eBPF verifier

源：https://www.kernel.org/doc/Documentation/networking/filter.txt

每个寄存器有3种状态 NOT_INIT SCALAR_VALUE pointer type
pointer type有分下面几种
PTR_TO_CTX Pointer to bpf_context.
CONST_PTR_TO_MAP Pointer to struct bpf_map. "Const" because arithmetic on these pointers is forbidden.
PTR_TO_MAP_VALUE Pointer to the value stored in a map element.
PTR_TO_MAP_VALUE_OR_NULL
Either a pointer to a map value, or NULL; map accesses
(see section 'eBPF maps', below) return this type,
which becomes a PTR_TO_MAP_VALUE when checked != NULL.
Arithmetic on these pointers is forbidden.
PTR_TO_STACK Frame pointer.
PTR_TO_PACKET skb->data.
PTR_TO_PACKET_END skb->data + headlen; arithmetic forbidden.
PTR_TO_SOCKET Pointer to struct bpf_sock_ops, implicitly refcounted.
PTR_TO_SOCKET_OR_NULL
Either a pointer to a socket, or NULL; socket lookup
returns this type, which becomes a PTR_TO_SOCKET when
checked != NULL. PTR_TO_SOCKET is reference-counted,
so programs must release the reference through the
socket release function before the end of the program.
Arithmetic on these pointers is forbidden.