Issue with bpf_xdp_adjust_head - ebpf

Question

Working on a small program with bpf to extend the packet ( ie: add mpls encap).
Managed to extend without issues but I write back the header the s_mac and d_mac are not correct. I'm assuming because the new space is smaller I'm overlapping with the original eth header and thus I get the wrong values.
/* extend the packet for mpls header encapsulation */
if (bpf_xdp_adjust_head(ctx, 0 - (int)sizeof(struct mpls_hdr)))
return XDP_DROP;
data = (void *)(long)ctx->data;
data_end = (void *)(long)ctx->data_end;
/* relocate ethernet header to start of packet and set MACs */
new_eth = data;
old_eth = data + (int)sizeof(struct mpls_hdr);
//set new header while swaping src/dst mac
set_ethhdr(new_eth, old_eth, bpf_htons(ETH_P_MPLS_UC));
This is where I believe the problem happens, when I memcpy into new_eth I'm biting into the old_eth space. Is there a way I can solve this by copying the values of the old_eth into another structure and then use that one to create the new_eth ?
static __always_inline void set_ethhdr(struct ethhdr *new_eth,
const struct ethhdr *old_eth,
__be16 h_proto)
{
memcpy(new_eth->h_source, old_eth->h_source, ETH_ALEN);
memcpy(new_eth->h_dest, old_eth->h_dest, ETH_ALEN);
new_eth->h_proto = h_proto;
}
I'm very new to BPF and C in general so not sure if that's indeed the issue.
Thx.

Answer 1

I've changed as per below. Unsure if this is the best way to solve it but it works for my usecase ( poc ).
static __always_inline void set_ethhdr(struct ethhdr *new_eth,
const struct ethhdr *old_eth,
__be16 h_proto)
{
__u8 h_tmp_src[ETH_ALEN];
__u8 h_tmp_dst[ETH_ALEN];
__builtin_memcpy(h_tmp_src, old_eth->h_source, ETH_ALEN);
__builtin_memcpy(h_tmp_dst, old_eth->h_dest, ETH_ALEN);
__builtin_memcpy(new_eth->h_dest, h_tmp_src, ETH_ALEN);
__builtin_memcpy(new_eth->h_source, h_tmp_dst, ETH_ALEN);
new_eth->h_proto = h_proto;
}