Re: [RFC nf-next v2 1/2] netfilter: bpf: support prog update

D. Wythe
2023-12-19T12:50:10Z

On 12/19/23 3:06 AM, Simon Horman wrote:
> On Mon, Dec 18, 2023 at 12:18:20PM +0800, D. Wythe 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
>> access the prog under rcu.
>>
>> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
> ...
>
>> @@ -26,8 +17,20 @@ struct bpf_nf_link {
>> struct net *net;
>> u32 dead;
>> const struct nf_defrag_hook *defrag_hook;
>> + struct rcu_head head;
>> };
>>
>> +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(nf_link->link.prog), &ctx);
> Hi,
>
> AFAICT nf_link->link.prog isn't annotated as __rcu,
> so perhaps rcu_dereference() is not correct here?
>
> In any case, sparse seems a bit unhappy:
>
> .../nf_bpf_link.c:31:29: error: incompatible types in comparison expression (different address spaces):
> .../nf_bpf_link.c:31:29: struct bpf_prog [noderef] __rcu *
> .../nf_bpf_link.c:31:29: struct bpf_prog *

Hi Simon,

thanks for the reporting.

Yes, I had anticipated that sparse would report an error. I tried to
cast the type,
but it would compile an error likes that:

net/netfilter/nf_bpf_link.c: In function ‘nf_hook_run_bpf’:
./include/asm-generic/rwonce.h:44:70: error: lvalue required as unary
‘&’ operand
44 | #define __READ_ONCE(x) (*(const volatile
__unqual_scalar_typeof(x) *)&(x))
| ^
./include/asm-generic/rwonce.h:50:2: note: in expansion of macro
‘__READ_ONCE’
50 |  __READ_ONCE(x);       \
|  ^~~~~~~~~~~
./include/linux/rcupdate.h:436:43: note: in expansion of macro ‘READ_ONCE’
436 |  typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
|                                           ^~~~~~~~~
./include/linux/rcupdate.h:584:2: note: in expansion of macro
‘__rcu_dereference_check’
584 |  __rcu_dereference_check((p), __UNIQUE_ID(rcu), \
|  ^~~~~~~~~~~~~~~~~~~~~~~
./include/linux/rcupdate.h:656:28: note: in expansion of macro
‘rcu_dereference_check’
656 | #define rcu_dereference(p) rcu_dereference_check(p, 0)
|                            ^~~~~~~~~~~~~~~~~~~~~
net/netfilter/nf_bpf_link.c:31:22: note: in expansion of macro
‘rcu_dereference’
31 |  return bpf_prog_run(rcu_dereference((const struct bpf_prog
__rcu *)nf_link->link.prog), &ctx);
|                      ^~~~~~~~~~~~~~~

So, I think we might need to go back to version 1.

@ Florian , what do you think ?

D. Wythe

>> +}
>> +
>> #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,
> ...