net: support network-config:disabled on the kernel commandline (#232)

Allow disabling cloud-init's network configuration via a plain-text kernel cmdline

Cloud-init docs indicate that users can disable cloud-init networking via kernel
command line parameter 'network-config=<YAML>'.  This does not work unless 
the <YAML> payload base64 encoded.  Document the base64 encoding
requirement and add a plain-text value for disabling cloud-init network config:

    network-config=disabled

Also:
 - Log an error and ignore any plain-text network-config payloads that are
   not specifically 'network-config=disabled'.
 - Log a warning if network-config kernel param is invalid yaml but do not
   raise an exception, allowing boot to continue and use fallback networking.

LP: #1862702

1 files changed, 20 insertions, 14 deletions

diff --git a/cloudinit/net/cmdline.py b/cloudinit/net/cmdline.py
index 64e1c699..814ce411 100755
--- a/cloudinit/net/cmdline.py
+++ b/cloudinit/net/cmdline.py
@@ -10,6 +10,7 @@ import base64
 import glob
 import gzip
 import io
+import logging
 import os
 
 from cloudinit import util
@@ -19,6 +20,8 @@ from . import read_sys_net_safe
 
 _OPEN_ISCSI_INTERFACE_FILE = "/run/initramfs/open-iscsi.interface"
 
+KERNEL_CMDLINE_NETWORK_CONFIG_DISABLED = "disabled"
+
 
 class InitramfsNetworkConfigSource(metaclass=abc.ABCMeta):
     """ABC for net config sources that read config written by initramfses"""
@@ -233,34 +236,35 @@ def read_initramfs_config():
     return None
 
 
-def _decomp_gzip(blob, strict=True):
-    # decompress blob. raise exception if not compressed unless strict=False.
+def _decomp_gzip(blob):
+    # decompress blob or return original blob
     with io.BytesIO(blob) as iobuf:
         gzfp = None
         try:
             gzfp = gzip.GzipFile(mode="rb", fileobj=iobuf)
             return gzfp.read()
         except IOError:
-            if strict:
-                raise
             return blob
         finally:
             if gzfp:
                 gzfp.close()
 
 
-def _b64dgz(b64str, gzipped="try"):
-    # decode a base64 string.  If gzipped is true, transparently uncompresss
-    # if gzipped is 'try', then try gunzip, returning the original on fail.
-    try:
-        blob = base64.b64decode(b64str)
-    except TypeError:
-        raise ValueError("Invalid base64 text: %s" % b64str)
+def _b64dgz(data):
+    """Decode a string base64 encoding, if gzipped, uncompress as well
 
-    if not gzipped:
-        return blob
+    :return: decompressed unencoded string of the data or empty string on
+       unencoded data.
+    """
+    try:
+        blob = base64.b64decode(data)
+    except (TypeError, ValueError):
+        logging.error(
+            "Expected base64 encoded kernel commandline parameter"
+            " network-config. Ignoring network-config=%s.", data)
+        return ''
 
-    return _decomp_gzip(blob, strict=gzipped != "try")
+    return _decomp_gzip(blob)
 
 
 def read_kernel_cmdline_config(cmdline=None):
@@ -273,6 +277,8 @@ def read_kernel_cmdline_config(cmdline=None):
             if tok.startswith("network-config="):
                 data64 = tok.split("=", 1)[1]
         if data64:
+            if data64 == KERNEL_CMDLINE_NETWORK_CONFIG_DISABLED:
+                return {"config": "disabled"}
             return util.load_yaml(_b64dgz(data64))
 
     return None