bpf: test case to check whether src/dst regs got mangled by xadd

We currently do not have such a test case in test_verifier selftests
but it's important to test under bpf_jit_enable=1 to make sure JIT
implementations do not mistakenly mess with src/dst reg for xadd/{w,dw}.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_verifier.c

@@ -12004,6 +12004,46 @@ static struct bpf_test tests[] = {
 		.errstr = "BPF_XADD stores into R2 packet",
 		.prog_type = BPF_PROG_TYPE_XDP,
 	},
+	{
+		"xadd/w check whether src/dst got mangled, 1",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+			BPF_MOV64_REG(BPF_REG_7, BPF_REG_10),
+			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
+			BPF_STX_XADD(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
+			BPF_STX_XADD(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
+			BPF_JMP_REG(BPF_JNE, BPF_REG_6, BPF_REG_0, 3),
+			BPF_JMP_REG(BPF_JNE, BPF_REG_7, BPF_REG_10, 2),
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
+			BPF_EXIT_INSN(),
+			BPF_MOV64_IMM(BPF_REG_0, 42),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+		.retval = 3,
+	},
+	{
+		"xadd/w check whether src/dst got mangled, 2",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+			BPF_MOV64_REG(BPF_REG_7, BPF_REG_10),
+			BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_0, -8),
+			BPF_STX_XADD(BPF_W, BPF_REG_10, BPF_REG_0, -8),
+			BPF_STX_XADD(BPF_W, BPF_REG_10, BPF_REG_0, -8),
+			BPF_JMP_REG(BPF_JNE, BPF_REG_6, BPF_REG_0, 3),
+			BPF_JMP_REG(BPF_JNE, BPF_REG_7, BPF_REG_10, 2),
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_10, -8),
+			BPF_EXIT_INSN(),
+			BPF_MOV64_IMM(BPF_REG_0, 42),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
+		.retval = 3,
+	},
 	{
 		"bpf_get_stack return R0 within range",
 		.insns = {