selftests/bpf: add a test to test_align

New test adds 14 to the unknown value before adding to the packet pointer,
 meaning there's no 'fixed offset' field and instead we add into the
 var_off, yielding a '4n+2' value.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/test_align.c

@@ -374,6 +374,73 @@ static struct bpf_align_test tests[] = {
 			{33, "R5=pkt(id=4,off=18,r=22,umax_value=2040,var_off=(0x0; 0x7fc))"},
 		},
 	},
+	{
+		.descr = "packet variable offset 2",
+		.insns = {
+			/* Create an unknown offset, (4n+2)-aligned */
+			LOAD_UNKNOWN(BPF_REG_6),
+			BPF_ALU64_IMM(BPF_LSH, BPF_REG_6, 2),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_6, 14),
+			/* Add it to the packet pointer */
+			BPF_MOV64_REG(BPF_REG_5, BPF_REG_2),
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_5, BPF_REG_6),
+			/* Check bounds and perform a read */
+			BPF_MOV64_REG(BPF_REG_4, BPF_REG_5),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_4, 4),
+			BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_4, 1),
+			BPF_EXIT_INSN(),
+			BPF_LDX_MEM(BPF_W, BPF_REG_6, BPF_REG_5, 0),
+			/* Make a (4n) offset from the value we just read */
+			BPF_ALU64_IMM(BPF_AND, BPF_REG_6, 0xff),
+			BPF_ALU64_IMM(BPF_LSH, BPF_REG_6, 2),
+			/* Add it to the packet pointer */
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_5, BPF_REG_6),
+			/* Check bounds and perform a read */
+			BPF_MOV64_REG(BPF_REG_4, BPF_REG_5),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_4, 4),
+			BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_4, 1),
+			BPF_EXIT_INSN(),
+			BPF_LDX_MEM(BPF_W, BPF_REG_6, BPF_REG_5, 0),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+		.matches = {
+			/* Calculated offset in R6 has unknown value, but known
+			 * alignment of 4.
+			 */
+			{8, "R2=pkt(id=0,off=0,r=8,imm=0)"},
+			{8, "R6=inv(id=0,umax_value=1020,var_off=(0x0; 0x3fc))"},
+			/* Adding 14 makes R6 be (4n+2) */
+			{9, "R6=inv(id=0,umin_value=14,umax_value=1034,var_off=(0x2; 0x7fc))"},
+			/* Packet pointer has (4n+2) offset */
+			{11, "R5=pkt(id=1,off=0,r=0,umin_value=14,umax_value=1034,var_off=(0x2; 0x7fc))"},
+			{13, "R4=pkt(id=1,off=4,r=0,umin_value=14,umax_value=1034,var_off=(0x2; 0x7fc))"},
+			/* At the time the word size load is performed from R5,
+			 * its total fixed offset is NET_IP_ALIGN + reg->off (0)
+			 * which is 2.  Then the variable offset is (4n+2), so
+			 * the total offset is 4-byte aligned and meets the
+			 * load's requirements.
+			 */
+			{15, "R5=pkt(id=1,off=0,r=4,umin_value=14,umax_value=1034,var_off=(0x2; 0x7fc))"},
+			/* Newly read value in R6 was shifted left by 2, so has
+			 * known alignment of 4.
+			 */
+			{18, "R6=inv(id=0,umax_value=1020,var_off=(0x0; 0x3fc))"},
+			/* Added (4n) to packet pointer's (4n+2) var_off, giving
+			 * another (4n+2).
+			 */
+			{19, "R5=pkt(id=2,off=0,r=0,umin_value=14,umax_value=2054,var_off=(0x2; 0xffc))"},
+			{21, "R4=pkt(id=2,off=4,r=0,umin_value=14,umax_value=2054,var_off=(0x2; 0xffc))"},
+			/* At the time the word size load is performed from R5,
+			 * its total fixed offset is NET_IP_ALIGN + reg->off (0)
+			 * which is 2.  Then the variable offset is (4n+2), so
+			 * the total offset is 4-byte aligned and meets the
+			 * load's requirements.
+			 */
+			{23, "R5=pkt(id=2,off=0,r=4,umin_value=14,umax_value=2054,var_off=(0x2; 0xffc))"},
+		},
+	},
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)