helpers: Add get_config helper for getting kconfig map

drgn/helpers/linux/kconfig.py

@@ -0,0 +1,66 @@
+# SPDX-License-Identifier: GPL-3.0+
+
+"""
+Kconfig
+-------
+
+The ``drgn.helpers.linux.kconfig`` module provides helpers for reading the
+Linux kernel build configuration.
+"""
+
+import gzip
+import types
+from typing import Mapping
+
+__all__ = ("get_kconfig",)
+
+
+def get_kconfig(prog) -> Mapping[str, str]:
+    """
+    Get the kernel build configuration as a mapping from the option name to the
+    value.
+
+    >>> get_kconfig(prog)['CONFIG_SMP']
+    'y'
+    >>> get_kconfig(prog)['CONFIG_HZ']
+    '300'
+
+    This is only supported if the kernel was compiled with ``CONFIG_IKCONFIG``.
+    Note that most Linux distributions do not enable this option.
+    """
+    try:
+        return prog.cache["kconfig_map"]
+    except KeyError:
+        pass
+
+    try:
+        start = prog.symbol("kernel_config_data").address
+        size = prog.symbol("kernel_config_data_end").address - start
+    except LookupError:
+        # Before Linux kernel commit 13610aa908dc ("kernel/configs: use .incbin
+        # directive to embed config_data.gz") (in v5.1), the data is a variable
+        # rather than two symbols.
+        try:
+            kernel_config_data = prog["kernel_config_data"]
+        except KeyError:
+            raise LookupError(
+                "kernel configuration data not found; kernel must be compiled with CONFIG_IKCONFIG"
+            )
+        # The data is delimited by the magic strings "IKCFG_ST" and "IKCFG_ED"
+        # plus a NUL byte.
+        start = kernel_config_data.address_ + 8
+        size = len(kernel_config_data) - 17
+
+    data = prog.read(start, size)
+    result = {}
+    for line in gzip.decompress(data).decode().splitlines():
+        if not line or line.startswith("#"):
+            continue
+        name, _, value = line.partition("=")
+        if value:
+            result[name] = value
+
+    # Make result mapping 'immutable', so changes cannot propagate to the cache
+    result = types.MappingProxyType(result)
+    prog.cache["kconfig_map"] = result
+    return result

tests/helpers/linux/test_kconfig.py

@@ -0,0 +1,13 @@
+import os.path
+
+from drgn.helpers.linux.kconfig import get_kconfig
+
+from tests.helpers.linux import LinuxHelperTestCase
+
+
+class TestKconfig(LinuxHelperTestCase):
+    def test_get_kconfig(self):
+        if not os.path.isfile("/proc/config.gz"):
+            self.skipTest("kernel not built with CONFIG_IKCONFIG_PROC")
+        m = get_kconfig(self.prog)
+        self.assertIn(m["CONFIG_IKCONFIG"], {"y", "m"})