Use configuration file under /boot instead of /proc/config.gz

kern(1) has been using /proc/config.gz as the source of the currently
running kernel's config since its initial commit. Whilst this location
is convenient because of its static nature, it does need a specific
kernel option, IKCONFIG_PROC, to be set.

Furthermore, it introduces a dependency on gzip(1) to decompress the
file on the fly. Consequently, we need to stream gzip's output to
commands or save it in a temporary file to access it neatly.

Since Gentoo's default install script for the kernel, installkernel(8),
places each kernel's configuration file in /boot, we can use this
location to retrieve the currently running kernel's configuration
instead and work around all of the above limitations.

Of course we need to make sure that /boot is mounted before we try
accessing any configuration files on there. We use the already existing
code from install_kernel() for that.

3 files changed, 23 insertions, 11 deletions

diff --git a/TODO b/TODO
deleted file mode 100644
index 430f084..0000000
--- a/TODO
+++ /dev/null
@@ -1,2 +0,0 @@
-- Consider falling back to /boot/config-<kernel> if /proc/config.gz
-  does not exist
diff --git a/kern b/kern
index 42f1ac6..1f49a67 100755
--- a/kern
+++ b/kern
@@ -21,6 +21,12 @@ is_boot_mounted() {
 	mountpoint -q /boot
 }
 
+mount_boot_on_demand() {
+	if has_boot_mount && ! is_boot_mounted; then
+		sudo mount /boot
+	fi
+}
+
 get_latest_kernel() {
 	eselect --brief kernel list | tail -n1
 }
@@ -29,6 +35,17 @@ get_selected_kernel() {
 	readlink /usr/src/linux
 }
 
+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)
@@ -59,7 +76,7 @@ config_kernel() {
 }
 
 diff_kernel() {
-	diff=${1:-/proc/config.gz}
+	diff=${1:-$(get_running_config)}
 	nvim -d .config "$diff"
 }
 
@@ -68,9 +85,7 @@ build_kernel() {
 }
 
 install_kernel() {
-	if has_boot_mount && ! is_boot_mounted; then
-		sudo mount /boot
-	fi
+	mount_boot_on_demand
 	sudo make install modules_install
 }
 
@@ -84,7 +99,7 @@ clean_kernel() {
 
 test_diff() {
 	dt=$(mktemp)
-	zcat /proc/config.gz | sed -nE '/^(# )?CONFIG_.*/p' > "$dt"
+	sed -nE '/^(# )?CONFIG_.*/p' "$(get_running_config)" > "$dt"
 	if ! sed -nE '/^(# )?CONFIG_.*/p' .config | diff -q "$dt" -; then
 		diff_kernel
 		printf "Continue? [y/N] "
diff --git a/kern.1 b/kern.1
index 79dcad2..ac3e4e2 100644
--- a/kern.1
+++ b/kern.1
@@ -1,4 +1,4 @@
-.Dd May 16, 2020
+.Dd June 4, 2020
 .Dt KERN 1
 .Os
 .Sh NAME
@@ -103,9 +103,8 @@ 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
-.Sy IKCONFIG_PROC
-to be enabled.
+This requires the configuration file to be present under
+.Pa /boot .
 .It Sy build
 Builds the selected kernel.
 .It Sy install