netdev - Re: [PATCH bpf-next 0/6] bpf: add netfilter program type

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Date:   Thu, 13 Apr 2023 11:26:00 +0200
From:   Quentin Deslandes <qde@...cy.de>
To:     Florian Westphal <fw@...len.de>
CC:     <netdev@...r.kernel.org>, <netfilter-devel@...r.kernel.org>,
        <bpf@...r.kernel.org>, <dxu@...uu.xyz>
Subject: Re: [PATCH bpf-next 0/6] bpf: add netfilter program type

On 12/04/2023 11:45, Florian Westphal wrote:
> Quentin Deslandes <qde@...cy.de> wrote:
>> On 05/04/2023 18:11, Florian Westphal wrote:
>>> Add minimal support to hook bpf programs to netfilter hooks, e.g.
>>> PREROUTING or FORWARD.
>>>
>>> For this the most relevant parts for registering a netfilter
>>> hook via the in-kernel api are exposed to userspace via bpf_link.
>>>
>>> The new program type is 'tracing style', i.e. there is no context
>>> access rewrite done by verifier, the function argument (struct bpf_nf_ctx)
>>> isn't stable.
>>> There is no support for direct packet access, dynptr api should be used
>>> instead.
>>
>> Does this mean the verifier will reject any program accessing ctx->skb
>> (e.g. ctx->skb + X)?
> 
> Do you mean access to ctx->skb->data + X?  If so, yes, that won't work.
> 
> Otherwise, then no, it just means that programs might have to be recompiled
> if they lack needed relocation information, but only if bpf_nf_ctx structure is
> changed.
> 
> Initial version used "__sk_buff *skb", like e.g. clsact.  I was told
> to not do that and expose the real kernel-side structure instead and to
> not bother with direct packet access (skb->data access) support.
> 

That's exactly what I meant, thanks.

>>>  #include "vmlinux.h"
>>> extern int bpf_dynptr_from_skb(struct __sk_buff *skb, __u64 flags,
>>>                                struct bpf_dynptr *ptr__uninit) __ksym;
>>> extern void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, uint32_t offset,
>>>                                    void *buffer, uint32_t buffer__sz) __ksym;
>>> SEC("netfilter")
>>> int nf_test(struct bpf_nf_ctx *ctx)
>>> {
>>> 	struct nf_hook_state *state = ctx->state;
>>> 	struct sk_buff *skb = ctx->skb;
> 
> ctx->skb is dereferenced...
> 
>>> 	if (bpf_dynptr_from_skb(skb, 0, &ptr))
>>> 		return NF_DROP;
> 
> ... dynptr is created ...
> 
>>> 	iph = bpf_dynptr_slice(&ptr, 0, &_iph, sizeof(_iph));
>>> 	if (!iph)
>>> 		return NF_DROP;
>>> 	th = bpf_dynptr_slice(&ptr, iph->ihl << 2, &_th, sizeof(_th));
> 
> ip header access.