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Message-ID: <d5879c57-634f-4973-b52d-4994d0929de6@linux.alibaba.com> Date: Wed, 27 Dec 2023 16:20:09 +0800 From: "D. Wythe" <alibuda@...ux.alibaba.com> To: Alexei Starovoitov <alexei.starovoitov@...il.com> Cc: Pablo Neira Ayuso <pablo@...filter.org>, Jozsef Kadlecsik <kadlec@...filter.org>, Florian Westphal <fw@...len.de>, bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, coreteam@...filter.org, netfilter-devel <netfilter-devel@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org> Subject: Re: [RFC nf-next v3 1/2] netfilter: bpf: support prog update On 12/23/23 6:23 AM, Alexei Starovoitov wrote: > On Thu, Dec 21, 2023 at 11:06 PM D. Wythe <alibuda@...ux.alibaba.com> wrote: >> >> >> On 12/21/23 5:11 AM, Alexei Starovoitov wrote: >>> On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@...ux.alibaba.com> wrote: >>>> From: "D. Wythe" <alibuda@...ux.alibaba.com> >>>> >>>> To support the prog update, we need to ensure that the prog seen >>>> within the hook is always valid. Considering that hooks are always >>>> protected by rcu_read_lock(), which provide us the ability to >>>> access the prog under rcu. >>>> >>>> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com> >>>> --- >>>> net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++----------- >>>> 1 file changed, 48 insertions(+), 15 deletions(-) >>>> >>>> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c >>>> index e502ec0..9bc91d1 100644 >>>> --- a/net/netfilter/nf_bpf_link.c >>>> +++ b/net/netfilter/nf_bpf_link.c >>>> @@ -8,17 +8,8 @@ >>>> #include <net/netfilter/nf_bpf_link.h> >>>> #include <uapi/linux/netfilter_ipv4.h> >>>> >>>> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, >>>> - const struct nf_hook_state *s) >>>> -{ >>>> - const struct bpf_prog *prog = bpf_prog; >>>> - struct bpf_nf_ctx ctx = { >>>> - .state = s, >>>> - .skb = skb, >>>> - }; >>>> - >>>> - return bpf_prog_run(prog, &ctx); >>>> -} >>>> +/* protect link update in parallel */ >>>> +static DEFINE_MUTEX(bpf_nf_mutex); >>>> >>>> struct bpf_nf_link { >>>> struct bpf_link link; >>>> @@ -26,8 +17,20 @@ struct bpf_nf_link { >>>> struct net *net; >>>> u32 dead; >>>> const struct nf_defrag_hook *defrag_hook; >>>> + struct rcu_head head; >>> I have to point out the same issues as before, but >>> will ask them differently... >>> >>> Why do you think above rcu_head is necessary? >>> >>>> }; >>>> >>>> +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb, >>>> + const struct nf_hook_state *s) >>>> +{ >>>> + const struct bpf_nf_link *nf_link = bpf_link; >>>> + struct bpf_nf_ctx ctx = { >>>> + .state = s, >>>> + .skb = skb, >>>> + }; >>>> + return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx); >>>> +} >>>> + >>>> #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6) >>>> static const struct nf_defrag_hook * >>>> get_proto_defrag_hook(struct bpf_nf_link *link, >>>> @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link) >>>> static void bpf_nf_link_dealloc(struct bpf_link *link) >>>> { >>>> struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >>>> - >>>> - kfree(nf_link); >>>> + kfree_rcu(nf_link, head); >>> Why is this needed ? >>> Have you looked at tcx_link_lops ? >> Introducing rcu_head/kfree_rcu is to address the situation where the >> netfilter hooks might >> still access the link after bpf_nf_link_dealloc. > Why do you think so? > Hi Alexei, IMMO, nf_unregister_net_hook does not wait for the completion of the execution of the hook that is being removed, instead, it allocates a new array without the very hook to replace the old arrayvia rcu_assign_pointer() (in __nf_hook_entries_try_shrink), then it use call_rcu() to release the old one. You can find more details in commit 8c873e2199700c2de7dbd5eedb9d90d5f109462b. In other words, when nf_unregister_net_hook returns, there may still be contexts executing hooks on the old array, which means that the `link` may still be accessed after nf_unregister_net_hook returns. And that's the reason why we use kfree_rcu() to release the `link`. >> nf_hook_run_bpf >> const struct >> bpf_nf_link *nf_link = bpf_link; >> >> bpf_nf_link_release >> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); >> >> bpf_nf_link_dealloc >> free(link) >> bpf_prog_run(link->prog); >> >> >> I had checked the tcx_link_lops ,it's seems it use the synchronize_rcu() >> to solve the > Where do you see such code in tcx_link_lops ? I'm not certain if the reason that it choose to use synchronize_rcu()is the same as mine, but I did see it here: tcx_link_release() -> tcx_entry_sync() static inline void tcx_entry_sync(void) { /* bpf_mprog_entry got a/b swapped, therefore ensure that * there are no inflight users on the old one anymore. */ synchronize_rcu(); } >> same problem, which is also the way we used in the first version. >> >> https://lore.kernel.org/bpf/1702467945-38866-1-git-send-email-alibuda@linux.alibaba.com/ >> >> However, we have received some opposing views, believing that this is a >> bit overkill, >> so we decided to use kfree_rcu. >> >> https://lore.kernel.org/bpf/20231213222415.GA13818@breakpoint.cc/ >> >>>> } >>>> >>>> static int bpf_nf_link_detach(struct bpf_link *link) >>>> @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link, >>>> static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog, >>>> struct bpf_prog *old_prog) >>>> { >>>> - return -EOPNOTSUPP; >>>> + struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link); >>>> + int err = 0; >>>> + >>>> + mutex_lock(&bpf_nf_mutex); >>> Why do you need this mutex? >>> What race does it solve? >> To avoid user update a link with differ prog at the same time. I noticed >> that sys_bpf() >> doesn't seem to prevent being invoked by user at the same time. Have I >> missed something? > You're correct that sys_bpf() doesn't lock anything. > But what are you serializing in this bpf_nf_link_update() ? > What will happen if multiple bpf_nf_link_update() > without mutex run on different CPUs in parallel ? I must admit that it is indeed feasible if we eliminate the mutex and use cmpxchg to swap the prog (we need to ensure that there is only one bpf_prog_put() on the old prog). However, when cmpxchg fails, it means that this context has not outcompeted the other one, and we have to return a failure. Maybe something like this: if (!cmpxchg(&link->prog, old_prog, new_prog)) { /* already replaced by another link_update */ return -xxx; } As a comparison, The version with the mutex wouldn't encounter this error, every update would succeed. I think that it's too harsh for the user to receive a failure in that case since they haven't done anything wrong. Best wishes, D. Wythe
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