usertest for exec() out of memory recovery

and fix a few exec() bugs

kernel/exec.c

@@ -67,8 +67,10 @@ exec(char *path, char **argv)
   // Allocate two pages at the next page boundary.
   // Use the second as the user stack.
   sz = PGROUNDUP(sz);
-  if((sz = uvmalloc(pagetable, sz, sz + 2*PGSIZE)) == 0)
+  uint64 sz1;
+  if((sz1 = uvmalloc(pagetable, sz, sz + 2*PGSIZE)) == 0)
     goto bad;
+  sz = sz1;
   uvmclear(pagetable, sz-2*PGSIZE);
   sp = sz;
   stackbase = sp - PGSIZE;

kernel/sysfile.c

@@ -436,10 +436,9 @@ sys_exec(void)
     }
     argv[i] = kalloc();
     if(argv[i] == 0)
-      panic("sys_exec kalloc");
-    if(fetchstr(uarg, argv[i], PGSIZE) < 0){
       goto bad;
-    }
+    if(fetchstr(uarg, argv[i], PGSIZE) < 0)
+      goto bad;
   }
 
   int ret = exec(path, argv);

user/usertests.c

@@ -2452,6 +2452,42 @@ badarg(char *s)
   exit(0);
 }
 
+// test the exec() code that cleans up if it runs out
+// of memory. it's really a test that such a condition
+// doesn't cause a panic.
+void
+execout(char *s)
+{
+  for(int avail = 0; avail < 15; avail++){
+    int pid = fork();
+    if(pid < 0){
+      printf("fork failed\n");
+      exit(1);
+    } else if(pid == 0){
+      // allocate all of memory.
+      while(1){
+        uint64 a = (uint64) sbrk(4096);
+        if(a == 0xffffffffffffffffLL)
+          break;
+      }
+
+      // free a few pages, in order to let exec() make some
+      // progress.
+      for(int i = 0; i < avail; i++)
+        sbrk(-4096);
+      
+      close(1);
+      char *args[] = { "echo", "x", 0 };
+      exec("echo", args);
+      exit(0);
+    } else {
+      wait((int*)0);
+    }
+  }
+
+  exit(0);
+}
+
 //
 // use sbrk() to count how many free physical memory pages there are.
 //
@@ -2520,6 +2556,7 @@ main(int argc, char *argv[])
     void (*f)(char *);
     char *s;
   } tests[] = {
+    {execout, "execout"},
     {copyin, "copyin"},
     {copyout, "copyout"},
     {copyinstr1, "copyinstr1"},