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Message-ID: <20230412094554.GD6670@breakpoint.cc> Date: Wed, 12 Apr 2023 11:45:54 +0200 From: Florian Westphal <fw@...len.de> To: Quentin Deslandes <qde@...cy.de> Cc: Florian Westphal <fw@...len.de>, 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 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. > > #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.
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