Use /proc/config.gz for config diffs

diff_kernel relies on the existence of the /boot/config-{kernel} file
which is usually installed by the install-kernel script. This file may
be missing on systems with a custom install workflow or EFI systems that
use the systemd install_kernel functionality. The latter installs the
EFI stub kernel only, no auxiliary files of any kind. Comparisons
against the /boot/config-{kernel} file therefore fail on such systems.

This commit attempts to support a wider range of install workflows by
extracting the config from /proc/config.gz instead. Regardless of how
the kernel is installed, this particular file should exist as long as
CONFIG_IKCONFIG is enabled in the running kernel.

A word about "running config": The way that phrase was used previous to
this commit is technically inaccurate, since the "running config" could
change by installing a kernel and replacing the config file under /boot.
Now we *always* refer to the configuration saved within the currently
running kernel.

1 files changed, 5 insertions, 2 deletions

diff --git a/kern.1 b/kern.1
index e1826f2..eb963dc 100644
--- a/kern.1
+++ b/kern.1
@@ -105,8 +105,11 @@ to display the differences between the generated kernel configuration and
 .Pp
 If no config is given, this command compares against the configuration of the currently
 running kernel.
-This requires the configuration file to be present under
-.Pa /boot .
+This requires a kernel configuration to be present under
+.Pa /proc/config.gz .
+See the
+.Sy CONFIG_IKCONFIG
+configuration option in the kernel for more information.
 .It Sy build
 Builds the selected kernel.
 .It Sy install