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diff --git a/content/2019-10-27-kernel-debugging-qemu.md b/content/2019-10-27-kernel-debugging-qemu.md
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-+++
-title = "Linux Kernel debugging with QEMU"
-
-[taxonomies]
-tags = ["linux", "qemu"]
-+++
-
-**EDIT**:
-- 2021-07-15: Added `Appendix: Dockerfile for Kernel development` and updated
-  busybox + Kernel versions.
-
-The other evening while starring at some Linux kernel code I thought, let me
-setup a minimal environment so I can easily step through the code and examine
-the state.
-
-I ended up creating:
-- a [Linux kernel][linux-kernel] with minimal configuration
-- a minimal [ramdisk][initrd] to boot into which is based on [busybox][busybox]
-
-In the remaing part of this article we will go through each step by first
-building the kernel, then building the initrd and then running the kernel using
-[QEMU][qemu] and debugging it with [GDB][gdb].
-
-## $> make kernel
-
-Before building the kernel we first need to generate a configuration. As a
-starting point we generate a minimal config with the `make tinyconfig` make
-target. Running this command will generate a `.config` file. After generating
-the initial config file we customize the kernel using the merge fragment flow.
-This allows us to merge a fragment file into the current configuration by
-running the `scripts/kconfig/merge_config.sh` script.
-
-Let's quickly go over some customizations we do.
-The following two lines enable support for gzipped initramdisks:
-```config
-CONFIG_BLK_DEV_INITRD=y
-CONFIG_RD_GZIP=y
-```
-The next two configurations are important as they enable the binary loaders for
-[ELF][binfmt-elf] and [script #!][binfmt-script] files.
-```config
-CONFIG_BINFMT_ELF=y
-CONFIG_BINFMT_SCRIPT=y
-```
-
-> Note: In the cursed based configuration `make menuconfig` we can search for
-> configurations using the `/` key and then select a match using the number keys.
-> After selecting a match we can check the `Help` to get a description for the
-> configuration parameter.
-
-Building the kernel with the default make target will give us the following two
-files:
-- `vmlinux` statically linked kernel (ELF file) containing symbol information for debugging
-- `arch/x86_64/boot/bzImage` compressed kernel image for booting
-
-Full configure & build script:
-```sh
-{{ include(path="content/2019-10-27-kernel-debugging-qemu/build_kernel.sh") }}
-```
-
-## $> make initrd
-
-Next step is to build the initrd which we base on [busybox][busybox]. Therefore
-we first build the busybox project in its default configuration with one
-change, we enable following configuration to build a static binary so it can be
-used stand-alone:
-```sh
-sed -i 's/# CONFIG_STATIC .*/CONFIG_STATIC=y/' .config
-```
-
-One important step before creating the final initrd is to create an init
-process. This will be the first process executed in userspace after the kernel
-finished its initialization. We just create a script that drops us into a
-shell:
-```sh
-cat <<EOF > init
-#!/bin/sh
-
-mount -t proc none /proc
-mount -t sysfs none /sys
-
-exec setsid cttyhack sh
-EOF
-```
-> By default the kernel looks for `/sbin/init` in the root file system, but the
-> location can optionally be specified with the [`init=`][kernel-param] kernel
-> parameter.
-
-Full busybox & initrd build script:
-```sh
-{{ include(path="content/2019-10-27-kernel-debugging-qemu/build_initrd.sh") }}
-```
-
-## Running QEMU && GDB
-
-After finishing the previous steps we have all we need to run and debug the
-kernel. We have `arch/x86/boot/bzImage` and `initramfs.cpio.gz` to boot the
-kernel into a shell and we have `vmlinux` to feed the debugger with debug
-symbols.
-
-We start QEMU as follows, thanks to the `-S` flag the CPU will freeze until we
-connected the debugger:
-```sh
-# -S    freeze CPU until debugger connected
-> qemu-system-x86_64                                                 \
-  -kernel ./linux-5.3.7/arch/x86/boot/bzImage                        \
-  -nographic                                                         \
-  -append "earlyprintk=ttyS0 console=ttyS0 nokaslr init=/init debug" \
-  -initrd ./initramfs.cpio.gz                                        \
-  -gdb tcp::1234                                                     \
-  -S
-```
-
-Then we can start GDB and connect to the GDB server running in QEMU (configured
-via `-gdb tcp::1234`). From now on we can start to debug through the
-kernel.
