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.

2 files changed, 15 insertions, 11 deletions

diff --git a/kern b/kern
index 0f19772..f24b573 100755
--- a/kern
+++ b/kern
@@ -4,6 +4,14 @@ set -e
 
 KERNFRAG=${KERNFRAG:-/etc/kernfrag}
 
+RUNNING_CONFIG=$(mktemp --suffix=-kern-config)
+zcat /proc/config.gz > "$RUNNING_CONFIG"
+
+# We do want to expand tmpdir now rather than later,
+# and mktemp should give us a path without spaces.
+# shellcheck disable=SC2064
+trap "{ rm $RUNNING_CONFIG; }" EXIT
+
 err() {
 	printf "kern: %s\n" "$@" >&2
 }
@@ -43,13 +51,6 @@ get_running_kernel() {
 	uname -r
 }
 
-get_running_config() {
-	mount_boot_on_demand
-	config=/boot/config-$(get_running_kernel)
-	test -r "$config" || errx "Could not read configuration file: $config"
-	echo "$config"
-}
-
 set_kernel() {
 	kern=${1:-$(get_latest_kernel)}
 	selected=$(get_selected_kernel)
@@ -82,7 +83,7 @@ config_kernel() {
 
 diff_kernel() {
 	need_kernel_source
-	diff=${1:-$(get_running_config)}
+	diff=${1:-$RUNNING_CONFIG}
 
 	if command -v git >/dev/null; then
 		git diff --no-index "$diff" .config
@@ -116,7 +117,7 @@ strip_config() {
 
 test_diff() {
 	dt=$(mktemp)
-	strip_config "$(get_running_config)" > "$dt"
+	strip_config "$RUNNING_CONFIG" > "$dt"
 
 	if ! strip_config .config | diff -q "$dt" -; then
 		diff_kernel || true
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