selftests/bpf: check if BPF_ST with variable offset preserves STACK_ZERO

A test case to verify that variable offset BPF_ST instruction
preserves STACK_ZERO marks when writes zeros, e.g. in the following
situation:

  *(u64*)(r10 - 8) = 0   ; STACK_ZERO marks for fp[-8]
  r0 = random(-7, -1)    ; some random number in range of [-7, -1]
  r0 += r10              ; r0 is now variable offset pointer to stack
  *(u8*)(r0) = 0         ; BPF_ST writing zero, STACK_ZERO mark for
                         ; fp[-8] should be preserved.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230214232030.1502829-5-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -35,3 +35,33 @@
 	.expected_attach_type = BPF_SK_LOOKUP,
 	.runs = -1,
 },
+{
+	"BPF_ST_MEM stack imm zero, variable offset",
+	.insns = {
+	/* set fp[-16], fp[-24] to zeros */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0),
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -24, 0),
+	/* r0 = random value in range [-32, -15] */
+	BPF_EMIT_CALL(BPF_FUNC_get_prandom_u32),
+	BPF_JMP_IMM(BPF_JLE, BPF_REG_0, 16, 2),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_0, 32),
+	/* fp[r0] = 0, make a variable offset write of zero,
+	 *             this should preserve zero marks on stack.
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_10),
+	BPF_ST_MEM(BPF_B, BPF_REG_0, 0, 0),
+	/* r0 = fp[-20], if variable offset write was tracked correctly
+	 *               r0 would be a known zero.
+	 */
+	BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_10, -20),
+	/* Would fail return code verification if r0 range is not tracked correctly. */
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	/* Use prog type that requires return value in range [0, 1] */
+	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
+	.expected_attach_type = BPF_SK_LOOKUP,
+	.runs = -1,
+},