netdev - Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in __bpf_prog

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Date: Wed, 24 May 2023 12:44:16 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Yonghong Song <yhs@...a.com>
Cc: Teng Qi <starmiku1207184332@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in __bpf_prog_put()

On Wed, May 24, 2023 at 12:34 PM Yonghong Song <yhs@...a.com> wrote:
>
>
>
> On 5/24/23 5:42 AM, Teng Qi wrote:
> > Thank you.
> >
> >> We cannot use rcu_read_lock_held() in the 'if' statement. The return
> >> value rcu_read_lock_held() could be 1 for some configurations regardless
> >> whether rcu_read_lock() is really held or not. In most cases,
> >> rcu_read_lock_held() is used in issuing potential warnings.
> >> Maybe there are other ways to record whether rcu_read_lock() is held or not?
> >
> > Sorry. I was not aware of the dependency of configurations of
> > rcu_read_lock_held().
> >
> >> If we cannot resolve rcu_read_lock() presence issue, maybe the condition
> >> can be !in_interrupt(), so any process-context will go to a workqueue.
> >
> > I agree that using !in_interrupt() as a condition is an acceptable solution.
>
> This should work although it could be conservative.
>
> >
> >> Alternatively, we could have another solution. We could add another
> >> function e.g., bpf_prog_put_rcu(), which indicates that bpf_prog_put()
> >> will be done in rcu context.
> >
> > Implementing a new function like bpf_prog_put_rcu() is a solution that involves
> > more significant changes.
>
> Maybe we can change signature of bpf_prog_put instead? Like
>     void bpf_prog_put(struct bpf_prog *prog, bool in_rcu)
> and inside bpf_prog_put we can add
>     WARN_ON_ONCE(in_rcu && !bpf_rcu_lock_held());

bpf_rcu_lock_held() is used for different cases.

Here s/in_irq/in_interrupt/ inside bpf_prog_put() is enough
to address this theoretical issue.