scripts/gdb: add kernel config dumping command

lx-configdump <file> dumps the contents of the gzipped .config to a text
file when the config is included in the kernel with CONFIG_IKCONFIG.  By
default, the file written is called config.txt, but it can be any user
supplied filename as well.  If the kernel config is in a module
(configs.ko), then it can be loaded along with symbols for the module
loaded with 'lx-symbols' and then this command will still work.

Obviously if you have the whole vmlinux then this can also be achieved
with scripts/extract-ikconfig, but this gdb script can be useful to
confirm that the memory contents of the config in memory and the vmlinux
contents on disk match what is expected.

[swboyd@chromium.org: v2]
  Link: http://lkml.kernel.org/r/20190329220844.38234-3-swboyd@chromium.org
Link: http://lkml.kernel.org/r/20190325184522.260535-3-swboyd@chromium.org
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jackie Liu <liuyun01@kylinos.cn>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb/linux/config.py

@@ -0,0 +1,44 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright 2019 Google LLC.
+
+import gdb
+import zlib
+
+from linux import utils
+
+
+class LxConfigDump(gdb.Command):
+    """Output kernel config to the filename specified as the command
+       argument. Equivalent to 'zcat /proc/config.gz > config.txt' on
+       a running target"""
+
+    def __init__(self):
+        super(LxConfigDump, self).__init__("lx-configdump", gdb.COMMAND_DATA,
+                                           gdb.COMPLETE_FILENAME)
+
+    def invoke(self, arg, from_tty):
+        if len(arg) == 0:
+            filename = "config.txt"
+        else:
+            filename = arg
+
+        try:
+            py_config_ptr = gdb.parse_and_eval("kernel_config_data + 8")
+            py_config_size = gdb.parse_and_eval(
+                    "sizeof(kernel_config_data) - 1 - 8 * 2")
+        except gdb.error as e:
+            raise gdb.GdbError("Can't find config, enable CONFIG_IKCONFIG?")
+
+        inf = gdb.inferiors()[0]
+        zconfig_buf = utils.read_memoryview(inf, py_config_ptr,
+                                            py_config_size).tobytes()
+
+        config_buf = zlib.decompress(zconfig_buf, 16)
+        with open(filename, 'wb') as f:
+            f.write(config_buf)
+
+        gdb.write("Dumped config to " + filename + "\n")
+
+
+LxConfigDump()

scripts/gdb/vmlinux-gdb.py

@@ -27,6 +27,7 @@ else:
     import linux.modules
     import linux.dmesg
     import linux.tasks
+    import linux.config
     import linux.cpus
     import linux.lists
     import linux.proc