netdev - Re: [PATCH v5 bpf-next 2/5] bpf: Introduce sleepable BPF programs

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Date:   Tue, 30 Jun 2020 16:41:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     davem@...emloft.net, paulmck@...nel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v5 bpf-next 2/5] bpf: Introduce sleepable BPF programs

On Wed, Jul 01, 2020 at 01:26:44AM +0200, Daniel Borkmann wrote:
> On 6/30/20 6:33 AM, Alexei Starovoitov wrote:
> [...]
> > +/* list of non-sleepable kernel functions that are otherwise
> > + * available to attach by bpf_lsm or fmod_ret progs.
> > + */
> > +static int check_sleepable_blacklist(unsigned long addr)
> > +{
> > +#ifdef CONFIG_BPF_LSM
> > +	if (addr == (long)bpf_lsm_task_free)
> > +		return -EINVAL;
> > +#endif
> > +#ifdef CONFIG_SECURITY
> > +	if (addr == (long)security_task_free)
> > +		return -EINVAL;
> > +#endif
> > +	return 0;
> > +}
> 
> Would be nice to have some sort of generic function annotation to describe
> that code cannot sleep inside of it, and then filter based on that. Anyway,
> is above from manual code inspection?

yep. all manual. I don't think there is a way to automate it.
At least I cannot think of one.

> What about others like security_sock_rcv_skb() for example which could be
> bh_lock_sock()'ed (or, generally hooks running in softirq context)?

ahh. it's in running in bh at that point? then it should be added to blacklist.

The rough idea I had is to try all lsm_* and security_* hooks with all
debug kernel flags and see which ones will complain. Then add them to blacklist.
Unfortunately I'm completely swamped and cannot promise to do that
in the coming months.
So either we wait for somebody to do due diligence or land it knowing
that blacklist is incomplete and fix it up one by one.
I think the folks who're waiting on sleepable work would prefer the latter.
I'm fine whichever way.