Gcc expects to be able to pick up the return

address off the stack, so put one there for it. (Bug was hidden by bad segment limits.)

Gcc expects to be able to pick up the return
8c4b5fc5 · rsc · 2ef3a64b · 8c4b5fc5
--- a/main.c
+++ b/main.c
@@ -128,12 +128,12 @@ process0(void)
  p0->cwd = iget(rootdev, 1);
  iunlock(p0->cwd);

-  // dummy user memory to make copyproc() happy.
-  // must be big enough to hold the init binary.
-  p0->sz = PAGE;
+  // Dummy user memory to make copyproc() happy.
+  // Must be big enough to hold the init binary and stack.
+  p0->sz = 2*PAGE;
  p0->mem = kalloc(p0->sz);

-  // fake a trap frame as if a user process had made a system
+  // Fake a trap frame as if a user process had made a system
  // call, so that copyproc will have a place for the new
  // process to return to.
  p0->tf = &tf;
@@ -142,6 +142,13 @@ process0(void)
  p0->tf->cs = (SEG_UCODE << 3) | DPL_USER;
  p0->tf->eflags = FL_IF;
  p0->tf->esp = p0->sz;
+  
+  // Push bogus return address, both to cause problems
+  // if main returns and also because gcc can generate
+  // function prologs that expect to be able to read the
+  // return address off the stack without causing a fault.
+  p0->tf->esp -= 4;
+  *(uint*)(p0->mem + p0->tf->esp) = 0xefefefef;

  p1 = copyproc(p0);