Use MIPS N32 abi for syscall handling

Signed-off-by: Harry Chen <i@harrychen.xyz>

kernel/src/arch/mipsel/interrupt.rs

@@ -129,8 +129,17 @@ fn timer() {
 
 fn syscall(tf: &mut TrapFrame) {
     tf.epc += 4;   // Must before syscall, because of fork.
-    let ret = crate::syscall::syscall(tf.t0, [tf.a0, tf.a1, tf.a2, tf.a3, tf.s0, tf.s1], tf);
-    tf.v0 = ret as usize;
+    let arguments = [tf.a0, tf.a1, tf.a2, tf.a3, tf.t0, tf.t1];
+    trace!("MIPS syscall {} invoked with {:?}", tf.v0, arguments);
+
+    let ret = crate::syscall::syscall(tf.v0, arguments, tf);
+    let ret = tf.v0 as isize;
+    // comply with mips n32 abi, always return a positive value
+    // https://git.musl-libc.org/cgit/musl/tree/arch/mipsn32/syscall_arch.h
+    if (ret < 0) {
+        tf.v0 = (-ret) as usize;
+        tf.a3 = 1;
+    }
 }
 
 fn page_fault(tf: &mut TrapFrame) {

user

@@ -1 +1 @@
-Subproject commit a460c5b32744292ed50922a159fa22c830aa61b9
+Subproject commit 23cfbb6565377ee88ad0cec3c1fcd3a090ee88c6