13:32:45 +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=1792157567; cv=none; b=MuguDngkLf//YbhD3M3mWcaFWJ+OTWRDzsIcNMZ67ky7nlhPQvvRSrK179MVVl/nY9wufJUzUS0MVAHbW2UMFCXoVEGJ81+d1ogCyEvwfLvAaCUZLccMuIBjUrm8SgiIc+QSpsGZAjHqlysrw4nL96uLp/+s1QzbQJo6+t9hlEo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792157567; c=relaxed/simple; bh=IX6H8arMlYubY/lS89ufiC7y/RqcLUMFhlz3ioSxX8Q=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=Y/HYPTWCcGOANI16n1nldc3iNt25suBfF93xXLKZ3nReBU4gMteWBji7JCtvh5RahKCojHwvwRGUDxw01/jpXDDJf0HBHh8Ds8DpksxqPLzxUj16f98W18pAcfsTQp6czT/XZ1CV6i4JWTtur9ZhlS5cgJXZNiIliuC0dbESb3k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=cpvecSfy; 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="cpvecSfy" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 893091F000C6; Fri, 16 Oct 2026 13:32:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792157565; bh=D5xTKdNA3CQIHjqgOAUhRXvpc0K+S7w4M//wDUxIodY=; h=Date:From:To:Cc:Subject:References; b=cpvecSfyAn20+W23rc0NSNYxTcvp3PyyStXuM7bPRfVxzcDxvrDgTRHU+3dtNp+6w SyzV40EA8XleH7hbO7bSdGgbxnvqN9jKhs4f9e8UDOi6K0071pU8fKp5EFF6gdJie0 bYLfNmIWHr8W80moeblPMRSRQ37WNSdhRIzsiHHCkZghDXQ8oRB9eOcucsvTPn7QtA xn3oRtRToyMSLdROyw8psNpsW55XvMmR6971JqHsqmglxyIjP1/NiiuvJGbuhtlrw2 3JDHmyDk/EojDw8Pv5CMNaP2SnALsafVoJfqVnhMxXIcvGKNmfCRseMSGRHhQuZTi1 z7AvxL34hcLPA== Received: from rostedt by gandalf with local (Exim 4.100.1) (envelope-from ) id 1xHi3A-00000000WUS-3PQB; Fri, 16 Oct 2026 09:32:44 -0400 Message-ID: <20261016133244.620040185@kernel.org> User-Agent: quilt/0.69 Date: Fri, 16 Oct 2026 09:32:19 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers , Andrew Morton , stable@vger.kernel.org, Hao Ge Subject: [for-linus][PATCH 1/3] tracing: Fix crash in print_graph_function_event() References: <20261016133218.800011513@kernel.org> 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 From: Hao Ge 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. Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20261015130658.221198-1-hao.ge@linux.dev Fixes: e29aa918a928 ("tracing: Have function graph tracer define options per instance") Suggested-by: Steven Rostedt Signed-off-by: Hao Ge Signed-off-by: Steven Rostedt --- kernel/trace/trace.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 3b22e70c6016..97a058623f94 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 @@ -1318,7 +1324,12 @@ static int run_tracer_selftest(struct tracer *type) tracing_reset_online_cpus(&tr->array_buffer); tr->current_trace = type; - tr->current_trace_flags = type->flags ? : type->default_flags; + if (type->flags) + tr->current_trace_flags = type->flags; + else if (type->default_flags) + tr->current_trace_flags = type->default_flags; + else + tr->current_trace_flags = &no_tracer_flags; #ifdef CONFIG_TRACER_MAX_TRACE if (tracer_uses_snapshot(type)) { @@ -5089,7 +5100,12 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf) return ret; } - tr->current_trace_flags = t->flags ? : t->tracer->flags; + if (t->flags) + tr->current_trace_flags = t->flags; + else if (t->tracer->flags) + tr->current_trace_flags = t->tracer->flags; + else + tr->current_trace_flags = &no_tracer_flags; if (trace->init) { ret = tracer_init(trace, tr); -- 2.53.0