bpf, tests: Add word-order tests for load/store of double words

A double word (64-bit) load/store may be implemented as two successive
32-bit operations, one for each word. Check that the order of those
operations is consistent with the machine endianness.
Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210809091829.810076-10-johan.almbladh@anyfinetworks.com

lib/test_bpf.c

@@ -5420,6 +5420,42 @@ static struct bpf_test tests[] = {
 		{ { 0, 0xffffffff } },
 		.stack_depth = 40,
 	},
+	{
+		"STX_MEM_DW: Store double word: first word in memory",
+		.u.insns_int = {
+			BPF_LD_IMM64(R0, 0),
+			BPF_LD_IMM64(R1, 0x0123456789abcdefLL),
+			BPF_STX_MEM(BPF_DW, R10, R1, -40),
+			BPF_LDX_MEM(BPF_W, R0, R10, -40),
+			BPF_EXIT_INSN(),
+		},
+		INTERNAL,
+		{ },
+#ifdef __BIG_ENDIAN
+		{ { 0, 0x01234567 } },
+#else
+		{ { 0, 0x89abcdef } },
+#endif
+		.stack_depth = 40,
+	},
+	{
+		"STX_MEM_DW: Store double word: second word in memory",
+		.u.insns_int = {
+			BPF_LD_IMM64(R0, 0),
+			BPF_LD_IMM64(R1, 0x0123456789abcdefLL),
+			BPF_STX_MEM(BPF_DW, R10, R1, -40),
+			BPF_LDX_MEM(BPF_W, R0, R10, -36),
+			BPF_EXIT_INSN(),
+		},
+		INTERNAL,
+		{ },
+#ifdef __BIG_ENDIAN
+		{ { 0, 0x89abcdef } },
+#else
+		{ { 0, 0x01234567 } },
+#endif
+		.stack_depth = 40,
+	},
 	/* BPF_STX | BPF_ATOMIC | BPF_W/DW */
 	{
 		"STX_XADD_W: Test: 0x12 + 0x10 = 0x22",