kernel: Fix DEBUG builds and add default debug targets/images

Building debug kernels (with additional run time checks and debugging)
was broken a few commits back. This adds back support for building debug
kernels.

In addition, it builds and uploads debug kernels for selected kernel
series (4.9.x LTS and latest stable). The tag for these kernels has
a "_dbg" suffix.

Update documentation.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

docs/kernels.md

@@ -15,7 +15,10 @@ image is tagged with the full kernel version plus the hash of the
 files it was created from (git tree hash of the `./kernel`
 directory). For convenience, the latest kernel of each stable series
 is also available under a shorthand tag, e.g. `linuxkit/kernel:4.9.x`
-for the latest `4.9` kernel.
+for the latest `4.9` kernel. For selected kernels (mostly the LTS
+kernels and latest stable kernels) we also compile/push kernels with
+additional debugging enabled. The hub images for these kernels have
+the `_dbg` suffix in the tag.
 
 In addition to the official kernel images, LinuxKit offers the ability
 to build bootable Linux images with kernels from various

kernel/Dockerfile

@@ -2,7 +2,7 @@ FROM linuxkit/kernel-compile:1b396c221af673757703258159ddc8539843b02b@sha256:6b3
 
 ARG KERNEL_VERSION
 ARG KERNEL_SERIES
-ARG DEBUG=0
+ARG DEBUG
 
 ENV KERNEL_SOURCE=https://www.kernel.org/pub/linux/kernel/v4.x/linux-${KERNEL_VERSION}.tar.xz
 
@@ -13,7 +13,7 @@ RUN cat linux-${KERNEL_VERSION}.tar.xz | tar --absolute-names -xJ &&  mv /linux-
 COPY kernel_config-${KERNEL_SERIES} /linux/arch/x86/configs/x86_64_defconfig
 COPY kernel_config.debug /linux/debug_config
 
-RUN if [ $DEBUG -ne "0" ]; then \
+RUN if [ -n "${DEBUG}" ]; then \
     sed -i 's/CONFIG_PANIC_ON_OOPS=y/# CONFIG_PANIC_ON_OOPS is not set/' /linux/arch/x86/configs/x86_64_defconfig; \
     cat /linux/debug_config >> /linux/arch/x86/configs/x86_64_defconfig; \
     fi

kernel/Makefile

@@ -8,8 +8,6 @@
 # of the current directory. The build will only rebuild the kernel
 # image if the git tree hash changed.
 
-# Enable a debug kernel
-DEBUG?=0
 # Git tree hash of this directory. Override to force build
 HASH?=$(shell git ls-tree HEAD -- ../$(notdir $(CURDIR)) | awk '{print $$3}')
 # Name on Hub
@@ -28,37 +26,44 @@ sign:
 # Arguments:
 # $1: Full kernel version, e.g., 4.9.22
 # $2: Kernel "series", e.g., 4.9.x
+# $3: Build a debug kernel (used as suffix for image)
 # This defines targets like:
-# tag_4.9.x, push_4.9.x and sign_4.9.x
-# and adds them as dependencies to the global targets
+# build_4.9.x, push_4.9.x and sign_4.9.x and adds them as dependencies
+# to the global targets
+# Set $3 to "_dbg", to build debug kernels. This defines targets like
+# build_4.9.x_dbg and adds "_dbg" to the hub image name.
 define kernel
-build_$(2): Dockerfile Makefile $(wildcard patches-$(2)/*) kernel_config-$(2) kernel_config.debug
-	docker pull linuxkit/$(IMAGE):$(1)-$(HASH) || \
+build_$(2)$(3): Dockerfile Makefile $(wildcard patches-$(2)/*) kernel_config-$(2) kernel_config.debug
+	docker pull linuxkit/$(IMAGE):$(1)$(3)-$(HASH) || \
 		docker build \
 			--build-arg KERNEL_VERSION=$(1) \
 			--build-arg KERNEL_SERIES=$(2) \
-			--no-cache -t linuxkit/$(IMAGE):$(1)-$(HASH) .
+			--build-arg DEBUG=$(3) \
+			--no-cache -t linuxkit/$(IMAGE):$(1)$(3)-$(HASH) .
 
-push_$(2): build_$(2)
-	docker pull linuxkit/$(IMAGE):$(1)-$(HASH) || \
-		(docker push linuxkit/$(IMAGE):$(1)-$(HASH) && \
-		 docker tag linuxkit/$(IMAGE):$(1)-$(HASH) linuxkit/$(IMAGE):$(2) && \
-		 docker push linuxkit/$(IMAGE):$(2))
+push_$(2)$(3): build_$(2)$(3)
+	docker pull linuxkit/$(IMAGE):$(1)$(3)-$(HASH) || \
+		(docker push linuxkit/$(IMAGE):$(1)$(3)-$(HASH) && \
+		 docker tag linuxkit/$(IMAGE):$(1)$(3)-$(HASH) linuxkit/$(IMAGE):$(2)$(3) && \
+		 docker push linuxkit/$(IMAGE):$(2)$(3))
 
-sign_$(2): build_$(2)
-	DOCKER_CONTENT_TRUST=1 docker pull linuxkit/$(IMAGE):$(1)-$(HASH) || \
-		(DOCKER_CONTENT_TRUST=1 docker push linuxkit/$(IMAGE):$(1)-$(HASH) && \
-		 docker tag linuxkit/$(IMAGE):$(1)-$(HASH) linuxkit/$(IMAGE):$(2) && \
-		 DOCKER_CONTENT_TRUST=1 docker push linuxkit/$(IMAGE):$(2))
+sign_$(2)$(3): build_$(2)$(3)
+	DOCKER_CONTENT_TRUST=1 docker pull linuxkit/$(IMAGE):$(1)$(3)-$(HASH) || \
+		(DOCKER_CONTENT_TRUST=1 docker push linuxkit/$(IMAGE):$(1)$(3)-$(HASH) && \
+		 docker tag linuxkit/$(IMAGE):$(1)$(3)-$(HASH) linuxkit/$(IMAGE):$(2)$(3) && \
+		 DOCKER_CONTENT_TRUST=1 docker push linuxkit/$(IMAGE):$(2)$(3))
 
-build: build_$(2)
-push: push_$(2)
-sign: sign_$(2)
+build: build_$(2)$(3)
+push: push_$(2)$(3)
+sign: sign_$(2)$(3)
 endef
 
 #
 # Build Targets
+# Debug targets only for latest stable and LTS stable
 #
 $(eval $(call kernel,4.10.13,4.10.x))
+$(eval $(call kernel,4.10.13,4.10.x,_dbg))
 $(eval $(call kernel,4.9.25,4.9.x))
+$(eval $(call kernel,4.9.25,4.9.x,_dbg))
 $(eval $(call kernel,4.4.64,4.4.x))

kernel/kernel_config.debug

@@ -1,6 +1,6 @@
 
 
-## MOBY DEBUG OPTIONS ##
+## LinuxKit DEBUG OPTIONS ##
 
 CONFIG_LOCKDEP=y
 CONFIG_FRAME_POINTER=y