bpf: Selftests, verifier case for non null pointer map value branch

When we have pointer type that is known to be non-null we only follow
the non-null branch. This adds tests to cover the map_value pointer
returned from a map lookup. To force an error if both branches are
followed we do an ALU op on R10.

Signed-off-by: John Fastabend <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/159009168650.6313.7434084136067263554.stgit@john-Precision-5820-Tower

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

···
 	.result_unpriv = REJECT,
 	.flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
 },
+{
+	"map lookup and null branch prediction",
+	.insns = {
+	BPF_MOV64_IMM(BPF_REG_1, 10),
+	BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_6, 0, 2),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_6, 0, 1),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_10, 10),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 4 },
+	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	.result = ACCEPT,
+},