Partially implement chroot(2) (#891)

* Partially implement chroot(2)

Really this return EPERM (permission denied), which seems to be absolutely fine
since a non-privileged user is currently assumed. This is what would normally
be returned in this scenario.

* update chroot implementation

tests/test_linux.py

@@ -1,4 +1,5 @@
 import os
+import errno
 import shutil
 import tempfile
 import unittest
@@ -184,3 +185,21 @@ class LinuxTest(unittest.TestCase):
         self.assertLess(_min, len(platform.files))
         self.assertGreater(_max, len(platform.files)-1)
 
+
+    def test_chroot(self):
+        # Create a minimal state
+        platform = self.symbolic_linux
+        platform.current.memory.mmap(0x1000, 0x1000, 'rw ')
+        platform.current.SP = 0x2000-4
+
+        # should error with ENOENT
+        this_file = os.path.realpath(__file__)
+        path = platform.current.push_bytes('{}\x00'.format(this_file))
+        fd = platform.sys_chroot(path)
+        self.assertEqual(fd, -errno.ENOTDIR)
+
+        # valid dir, but should always fail with EPERM
+        this_dir = os.path.dirname(this_file)
+        path = platform.current.push_bytes('{}\x00'.format(this_dir))
+        fd = platform.sys_chroot(path)
+        self.assertEqual(fd, -errno.EPERM)