linux-kernel - Re: [syzbot] [kernel?] possible deadlock in console_flush

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Message-ID: <ab273ae2-80ec-45da-910a-e74298c71d50@I-love.SAKURA.ne.jp>
Date: Wed, 20 Mar 2024 15:50:02 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot <syzbot+f78380e4eae53c64125c@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
        John Ogness
 <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bpf <bpf@...r.kernel.org>
Subject: Re: [syzbot] [kernel?] possible deadlock in console_flush_all (2)

Hello, bpf developers.

syzbot is reporting circular locking dependency because bpf program is hitting
printk() path. To avoid this dependency, something like
https://syzkaller.appspot.com/text?tag=Patch&x=121c92fe180000 is needed.

Is it possible to call printk_deferred_{enter,exit}() from the bpf side (e.g.
bpf_prog_run()) so that we don't need to add overhead on the scheduler side
when bpf programs are not registered?

printk_deferred_{enter,exit}() requires that printk_deferred_exit() is called
on a CPU which printk_deferred_enter() was called, for percpu counter is used.
Is migrate_{disable,enable}() also needed around printk_deferred_{enter,exit}() ?

On 2023/10/08 15:28, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7d730f1bf6f3 Add linux-next specific files for 20231005
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=172d36e9680000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f532286be4fff4b5
> dashboard link: https://syzkaller.appspot.com/bug?extid=f78380e4eae53c64125c
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40