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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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