linux-kernel - Re: [PATCH] bpf: defer printk() inside __bpf_prog

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Date: Wed, 26 Jun 2024 00:07:37 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: John Ogness <john.ogness@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau
 <martin.lau@...ux.dev>,
        Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

On 2024/06/25 23:17, John Ogness wrote:
> On 2024-06-25, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
>> syzbot is reporting circular locking dependency inside __bpf_prog_run(),
>> for fault injection calls printk() despite rq lock is already held.
>>
>> Guard __bpf_prog_run() using printk_deferred_{enter,exit}() (and
>> preempt_{disable,enable}() if CONFIG_PREEMPT_RT=n) in order to defer any
>> printk() messages.
> 
> Why is the reason for disabling preemption?

Because since kernel/printk/printk_safe.c uses a percpu counter for deferring
printk(), printk_safe_enter() and printk_safe_exit() have to be called from
the same CPU. preempt_disable() before printk_safe_enter() and preempt_enable()
after printk_safe_exit() guarantees that printk_safe_enter() and
printk_safe_exit() are called from the same CPU.