bpf: Add PTR_TO_BTF_ID_OR_NULL support

Add bpf_reg_type PTR_TO_BTF_ID_OR_NULL support.
For tracing/iter program, the bpf program context
definition, e.g., for previous bpf_map target, looks like
  struct bpf_iter__bpf_map {
    struct bpf_iter_meta *meta;
    struct bpf_map *map;
  };

The kernel guarantees that meta is not NULL, but
map pointer maybe NULL. The NULL map indicates that all
objects have been traversed, so bpf program can take
proper action, e.g., do final aggregation and/or send
final report to user space.

Add btf_id_or_null_non0_off to prog->aux structure, to
indicate that if the context access offset is not 0,
set to PTR_TO_BTF_ID_OR_NULL instead of PTR_TO_BTF_ID.
This bit is set for tracing/iter program.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175912.2476576-1-yhs@fb.com

include/linux/bpf.h

@@ -320,6 +320,7 @@ enum bpf_reg_type {
 	PTR_TO_TP_BUFFER,	 /* reg points to a writable raw tp's buffer */
 	PTR_TO_XDP_SOCK,	 /* reg points to struct xdp_sock */
 	PTR_TO_BTF_ID,		 /* reg points to kernel struct */
+	PTR_TO_BTF_ID_OR_NULL,	 /* reg points to kernel struct or NULL */
 };
 
 /* The information passed from prog-specific *_is_valid_access
@@ -658,6 +659,7 @@ struct bpf_prog_aux {
 	bool offload_requested;
 	bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
 	bool func_proto_unreliable;
+	bool btf_id_or_null_non0_off;
 	enum bpf_tramp_prog_type trampoline_prog_type;
 	struct bpf_trampoline *trampoline;
 	struct hlist_node tramp_hlist;