synchronize usertests and exec with util lab

Author: Robert Morris

kernel/exec.c

@@ -75,17 +75,17 @@ exec(char *path, char **argv)
   p = myproc();
   uint64 oldsz = p->sz;
 
-  // Allocate two pages at the next page boundary.
+  // Allocate some pages at the next page boundary.
   // Make the first inaccessible as a stack guard.
-  // Use the second as the user stack.
+  // Use the rest as the user stack.
   sz = PGROUNDUP(sz);
   uint64 sz1;
-  if((sz1 = uvmalloc(pagetable, sz, sz + 2*PGSIZE, PTE_W)) == 0)
+  if((sz1 = uvmalloc(pagetable, sz, sz + (USERSTACK+1)*PGSIZE, PTE_W)) == 0)
     goto bad;
   sz = sz1;
-  uvmclear(pagetable, sz-2*PGSIZE);
+  uvmclear(pagetable, sz-(USERSTACK+1)*PGSIZE);
   sp = sz;
-  stackbase = sp - PGSIZE;
+  stackbase = sp - USERSTACK*PGSIZE;
 
   // Push argument strings, prepare rest of stack in ustack.
   for(argc = 0; argv[argc]; argc++) {

kernel/param.h

@@ -11,3 +11,5 @@
 #define NBUF         (MAXOPBLOCKS*3)  // size of disk block cache
 #define FSSIZE       2000  // size of file system in blocks
 #define MAXPATH      128   // maximum file path name
+#define USERSTACK    1     // user stack pages
+

user/usertests.c

@@ -2310,9 +2310,14 @@ bigargtest(char *s)
   if(pid == 0){
     static char *args[MAXARG];
     int i;
+    char big[400];
+    memset(big, ' ', sizeof(big));
+    big[sizeof(big)-1] = '\0';
     for(i = 0; i < MAXARG-1; i++)
-      args[i] = "bigargs test: failed\n                                                                                                                                                                                                       ";
+      args[i] = big;
     args[MAXARG-1] = 0;
+    // this exec() should fail (and return) because the
+    // arguments are too large.
     exec("echo", args);
     fd = open("bigarg-ok", O_CREATE);
     close(fd);
@@ -2409,7 +2414,7 @@ stacktest(char *s)
   pid = fork();
   if(pid == 0) {
     char *sp = (char *) r_sp();
-    sp -= PGSIZE;
+    sp -= USERSTACK*PGSIZE;
     // the *sp should cause a trap.
     printf("%s: stacktest: read below stack %d\n", s, *sp);
     exit(1);