bpf: enable verifier to add 0 to packet ptr

The patch fixes the case when adding a zero value to the packet
pointer.  The zero value could come from src_reg equals type
BPF_K or CONST_IMM.  The patch fixes both, otherwise the verifer
reports the following error:
  [...]
    R0=imm0,min_value=0,max_value=0
    R1=pkt(id=0,off=0,r=4)
    R2=pkt_end R3=fp-12
    R4=imm4,min_value=4,max_value=4
    R5=pkt(id=0,off=4,r=4)
  269: (bf) r2 = r0     // r2 becomes imm0
  270: (77) r2 >>= 3
  271: (bf) r4 = r1     // r4 becomes pkt ptr
  272: (0f) r4 += r2    // r4 += 0
  addition of negative constant to packet pointer is not allowed

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Mihai Budiu <mbudiu@vmware.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -1397,7 +1397,7 @@ static int check_packet_ptr_add(struct bpf_verifier_env *env,
 		imm = insn->imm;
 
 add_imm:
-		if (imm <= 0) {
+		if (imm < 0) {
 			verbose("addition of negative constant to packet pointer is not allowed\n");
 			return -EACCES;
 		}

tools/testing/selftests/bpf/test_verifier.c

@@ -2403,6 +2403,29 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	},
+	{
+		"direct packet access: test14 (pkt_ptr += 0, CONST_IMM, good access)",
+		.insns = {
+			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+				    offsetof(struct __sk_buff, data)),
+			BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+				    offsetof(struct __sk_buff, data_end)),
+			BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 22),
+			BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_3, 7),
+			BPF_MOV64_IMM(BPF_REG_5, 12),
+			BPF_ALU64_IMM(BPF_RSH, BPF_REG_5, 4),
+			BPF_MOV64_REG(BPF_REG_6, BPF_REG_2),
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_6, BPF_REG_5),
+			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_6, 0),
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_EXIT_INSN(),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+	},
 	{
 		"helper access to packet: test1, valid packet_ptr range",
 		.insns = {