libxl: Record backend/frontend paths in /libxl/$DOMID

This gives us a record of all the backends we have set up for a
domain, which is separate from the frontends in
  /local/domain/$DOMID/device.

In particular:

1. A guest has write permission for the frontend path:
  /local/domain/$DOMID/device/$KIND/$DEVID
which means that the guest can completely delete the frontend.
(They can't recreate it because they don't have write permission
on the containing directory.)

2. A guest has write permission for the backend path recorded in the
frontend, ie, it can write to
  /local/domain/$DOMID/device/$KIND/$DEVID/backend
which means that the guest can break the association between
frontend and backend.

So we can't rely on iterating over the frontends to find all the
backends, or examining a frontend to discover how a device is
configured.

So, have libxl__device_generic_add record the frontend and backend
paths in /libxl/$DOMID/device, and have libxl__device_destroy remove
them again.

Create the containing directory /libxl/GUEST/device in
libxl__domain_make.  The already existing xs_rm in devices_destroy_cb
will take care of removing it.

This is part of XSA-175.

Backport note: Backported over 7472ced, which fixes a bug in driver
domain teardown.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

docs/misc/xenstore-paths.markdown

@@ -389,6 +389,21 @@ The guest's virtual time offset from UTC in seconds.
 
 ### libxl Specific Paths
 
+#### /libxl/$DOMID/device/$KIND/$DEVID
+
+Created by libxl for every frontend/backend pair created for $DOMID.
+Used by libxl for enumeration and management of the device.
+
+#### /libxl/$DOMID/device/$KIND/$DEVID/frontend
+
+Path in xenstore to the frontend, normally
+/local/domain/$DOMID/device/$KIND/$DEVID
+
+#### /libxl/$DOMID/device/$KIND/$DEVID/backend
+
+Path in xenstore to the backend, normally
+/local/domain/$BACKEND_DOMID/backend/$KIND/$DOMID/$DEVID
+
 #### /libxl/$DOMID/dm-version ("qemu\_xen"|"qemu\_xen\_traditional") = [n,INTERNAL]
 
 The device model version for a domain.

tools/libxl/libxl_create.c

@@ -586,6 +586,8 @@ retry_transaction:
 
     xs_rm(ctx->xsh, t, libxl_path);
     libxl__xs_mkdir(gc, t, libxl_path, noperm, ARRAY_SIZE(noperm));
+    libxl__xs_mkdir(gc, t, GCSPRINTF("%s/device", libxl_path),
+                    noperm, ARRAY_SIZE(noperm));
 
     xs_write(ctx->xsh, t, libxl__sprintf(gc, "%s/vm", dom_path), vm_path, strlen(vm_path));
     rc = libxl__domain_rename(gc, *domid, 0, info->name, t);

tools/libxl/libxl_device.c

@@ -40,6 +40,15 @@ char *libxl__device_backend_path(libxl__gc *gc, libxl__device *device)
                      device->domid, device->devid);
 }
 
+char *libxl__device_libxl_path(libxl__gc *gc, libxl__device *device)
+{
+    char *libxl_dom_path = libxl__xs_libxl_path(gc, device->domid);
+
+    return GCSPRINTF("%s/device/%s/%d", libxl_dom_path,
+                     libxl__device_kind_to_string(device->kind),
+                     device->devid);
+}
+
 /* Returns 1 if device exists, 0 if not, ERROR_* (<0) on error. */
 int libxl__device_exists(libxl__gc *gc, xs_transaction_t t,
                          libxl__device *device)
@@ -105,14 +114,16 @@ int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
         libxl__device *device, char **bents, char **fents, char **ro_fents)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
-    char *frontend_path, *backend_path;
+    char *frontend_path, *backend_path, *libxl_path;
     struct xs_permissions frontend_perms[2];
     struct xs_permissions ro_frontend_perms[2];
     struct xs_permissions backend_perms[2];
     int create_transaction = t == XBT_NULL;
+    int rc;
 
     frontend_path = libxl__device_frontend_path(gc, device);
     backend_path = libxl__device_backend_path(gc, device);
+    libxl_path = libxl__device_libxl_path(gc, device);
 
     frontend_perms[0].id = device->domid;
     frontend_perms[0].perms = XS_PERM_NONE;
@@ -127,8 +138,22 @@ int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
 retry_transaction:
     if (create_transaction)
         t = xs_transaction_start(ctx->xsh);
+
     /* FIXME: read frontend_path and check state before removing stuff */
 
+    rc = libxl__xs_rm_checked(gc, t, libxl_path);
+    if (rc) goto out;
+
+    rc = libxl__xs_write_checked(gc, t, GCSPRINTF("%s/frontend",libxl_path),
+                                 frontend_path);
+    if (rc) goto out;
+
+    rc = libxl__xs_write_checked(gc, t, GCSPRINTF("%s/backend",libxl_path),
+                                 backend_path);
+    if (rc) goto out;
+
+    /* xxx much of this function lacks error checks! */
+
     if (fents || ro_fents) {
         xs_rm(ctx->xsh, t, frontend_path);
         xs_mkdir(ctx->xsh, t, frontend_path);
@@ -174,6 +199,11 @@ retry_transaction:
         }
     }
     return 0;
+
+ out:
+    if (create_transaction && t)
+        libxl__xs_transaction_abort(gc, &t);
+    return rc;
 }
 
 typedef struct {
@@ -570,6 +600,7 @@ int libxl__device_destroy(libxl__gc *gc, libxl__device *dev)
 {
     const char *be_path = libxl__device_backend_path(gc, dev);
     const char *fe_path = libxl__device_frontend_path(gc, dev);
+    const char *libxl_path = libxl__device_libxl_path(gc, dev);
     const char *tapdisk_path = GCSPRINTF("%s/%s", be_path, "tapdisk-params");
     const char *tapdisk_params;
     xs_transaction_t t = 0;
@@ -594,6 +625,7 @@ int libxl__device_destroy(libxl__gc *gc, libxl__device *dev)
              */
             libxl__xs_path_cleanup(gc, t, fe_path);
             libxl__xs_path_cleanup(gc, t, be_path);
+            libxl__xs_path_cleanup(gc, t, libxl_path);
         } else if (dev->backend_domid == domid) {
             /*
              * The driver domain is in charge for removing what it can

tools/libxl/libxl_internal.h

@@ -1061,6 +1061,7 @@ _hidden int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
         libxl__device *device, char **bents, char **fents, char **ro_fents);
 _hidden char *libxl__device_backend_path(libxl__gc *gc, libxl__device *device);
 _hidden char *libxl__device_frontend_path(libxl__gc *gc, libxl__device *device);
+_hidden char *libxl__device_libxl_path(libxl__gc *gc, libxl__device *device);
 _hidden int libxl__parse_backend_path(libxl__gc *gc, const char *path,
                                       libxl__device *dev);
 _hidden int libxl__device_destroy(libxl__gc *gc, libxl__device *dev);