selftests/bpf: Test narrow load from bpf_sysctl.write

There are tests for full and narrows loads from bpf_sysctl.file_pos, but
for bpf_sysctl.write only full load is tested. Add the missing test.

Suggested-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrey Ignatov <rdna@fb.com>
Link: https://lore.kernel.org/bpf/20191029143027.28681-1-iii@linux.ibm.com

tools/testing/selftests/bpf/test_sysctl.c

@@ -120,6 +120,29 @@ static struct sysctl_test tests[] = {
 		.newval = "(none)", /* same as default, should fail anyway */
 		.result = OP_EPERM,
 	},
+	{
+		.descr = "ctx:write sysctl:write read ok narrow",
+		.insns = {
+			/* u64 w = (u16)write & 1; */
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+			BPF_LDX_MEM(BPF_H, BPF_REG_7, BPF_REG_1,
+				    offsetof(struct bpf_sysctl, write)),
+#else
+			BPF_LDX_MEM(BPF_H, BPF_REG_7, BPF_REG_1,
+				    offsetof(struct bpf_sysctl, write) + 2),
+#endif
+			BPF_ALU64_IMM(BPF_AND, BPF_REG_7, 1),
+			/* return 1 - w; */
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_7),
+			BPF_EXIT_INSN(),
+		},
+		.attach_type = BPF_CGROUP_SYSCTL,
+		.sysctl = "kernel/domainname",
+		.open_flags = O_WRONLY,
+		.newval = "(none)", /* same as default, should fail anyway */
+		.result = OP_EPERM,
+	},
 	{
 		.descr = "ctx:write sysctl:read write reject",
 		.insns = {