Handle U-mode ecall for syscall dispatch

Adds exception handler for U-mode ecall instructions to dispatch
system calls. Extracts syscall number and arguments from the saved
register context and returns results through the same mechanism.

This design enables privilege separation by allowing user-mode tasks
to request kernel services without direct access to privileged
operations, addressing the security issue where tasks could bypass
isolation by executing in M-mode.

Author: HeatCrab

arch/riscv/hal.c

@@ -321,6 +321,31 @@ uint32_t do_trap(uint32_t cause, uint32_t epc, uint32_t isr_sp)
     } else { /* Synchronous Exception */
         uint32_t code = MCAUSE_GET_CODE(cause);
 
+        /* Handle ecall from U-mode - system calls */
+        if (code == MCAUSE_ECALL_UMODE) {
+            /* Advance mepc past the ecall instruction (4 bytes) */
+            uint32_t new_epc = epc + 4;
+            write_csr(mepc, new_epc);
+
+            uint32_t *isr_frame = (uint32_t *) isr_sp;
+            isr_frame[31] = new_epc;
+
+            /* Extract syscall arguments from ISR frame */
+            int syscall_num = isr_frame[15];
+            void *arg1 = (void *) isr_frame[8];
+            void *arg2 = (void *) isr_frame[9];
+            void *arg3 = (void *) isr_frame[10];
+
+            /* Dispatch to syscall handler */
+            extern int syscall(int num, void *arg1, void *arg2, void *arg3);
+            int retval = syscall(syscall_num, arg1, arg2, arg3);
+
+            /* Store return value */
+            isr_frame[8] = retval;
+
+            return isr_sp;
+        }
+
         /* Handle ecall from M-mode - used for yielding in preemptive mode */
         if (code == MCAUSE_ECALL_MMODE) {
             /* Advance mepc past the ecall instruction (4 bytes) */