Merge pull request #1765 from rneugeba/kmod

Add support for building custom kernel modules

docs/kernels.md

@@ -36,6 +36,35 @@ In summary, LinuxKit offers a choice of the following kernels:
 - [linuxkit/kernel-fedora](https://hub.docker.com/r/linuxkit/kernel-fedora/): Selected Fedora kernels.
 
 
+## Compiling kernel modules
+
+The LinuxKit kernel packages include `kernel-dev.tar` which contains
+the headers and other files required to compile kernel modules against
+the specific version of the kernel. Currently, the headers are not
+included in the initial RAM disk, but it is possible to compile custom
+modules offline and include then include the modules in the initial
+RAM disk.
+
+There is a [example](../tests/kmod), but basically one can use a
+multi-stage build to compile the kernel modules:
+```
+FROM linuxkit/kernel:4.9.x AS ksrc
+# Extract headers and compile module
+FROM linuxkit/kernel-compile:1b396c221af673757703258159ddc8539843b02b@sha256:6b32d205bfc6407568324337b707d195d027328dbfec554428ea93e7b0a8299b AS build
+COPY --from=ksrc /kernel-dev.tar /
+RUN tar xf kernel-dev.tar
+
+# copy module source code and compile
+```
+
+To use the kernel module, we recommend adding a final stage to the
+Dockerfile above, which copies the kernel module from the `build`
+stage and performs a `insmod` as the entry point. You can add this
+package to the `onboot` section in your YAML
+file. [kmod.yml](../tests/kmod/kmod.yml) contains an example for the
+configuration.
+
+
 ## Working with Linux kernel patches for LinuxKit
 
 We may apply patches to the Linux kernel used in LinuxKit, primarily to

kernel/Dockerfile

@@ -53,9 +53,13 @@ RUN DVER=$(basename $(find /tmp/kernel-modules/lib/modules/ -mindepth 1 -maxdept
     dir=/tmp/usr/src/linux-headers-$DVER && \
     mkdir -p $dir && \
     cp /linux/.config $dir && \
-    cd /linux && \
-    cp -a include "$dir" && \
-    mkdir -p "$dir"/arch/x86 && cp -a arch/x86/include "$dir"/arch/x86/ && \
+    cp /linux/Module.symvers $dir && \
+    find . -path './include/*' -prune -o \
+           -path './arch/*/include' -prune -o \
+           -path './scripts/*' -prune -o \
+           -type f \( -name 'Makefile*' -o -name 'Kconfig*' -o -name 'Kbuild*' -o \
+                      -name '*.lds' -o -name '*.pl' -o -name '*.sh' \) | \
+         tar cf - -T - | (cd $dir; tar xf -) && \
     ( cd /tmp && tar cf /out/kernel-dev.tar usr/src )
 
 RUN printf "KERNEL_SOURCE=${KERNEL_SOURCE}\n" > /out/kernel-source-info

test/kmod/Dockerfile

@@ -0,0 +1,21 @@
+# This Dockerfile extracts the kernel headers from the kernel image
+# and then compiles a simple hello world kernel module against them.
+# In the last stage, it creates a package, which can be used for
+# testing.
+
+FROM linuxkit/kernel:4.9.x AS ksrc
+
+# Extract headers and compile module
+FROM linuxkit/kernel-compile:1b396c221af673757703258159ddc8539843b02b@sha256:6b32d205bfc6407568324337b707d195d027328dbfec554428ea93e7b0a8299b AS build
+COPY --from=ksrc /kernel-dev.tar /
+RUN tar xf kernel-dev.tar
+
+WORKDIR /kmod
+COPY ./src/* ./
+RUN make all
+
+# Package
+FROM alpine:3.5
+COPY --from=build /kmod/hello_world.ko /
+COPY check.sh /check.sh
+ENTRYPOINT ["/bin/sh", "/check.sh"]

test/kmod/check.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+function failed {
+	printf "Kernel module test suite FAILED\n"
+	/sbin/poweroff -f
+}
+
+uname -a
+modinfo hello_world.ko || failed
+insmod hello_world.ko || failed
+[ -n "$(dmesg | grep -o 'Hello LinuxKit')" ] || failed
+rmmod hello_world || failed
+
+printf "Kernel module test suite PASSED\n"
+
+/sbin/poweroff -f

test/kmod/kmod.yml

@@ -0,0 +1,17 @@
+kernel:
+  image: "linuxkit/kernel:4.9.x"
+  cmdline: "console=ttyS0"
+init:
+  - linuxkit/init:63eed9ca7a09d2ce4c0c5e7238ac005fa44f564b
+  - linuxkit/runc:b0fb122e10dbb7e4e45115177a61a3f8d68c19a9
+  - linuxkit/containerd:18eaf72f3f4f9a9f29ca1951f66df701f873060b
+onboot:
+  - name: check
+    image: "kmod-test"
+    binds:
+     - /dev:/dev
+     - /lib/modules:/lib/modules
+    capabilities:
+     - all
+outputs:
+  - format: kernel+initrd

test/kmod/run_test.sh

@@ -0,0 +1,10 @@
+#! /bin/sh
+
+# Make sure we have the latest kernel image
+docker pull linuxkit/kernel:4.9.x
+# Build a package
+docker build -t kmod-test .
+# Build a LinuxKit image with kernel module (and test script)
+moby build kmod
+# Run it
+linuxkit run kmod

test/kmod/src/Makefile

@@ -0,0 +1,6 @@
+obj-m += hello_world.o
+KVER=$(shell basename /usr/src/linux-headers-*)
+all:
+	make -C /usr/src/$(KVER) M=$(PWD) modules
+clean:
+	make -C /usr/src/$(KVER) M=$(PWD) clean

test/kmod/src/hello_world.c

@@ -0,0 +1,22 @@
+/*  
+ * A simple Hello World kernel module 
+ */
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+int init_hello(void)
+{
+	printk(KERN_INFO "Hello LinuxKit\n");
+	return 0;
+}
+
+void exit_hello(void)
+{
+	printk(KERN_INFO "Goodbye LinuxKit.\n");
+}
+
+module_init(init_hello);
+module_exit(exit_hello);
+MODULE_AUTHOR("Rolf Neugebauer <rolf.neugebauer@docker.com>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("A simple Hello World kernel module for testing");