1 files changed, 30 insertions, 0 deletions
diff --git a/kernel/cow.c b/kernel/cow.c
new file mode 100644
index 0000000..b3634fc
--- /dev/null
+++ b/kernel/cow.c
@@ -0,0 +1,30 @@
+// COW pagefault handler
+#include "types.h"
+#include "riscv.h"
+#include "defs.h"
+
+int
+cow_handler(pagetable_t pagetable, uint64 va)
+{
+  // you can't really write to rediculous pointers
+  if(va >= MAXVA || PGROUNDDOWN(va) == 0)
+    return -1;
+  pte_t *pte = walk(pagetable, va, 0);
+  if(pte == 0 || (*pte & PTE_V) == 0 || (*pte & PTE_U) == 0)
+    return -1;
+  if(*pte & PTE_C){
+    uint64 pa_orig = PTE2PA(*pte);
+    uint64 pa_new = (uint64)kalloc();
+    if(pa_new == 0){
+      printf("cow pagefault: kalloc failed\n");
+      return -1;
+    }
+    // copy the page and add write permission
+    memmove((void*)pa_new, (void*)pa_orig, PGSIZE);
+    uint64 flags = (PTE_FLAGS(*pte) | PTE_W) & ~PTE_C;
+    *pte = PA2PTE(pa_new) | flags;
+    kfree((void*)pa_orig);
+  } else if ((*pte & PTE_W) == 0)
+    return -1;
+  return 0;
+}