netdev - Re: [PATCH bpf-next v4 6/6] bpf, selftests: add redirect

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Date:   Wed, 30 Sep 2020 12:20:04 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, john.fastabend@...il.com, kafai@...com,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 6/6] bpf, selftests: add redirect_neigh
 selftest

On Wed, Sep 30, 2020 at 05:18:20PM +0200, Daniel Borkmann wrote:
> +
> +#ifndef barrier_data
> +# define barrier_data(ptr)	asm volatile("": :"r"(ptr) :"memory")
> +#endif
> +
> +#ifndef ctx_ptr
> +# define ctx_ptr(field)		(void *)(long)(field)
> +#endif

> +static __always_inline bool is_remote_ep_v4(struct __sk_buff *skb,
> +					    __be32 addr)
> +{
> +	void *data_end = ctx_ptr(skb->data_end);
> +	void *data = ctx_ptr(skb->data);

please consider adding:
        __bpf_md_ptr(void *, data);
        __bpf_md_ptr(void *, data_end);
to struct __sk_buff in a followup to avoid this casting headache.

> +SEC("dst_ingress") int tc_dst(struct __sk_buff *skb)
> +{
> +	int idx = dst_to_src_tmp;
> +	__u8 zero[ETH_ALEN * 2];
> +	bool redirect = false;
> +
> +	switch (skb->protocol) {
> +	case __bpf_constant_htons(ETH_P_IP):
> +		redirect = is_remote_ep_v4(skb, __bpf_constant_htonl(ip4_src));
> +		break;
> +	case __bpf_constant_htons(ETH_P_IPV6):
> +		redirect = is_remote_ep_v6(skb, (struct in6_addr)ip6_src);
> +		break;
> +	}
> +
> +	if (!redirect)
> +		return TC_ACT_OK;
> +
> +	barrier_data(&idx);
> +	idx = bpf_ntohl(idx);

I don't follow. Why force that constant into a register and force
actual swap instruction?

> +
> +	__builtin_memset(&zero, 0, sizeof(zero));
> +	if (bpf_skb_store_bytes(skb, 0, &zero, sizeof(zero), 0) < 0)
> +		return TC_ACT_SHOT;
> +
> +	return bpf_redirect_neigh(idx, 0);
> +}

> +xxd -p < test_tc_neigh.o   | sed "s/eeddddee/$veth_src/g" | xxd -r -p > test_tc_neigh.x.o
> +xxd -p < test_tc_neigh.x.o | sed "s/eeffffee/$veth_dst/g" | xxd -r -p > test_tc_neigh.y.o

So the inline asm is because of the above?
So after compiling you're hacking elf binary for this pattern ?
Ouch. Please use global data or something. This is fragile.
This type of hacks should be discouraged and having selftests do them
goes as counter example.