inaccessible page under the user stack page, to help exec deal w/ too-large args

defs.h

@@ -176,6 +176,7 @@ pde_t*          copyuvm(pde_t*, uint);
 void            switchuvm(struct proc*);
 void            switchkvm(void);
 int             copyout(pde_t*, uint, void*, uint);
+void            clear_pte_u(pde_t *pgdir, char *uva);
 
 // number of elements in fixed-size array
 #define NELEM(x) (sizeof(x)/sizeof((x)[0]))

exec.c

@@ -49,13 +49,16 @@ exec(char *path, char **argv)
   iunlockput(ip);
   ip = 0;
 
-  // Allocate a one-page stack at the next page boundary
+  // Allocate two pages at the next page boundary.
+  // Make the first inaccessible.
+  // Use the second as the user stack.
   sz = PGROUNDUP(sz);
-  if((sz = allocuvm(pgdir, sz, sz + PGSIZE)) == 0)
+  if((sz = allocuvm(pgdir, sz, sz + 2*PGSIZE)) == 0)
     goto bad;
+  clear_pte_u(pgdir, (char*)(sz-2*PGSIZE));
+  sp = sz;
 
   // Push argument strings, prepare rest of stack in ustack.
-  sp = sz;
   for(argc = 0; argv[argc]; argc++) {
     if(argc >= MAXARG)
       goto bad;

usertests.c

@@ -1525,7 +1525,7 @@ bigargtest(void)
     for(i = 0; i < MAXARG-1; i++)
       args[i] = "bigargs test: failed\n                                                                                                                                                                                                       ";
     args[MAXARG-1] = 0;
-    printf(stdout, "bigarg test %d\n", (MAXARG-1)*strlen(args[0]));
+    printf(stdout, "bigarg test\n");
     exec("echo", args);
     printf(stdout, "bigarg test ok\n");
     fd = open("bigarg-ok", O_CREATE);

vm.c

@@ -363,3 +363,16 @@ copyout(pde_t *pgdir, uint va, void *p, uint len)
   }
   return 0;
 }
+
+// Clear PTE_U on a page. Used to create an inaccessible
+// page beneath the user stack.
+void
+clear_pte_u(pde_t *pgdir, char *uva)
+{
+  pte_t *pte;
+
+  pte = walkpgdir(pgdir, uva, 0);
+  if(pte == 0)
+    panic("clear_pte_u");
+  *pte &= ~PTE_U;
+}