On Tue, Jun 25, 2024 at 9:05 AM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On 2024/06/26 0:47, John Ogness wrote:
> > On 2024-06-26, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> >> 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.
> >
> > 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?
>
> No, I'm not aware of such configuration.
>
> Does migrate_disable() imply preempt_disable() ?
> If yes, we don't need to also call preempt_disable().
> My understanding is that migration is about "on which CPU a process runs"
> and preemption is about "whether a different process runs on this CPU".
> That is, disabling migration and disabling preemption are independent.
>
> Is migrate_disable() alone sufficient for managing a percpu counter?
> If yes, we don't need to also call preempt_disable() in order to manage
> a percpu counter.
If you want to add printk_deferred_enter() it
probably should be in should_fail_ex(). Not here.
We will not be wrapping all bpf progs this way.
pw-bot: cr
