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

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Date: Wed, 26 Jun 2024 10:18:08 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	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>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

On Tue 2024-06-25 17:53:14, John Ogness wrote:
> On 2024-06-26, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
> > 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.
> 
> Yes, but we already have cant_migrate(). Are you suggesting there are
> configurations where cant_migrate() is true but the context can be
> migrated anyway?

IMHO, we want to enter printk_safe only with preemption disabled.
Otherwise, printk() would stay deferred on the given CPU for any
task scheduled in this section.

Best Regards,
Petr