selftests/bpf: Test for null-pointer-deref bugfix in resolve_prog_type()

This test verifies that resolve_prog_type() works as expected when
`attach_prog_fd` is not passed in.

`prog->aux->dst_prog` in resolve_prog_type() is assigned by
`attach_prog_fd`, and would be NULL if `attach_prog_fd` is not provided.

Loading EXT prog with bpf_dynptr_from_skb() kfunc call in this way will
lead to null-pointer-deref.

Verify that the null-pointer-deref bug in resolve_prog_type() is fixed.
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240711145819.254178-3-wutengda@huaweicloud.com

tools/testing/selftests/bpf/verifier/calls.c

@@ -275,6 +275,19 @@
 	.result_unpriv = REJECT,
 	.result = ACCEPT,
 },
+{
+	"calls: invalid kfunc call: must provide (attach_prog_fd, btf_id) pair when freplace",
+	.insns = {
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, BPF_PSEUDO_KFUNC_CALL, 0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_EXT,
+	.result = REJECT,
+	.errstr = "Tracing programs must provide btf_id",
+	.fixup_kfunc_btf_id = {
+		{ "bpf_dynptr_from_skb", 0 },
+	},
+},
 {
 	"calls: basic sanity",
 	.insns = {