Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

From: Mathieu Desnoyers <[email-masked]>
Date: Mon, 9 Jul 2018 17:35:34 -0400 (EDT)

----- On Jul 9, 2018, at 5:09 PM, Alexei Starovoitov [email-masked] wrote:

> On Sun, Jul 08, 2018 at 04:54:38PM -0400, Mathieu Desnoyers wrote:
>> ----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes [email-masked] wrote:
>> 
>> > On Fri, Jul 06, 2018 at 07:54:28PM -0700, Alexei Starovoitov wrote:
>> >> On Fri, Jul 06, 2018 at 06:56:16PM -0700, Daniel Colascione wrote:
>> >> > BPF_SYNCHRONIZE waits for any BPF programs active at the time of
>> >> > BPF_SYNCHRONIZE to complete, allowing userspace to ensure atomicity of
>> >> > RCU data structure operations with respect to active programs. For
>> >> > example, userspace can update a map->map entry to point to a new map,
>> >> > use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to
>> >> > complete, and then drain the old map without fear that BPF programs
>> >> > may still be updating it.
>> >> > 
>> >> > Signed-off-by: Daniel Colascione <[email-masked]>
>> >> > ---
>> >> >  include/uapi/linux/bpf.h |  1 +
>> >> >  kernel/bpf/syscall.c     | 14 ++++++++++++++
>> >> >  2 files changed, 15 insertions(+)
>> >> > 
>> >> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> >> > index b7db3261c62d..4365c50e8055 100644
>> >> > --- a/include/uapi/linux/bpf.h
>> >> > +++ b/include/uapi/linux/bpf.h
>> >> > @@ -98,6 +98,7 @@ enum bpf_cmd {
>> >> >  	BPF_BTF_LOAD,
>> >> >  	BPF_BTF_GET_FD_BY_ID,
>> >> >  	BPF_TASK_FD_QUERY,
>> >> > +	BPF_SYNCHRONIZE,
>> >> >  };
>> >> >  
>> >> >  enum bpf_map_type {
>> >> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> >> > index d10ecd78105f..60ec7811846e 100644
>> >> > --- a/kernel/bpf/syscall.c
>> >> > +++ b/kernel/bpf/syscall.c
>> >> > @@ -2272,6 +2272,20 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *,
>> >> > uattr, unsigned int, siz
>> >> >  	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
>> >> >  		return -EPERM;
>> >> >  
>> >> > +	if (cmd == BPF_SYNCHRONIZE) {
>> >> > +		if (uattr != NULL || size != 0)
>> >> > +			return -EINVAL;
>> >> > +		err = security_bpf(cmd, NULL, 0);
>> >> > +		if (err < 0)
>> >> > +			return err;
>> >> > +		/* BPF programs are run with preempt disabled, so
>> >> > +		 * synchronize_sched is sufficient even with
>> >> > +		 * RCU_PREEMPT.
>> >> > +		 */
>> >> > +		synchronize_sched();
>> >> > +		return 0;
>> >> 
>> >> I don't think it's necessary. sys_membarrier() can do this already
>> >> and some folks use it exactly for this use case.
>> > 
>> > Alexei, the use of sys_membarrier for this purpose seems kind of weird to me
>> > though. No where does the manpage say membarrier should be implemented this
>> > way so what happens if the implementation changes?
>> > 
>> > Further, membarrier manpage says that a memory barrier should be matched with
>> > a matching barrier. In this use case there is no matching barrier, so it
>> > makes it weirder.
>> > 
>> > Lastly, sys_membarrier seems will not work on nohz-full systems, so its a bit
>> > fragile to depend on it for this?
>> > 
>> >        case MEMBARRIER_CMD_GLOBAL:
>> >                /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */
>> >                if (tick_nohz_full_enabled())
>> >                        return -EINVAL;
>> >                if (num_online_cpus() > 1)
>> >                        synchronize_sched();
>> >                return 0;
>> > 
>> > 
>> > Adding Mathieu as well who I believe is author/maintainer of membarrier.
>> 
>> See commit 907565337
>> "Fix: Disable sys_membarrier when nohz_full is enabled"
>> 
>> "Userspace applications should be allowed to expect the membarrier system
>> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
>> nohz_full CPUs, but synchronize_sched() does not take those into
>> account."
>> 
>> So AFAIU you'd want to re-use membarrier to issue synchronize_sched, and you
>> only care about kernel preempt off critical sections.
>> 
>> Clearly bpf code does not run in user-space, so it would "work".
>> 
>> But the guarantees provided by membarrier are not to synchronize against
>> preempt off per se. It's just that the current implementation happens to
>> do that. The point of membarrier is to turn user-space memory barriers
>> into compiler barriers.
>> 
>> If what you need is to wait for a RCU grace period for whatever RCU flavor
>> ebpf is using, I would against using membarrier for this. I would rather
>> recommend adding a dedicated BPF_SYNCHRONIZE so you won't leak
>> implementation details to user-space, *and* you can eventually change you
>> RCU implementation for e.g. SRCU in the future if needed.
> 
> The point about future changes to underlying bpf mechanisms is valid.
> There is work already on the way to reduce the scope of preempt_off+rcu_lock
> that currently lasts the whole prog. We will have new prog types that won't
> have such wrappers and will do rcu_lock/unlock and preempt on/off only
> when necessary.
> So something like BPF_SYNCHRONIZE will break soon, since the kernel cannot have
> guarantees on when programs finish. Calling this command BPF_SYNCHRONIZE_PROG
> also won't make sense for the same reason.
> What we can do it instead is to define synchronization barrier for
> programs accessing maps. May be call it something like:
> BPF_SYNC_MAP_ACCESS ?
> uapi/bpf.h would need to have extensive comment what this barrier is doing.
> Implementation should probably call synchronize_rcu() and not play games
> with synchronize_sched(), since that's going too much into implementation.
> Also should such sys_bpf command be root only?
> I'm not sure whether dos attack can be made by spamming synchronize_rcu()
> and synchronize_sched() for that matter.

Adding Paul E. McKenney in CC. He may want to share his thoughts on the matter.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com