Messages in this thread Patch in this message | | | Date | Mon, 9 Jul 2018 17:35:34 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command |
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----- On Jul 9, 2018, at 5:09 PM, Alexei Starovoitov alexei.starovoitov@gmail.com wrote:
> On Sun, Jul 08, 2018 at 04:54:38PM -0400, Mathieu Desnoyers wrote: >> ----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes joelaf@google.com 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 <dancol@google.com> >> >> > --- >> >> > 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
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