Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

[Tetsuo Handa]

On 2024/06/25 23:17, John Ogness wrote:
> On 2024-06-25, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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.