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

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Date: Tue, 25 Jun 2024 17:53:14 +0206
From: John Ogness <john.ogness@...utronix.de>
To: 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>, 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 2024-06-26, 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?

John