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

371ab7fa · Robert Morris · 62e3b8a9 · 371ab7fa · 371ab7fa · 371ab7fa
--- a/defs.h
+++ b/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]))
--- a/exec.c
+++ b/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;

--- a/usertests.c
+++ b/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);

--- a/vm.c
+++ b/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;
+}