Task 4

kernel.c

@@ -173,7 +173,7 @@ void process_setup(pid_t pid, const char* program_name) {
     ptable[pid].regs.reg_rip = pgm.entry();
 
     // allocate and map stack segment
-    uintptr_t stack_addr = PROC_START_ADDR + PROC_SIZE * pid - PAGESIZE;
+    uintptr_t stack_addr = MEMSIZE_VIRTUAL - PAGESIZE;
     uintptr_t pa = (uintptr_t) kalloc(PAGESIZE);
     memory_map(process_pagetable, stack_addr, pa, PTE_PWU);
     ptable[pid].regs.reg_rsp = stack_addr + PAGESIZE;
@@ -325,6 +325,7 @@ uintptr_t syscall(regstate* regs) {
 
 int syscall_page_alloc(uintptr_t addr) {
     uintptr_t pa = (uintptr_t) kalloc(PAGESIZE);
+    if (pa == 0) return -1;
     memory_map(current->pagetable, addr, pa, PTE_PWU);
     memset(memory_virtual_to_physical(current->pagetable, addr), 0, PAGESIZE);
     return 0;