chatgpt solved this one for me

cpu/idt.c

@@ -118,17 +118,44 @@ void trap(registers_t *r) {
 }
 
 /* takes a ptr that is supposed to be from userspace. If it is not, we
- * return 0, if it is a valid userspace pointer, we return the usable
- * kernelspace pointer
+ * return 0, if it is a valid we return 1
  */
-uint32_t is_userspace_ptr_mapped(uint32_t ptr) {
-    // todo: implement
-    return ptr;
+bool is_userspace_ptr_mapped(uint32_t ptr) {
+    if (ptr >= KERNBASE) {
+        return 0;
+    }
+
+    pde_t *pgdir = get_user_proc_page_directory();
+    if (!pgdir) {
+        return 0;
+    }
+
+    pde_t pde = pgdir[PDX(ptr)];
+    if ((pde & (PTE_P | PTE_U)) != (PTE_P | PTE_U)) {
+        return 0;
+    }
+    if (pde & PDE_PS) {
+        return 0;
+    }
+
+    pte_t *table = (pte_t*)P2V(PTE_ADDR(pde));
+    pte_t pte = table[PTX(ptr)];
+    if ((pte & (PTE_P | PTE_U)) != (PTE_P | PTE_U)) {
+        return 0;
+    }
+
+    return 1;
 }
 
 static void* get_userspace_cstr(uint32_t ptr) {
-    // todo: implement
-    return (void*)(ptr);
+    for (uint32_t addr = ptr;; addr++) {
+        if (addr == 0 || !is_userspace_ptr_mapped(addr)) {
+            return 0;
+        }
+        if (*(const char*)addr == '\0') {
+            return (void*)ptr;
+        }
+    }
 }
 
 static int handle_puts(const char* s) {
@@ -140,6 +167,7 @@ static int handle_puts(const char* s) {
 }
 
 static void handle_syscall(registers_t* r) {
+    int ret;
     switch (r->eax) {
         case SYS_exit:
             if (r->ebx == 0) {
@@ -157,11 +185,16 @@ static void handle_syscall(registers_t* r) {
             r->eax = 0;
             break;
         case SYS_puts:
-            r->eax = handle_puts(get_userspace_cstr(r->ebx));
+            ret = handle_puts(get_userspace_cstr(r->ebx));
+            if (ret < 0) {
+                printk("SYS_puts panic: page fault\n");
+                killproc();
+            }
+            r->eax = 0;
             break;
         default:
-            printk("Unknown syscall\n");
-            r->eax = -1;
+            printk("Userspace panic: Unknown syscall\n");
+            killproc();
     }
 }
 

proc.c

@@ -28,6 +28,9 @@ void trapret();
 void swtch(void** oldstack, void* newstack);
 
 pde_t *get_user_proc_page_directory() {
+    // if (!vm.user_task) {
+    //     return 0;
+    // }
     return vm.user_task->pgdir;
 }
 

user/snake.c

@@ -0,0 +1,8 @@
+#include "../syscall.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+int main() {
+    return 0;
+
+}