-```sh
-> gdb linux-5.3.7/vmlinux -ex 'target remote :1234'
-(gdb) b do_execve
-Breakpoint 1 at 0xffffffff810a1a60: file fs/exec.c, line 1885.
-(gdb) c
-Breakpoint 1, do_execve (filename=0xffff888000060000, __argv=0xffffffff8181e160 <argv_init>, __envp=0xffffffff8181e040 <envp_init>) at fs/exec.c:1885
-1885          return do_execveat_common(AT_FDCWD, filename, argv, envp, 0);
-(gdb) bt
-#0  do_execve (filename=0xffff888000060000, __argv=0xffffffff8181e160 <argv_init>, __envp=0xffffffff8181e040 <envp_init>) at fs/exec.c:1885
-#1  0xffffffff81000498 in run_init_process (init_filename=<optimized out>) at init/main.c:1048
-#2  0xffffffff81116b75 in kernel_init (unused=<optimized out>) at init/main.c:1129
-#3  0xffffffff8120014f in ret_from_fork () at arch/x86/entry/entry_64.S:352
-#4  0x0000000000000000 in ?? ()
-(gdb)
-```
-
----
-
-## Appendix: Try to get around `<optimized out>`
-
-When debugging the kernel we often face following situation in gdb:
-```
-(gdb) frame
-#0  do_execveat_common (fd=fd@entry=-100, filename=0xffff888000120000, argv=argv@entry=..., envp=envp@entry=..., flags=flags@entry=0) at fs/exec.c
-
-(gdb) info args
-fd = <optimized out>
-filename = 0xffff888000060000
-argv = <optimized out>
-envp = <optimized out>
-flags = <optimized out>
-file = 0x0
-```
-The problem is that the Linux kernel requires certain code to be compiled with
-optimizations enabled.
-
-In this situation we can "try" to reduce the optimization for single compilation
-units or a subtree (try because, reducing the optimization could break the
-build). To do so we adapt the Makefile in the corresponding directory.
-```make
-# fs/Makefile
-
-# configure for single compilation unit
-CFLAGS_exec.o := -Og
-
-# configure for the whole subtree of where the Makefile resides
-ccflags-y := -Og
-```
-
-After enabling optimize for debug experience `-Og` we can see the following now
-in gdb:
-```
-(gdb) frame
-#0  do_execveat_common (fd=fd@entry=-100, filename=0xffff888000120000, argv=argv@entry=..., envp=envp@entry=..., flags=flags@entry=0) at fs/exec.c
-
-(gdb) info args
-fd = -100
-filename = 0xffff888000120000
-argv = {ptr = {native = 0x10c5980}}
-envp = {ptr = {native = 0x10c5990}}
-flags = 0
-
-(gdb) p *filename
-$3 = {name = 0xffff888000120020 "/bin/ls", uptr = 0x10c59b8 "/bin/ls", refcnt = 1, aname = 0x0, iname = 0xffff888000120020 "/bin/ls"}
-
-(gdb) ptype filename
-type = struct filename {
-    const char *name;
-    const char *uptr;
-    int refcnt;
-    struct audit_names *aname;
-    const char iname[];
-}
-```
-
-## Appendix: `Dockerfile` for Kernel development
-
-The following `Dockerfile` provides a development environment with all the
-required tools and dependencies, to re-produce all the steps of building and
-debugging the Linux kernel.
-```dockerfile
-{{ include(path="content/2019-10-27-kernel-debugging-qemu/Dockerfile") }}
-```
-
-Save the listing above in a file called `Dockerfile` and build the docker image
-as follows.
-```sh
-docker build -t kernel-dev
-```
-> Optionally set `DOCKER_BUILDKIT=1` to use the newer image builder.
-
-Once the image has been built, an interactive container can be launched as
-follows.
-```sh
-# Some options for conveniene:
-#   -v <HOST>:<GUEST>     Mount host path to guest path.
-#   --rm                  Remove the container after exiting.
-
-docker run -it kernel-dev
-```
-
-[linux-kernel]: https://www.kernel.org
-[initrd]: https://www.kernel.org/doc/html/latest/admin-guide/initrd.html
-[busybox]: https://busybox.net
-[qemu]: https://www.qemu.org
-[gdb]: https://www.gnu.org/software/gdb
-[binfmt-elf]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/binfmt_elf.c
-[binfmt-script]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/binfmt_script.c
-[kernel-param]: https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html