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Date: Fri, 22 Dec 2023 15:06:17 +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 12/21/23 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.
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
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?
Best wishes,
D. Wythe
>> +
>> + if (nf_link->dead) {
>> + err = -EPERM;
>> + goto out;
>> + }
>> +
>> + /* target old_prog mismatch */
>> + if (old_prog && link->prog != old_prog) {
>> + err = -EPERM;
>> + goto out;
>> + }
>> +
>> + old_prog = link->prog;
>> + if (old_prog == new_prog) {
>> + /* don't need update */
>> + bpf_prog_put(new_prog);
>> + goto out;
>> + }
>> +
>> + old_prog = xchg(&link->prog, new_prog);
>> + bpf_prog_put(old_prog);
>> +out:
>> + mutex_unlock(&bpf_nf_mutex);
>> + return err;
>> }
>>
>> static const struct bpf_link_ops bpf_nf_link_lops = {
>> @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>>
>> link->hook_ops.hook = nf_hook_run_bpf;
>> link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF;
>> - link->hook_ops.priv = prog;
>> +
>> + /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains
>> + * valid at all times within nf_hook_run_bpf().
>> + */
>> + link->hook_ops.priv = link;
>>
>> link->hook_ops.pf = attr->link_create.netfilter.pf;
>> link->hook_ops.priority = attr->link_create.netfilter.priority;
>> --
>> 1.8.3.1
>>
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