bpf: enhance verifier to understand stack pointer arithmetic

llvm 4.0 and above generates the code like below:
....
440: (b7) r1 = 15
441: (05) goto pc+73
515: (79) r6 = *(u64 *)(r10 -152)
516: (bf) r7 = r10
517: (07) r7 += -112
518: (bf) r2 = r7
519: (0f) r2 += r1
520: (71) r1 = *(u8 *)(r8 +0)
521: (73) *(u8 *)(r2 +45) = r1
....
and the verifier complains "R2 invalid mem access 'inv'" for insn #521.
This is because verifier marks register r2 as unknown value after #519
where r2 is a stack pointer and r1 holds a constant value.

Teach verifier to recognize "stack_ptr + imm" and
"stack_ptr + reg with const val" as valid stack_ptr with new offset.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -1932,16 +1932,22 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 	},
 	{
-		"unpriv: obfuscate stack pointer",
+		"stack pointer arithmetic",
 		.insns = {
-			BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
-			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
-			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+			BPF_MOV64_IMM(BPF_REG_1, 4),
+			BPF_JMP_IMM(BPF_JA, 0, 0, 0),
+			BPF_MOV64_REG(BPF_REG_7, BPF_REG_10),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, -10),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, -10),
+			BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1),
+			BPF_ST_MEM(0, BPF_REG_2, 4, 0),
+			BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, 8),
+			BPF_ST_MEM(0, BPF_REG_2, 4, 0),
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 			BPF_EXIT_INSN(),
 		},
-		.errstr_unpriv = "R2 pointer arithmetic",
-		.result_unpriv = REJECT,
 		.result = ACCEPT,
 	},
 	{