[kernel] Pass objects not handles to syscall impls

This commit contains a couple large, interdependent changes:

- In preparation for capability checking, the _syscall_verify_*
  functions now load most handles passed in, and verify that they exist
  and are of the correct type. Lists and out-handles are not converted
  to objects.
- Also in preparation for capability checking, the internal
  representation of handles has changed. j6_handle_t is now 32 bits, and
  a new j6_cap_t (also 32 bits) is added. Handles of a process are now a
  util::map<j6_handle_t, handle> where handle is a new struct containing
  the id, capabilities, and object pointer.
- The kernel object definition DSL gained a few changes to support auto
  generating the handle -> object conversion in the _syscall_verify_*
  functions, mostly knowing the object type, and an optional "cname"
  attribute on objects where their names differ from C++ code.
  (Specifically vma/vm_area)
- Kernel object code and other code under kernel/objects is now in a new
  obj:: namespace, because fuck you <cstdlib> for putting "system" in
  the global namespace. Why even have that header then?
- Kernel object types constructed with the construct_handle helper now
  have a creation_caps static member to declare what capabilities a
  newly created object's handle should have.

src/kernel/syscalls/endpoint.cpp

@@ -5,62 +5,61 @@
 #include "objects/endpoint.h"
 #include "syscalls/helpers.h"
 
+using namespace obj;
+
 namespace syscalls {
 
 j6_status_t
-endpoint_create(j6_handle_t *handle)
+endpoint_create(j6_handle_t *self)
 {
-    construct_handle<endpoint>(handle);
+    construct_handle<endpoint>(self);
     return j6_status_ok;
 }
 
 j6_status_t
-endpoint_send(j6_handle_t handle, uint64_t tag, const void * data, size_t data_len)
+endpoint_send(endpoint *self, uint64_t tag, const void * data, size_t data_len)
 {
     if (tag & j6_tag_system_flag)
         return j6_err_invalid_arg;
 
-    endpoint *e = get_handle<endpoint>(handle);
-    if (!e) return j6_err_invalid_arg;
-
-    return e->send(tag, data, data_len);
+    return self->send(tag, data, data_len);
 }
 
 j6_status_t
-endpoint_receive(j6_handle_t handle, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
+endpoint_receive(endpoint *self, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
 {
     // Data is marked optional, but we need the length, and if length > 0,
     // data is not optional.
     if (!data_len || (*data_len && !data))
         return j6_err_invalid_arg;
 
-    endpoint *e = get_handle<endpoint>(handle);
-    if (!e) return j6_err_invalid_arg;
-
+    // Use local variables instead of the passed-in pointers, since
+    // they may get filled in when the sender is running, which means
+    // a different user VM space would be active.
     j6_tag_t out_tag = j6_tag_invalid;
     size_t out_len = *data_len;
-    j6_status_t s = e->receive(&out_tag, data, &out_len, timeout);
+    j6_status_t s = self->receive(&out_tag, data, &out_len, timeout);
     *tag = out_tag;
     *data_len = out_len;
     return s;
 }
 
 j6_status_t
-endpoint_sendrecv(j6_handle_t handle, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
+endpoint_sendrecv(endpoint *self, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
 {
     if (*tag & j6_tag_system_flag)
         return j6_err_invalid_arg;
 
-    endpoint *e = get_handle<endpoint>(handle);
-    if (!e) return j6_err_invalid_arg;
-
-    j6_status_t status = e->send(*tag, data, *data_len);
+    j6_status_t status = self->send(*tag, data, *data_len);
     if (status != j6_status_ok)
         return status;
 
+    // Use local variables instead of the passed-in pointers, since
+    // they may get filled in when the sender is running, which means
+    // a different user VM space would be active.
     j6_tag_t out_tag = j6_tag_invalid;
     size_t out_len = *data_len;
-    j6_status_t s = e->receive(&out_tag, data, &out_len, timeout);
+    j6_status_t s = self->receive(&out_tag, data, &out_len, timeout);
     *tag = out_tag;
     *data_len = out_len;
     return s;