bpf, selftests: Add verifier test for mem_or_null register with offset.

Add a new test case with mem_or_null typed register with off > 0 to ensure
it gets rejected by the verifier:

  # ./test_verifier 1011
  #1009/u check with invalid reg offset 0 OK
  #1009/p check with invalid reg offset 0 OK
  Summary: 2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: borkmann

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

@@ -58,6 +58,34 @@
 	.result = ACCEPT,
 	.result_unpriv = ACCEPT,
 },
+{
+	"check with invalid reg offset 0",
+	.insns = {
+	/* reserve 8 byte ringbuf memory */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_MOV64_IMM(BPF_REG_2, 8),
+	BPF_MOV64_IMM(BPF_REG_3, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_ringbuf_reserve),
+	/* store a pointer to the reserved memory in R6 */
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+	/* add invalid offset to memory or NULL */
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 1),
+	/* check whether the reservation was successful */
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4),
+	/* should not be able to access *(R7) = 0 */
+	BPF_ST_MEM(BPF_W, BPF_REG_6, 0, 0),
+	/* submit the reserved ringbuf memory */
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+	BPF_MOV64_IMM(BPF_REG_2, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_ringbuf_submit),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_ringbuf = { 1 },
+	.result = REJECT,
+	.errstr = "R0 pointer arithmetic on mem_or_null prohibited",
+},
 {
 	"check corrupted spill/fill",
 	.insns = {