Messages in this thread | | | Date | Thu, 14 Dec 2023 13:31:22 +0800 | Subject | Re: [RFC nf-next 1/2] netfilter: bpf: support prog update | From | "D. Wythe" <> |
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On 12/14/23 6:24 AM, Florian Westphal wrote: > D. Wythe <alibuda@linux.alibaba.com> wrote: >> From: "D. Wythe" <alibuda@linux.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 use a >> new RCU-protected context to access the prog. >> >> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com> >> --- >> net/netfilter/nf_bpf_link.c | 124 +++++++++++++++++++++++++++++++++++++++----- >> 1 file changed, 111 insertions(+), 13 deletions(-) >> >> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c >> index e502ec0..918c470 100644 >> --- a/net/netfilter/nf_bpf_link.c >> +++ b/net/netfilter/nf_bpf_link.c >> @@ -8,17 +8,11 @@ >> #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) >> +struct bpf_nf_hook_ctx >> { >> - const struct bpf_prog *prog = bpf_prog; >> - struct bpf_nf_ctx ctx = { >> - .state = s, >> - .skb = skb, >> - }; >> - >> - return bpf_prog_run(prog, &ctx); >> -} >> + struct bpf_prog *prog; >> + struct rcu_head rcu; >> +}; > I don't understand the need for this structure. AFAICS bpf_prog_put() > will always release the program via call_rcu()? > > If it doesn't, we are probably already in trouble as-is without this > patch, I don't think anything that prevents us from ending up calling already > released bpf prog, or releasing it while another cpu is still running it > if bpf_prog_put releases the actual underlying prog instantly. > > A BPF expert could confirm bpf-prog-put-is-call-rcu.
Hi Florian,
I must admit that I did not realize that bpf_prog is released under RCU ...
>> struct bpf_nf_link { >> struct bpf_link link; >> @@ -26,8 +20,59 @@ struct bpf_nf_link { >> struct net *net; >> u32 dead; >> const struct nf_defrag_hook *defrag_hook; >> + /* protect link update in parallel */ >> + struct mutex update_lock; >> + struct bpf_nf_hook_ctx __rcu *hook_ctx; > What kind of replacements-per-second rate are you aiming for? > I think > > static DEFINE_MUTEX(bpf_nf_mutex); > > is enough.
I'm okay with that.
> > Then bpf_nf_link gains > > struct bpf_prog __rcu *prog > > and possibly a trailing struct rcu_head, see below.
Yes, that's what we need.
>> +static void bpf_nf_hook_ctx_free_rcu(struct bpf_nf_hook_ctx *hook_ctx) >> +{ >> + call_rcu(&hook_ctx->rcu, __bpf_nf_hook_ctx_free_rcu); >> +} > Don't understand the need for call_rcu either, see below. > >> +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 *link = bpf_link; >> + struct bpf_nf_hook_ctx *hook_ctx; >> + struct bpf_nf_ctx ctx = { >> + .state = s, >> + .skb = skb, >> + }; >> + >> + hook_ctx = rcu_dereference(link->hook_ctx); > This could then just rcu_deref link->prog. > >> + return bpf_prog_run(hook_ctx->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, >> @@ -120,6 +165,10 @@ static void bpf_nf_link_release(struct bpf_link *link) >> if (!cmpxchg(&nf_link->dead, 0, 1)) { >> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops); >> bpf_nf_disable_defrag(nf_link); >> + /* Wait for outstanding hook to complete before the >> + * link gets released. >> + */ >> + synchronize_rcu(); >> } > Could you convert bpf_nf_link_dealloc to release via kfree_rcu instead? > Got it. >> @@ -162,7 +212,42 @@ 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); >> + struct bpf_nf_hook_ctx *hook_ctx; >> + int err = 0; >> + >> + mutex_lock(&nf_link->update_lock); >> + > I think you need to check link->dead here too.
Got that. > >> + /* bpf_nf_link_release() ensures that after its execution, there will be >> + * no ongoing or upcoming execution of nf_hook_run_bpf() within any context. >> + * Therefore, within nf_hook_run_bpf(), the link remains valid at all times." >> + */ >> + link->hook_ops.priv = link; > ATM we only need to make sure the bpf prog itself stays alive until after > all concurrent rcu critical sections have completed. > > After this change, struct bpf_link gets passed instead, so we need to > keep that alive too. > > Which works with synchronize_rcu, sure, but that seems a bit overkill here.
Got it! Thank you very much for your suggestion. I will address those issues you mentioned in the next version.
Best wishes, D. Wythe
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