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.
