I managed to get bit by the same issue twice in about 24 hours so I guess it’s time for a blog post. If you’ve ever compiled a kernel, you have dealt with kconfig symbols (CONFIG_FOO) which are used to enable various options in the kernel. You can’t select every symbol out that exists in the kernel since that would be a nightmare. Configuration options may be limited by adding depends e.g.

config FOO
    depends on BAR
    depends on BAZ if HAS_BAZ

Something like depends on BAR is statically defined and isn’t going to vary across systems. It turns out, you can do some interesting tricks and have your Kconfig symbol depend on the output of a script. The first time I got bit by this was with CONFIG_GCC_PLUGINS:

menuconfig GCC_PLUGINS
    bool "GCC plugins"
    depends on HAVE_GCC_PLUGINS
    depends on PLUGIN_HOSTCC != ""

    default "$(shell,$(srctree)/scripts/gcc-plugin.sh "$(preferred-plugin-hostcc)" "$(HOSTCXX)" "$(CC)")" if CC_IS_GCC

Because of the way gcc plugins work, you need to have an external package installed to do anything useful so it makes sense to only show the symbol if the package is properly installed.

As a distro maintainer, I’m responsible for looking at new symbols as they are added to the kernel. The easiest way to do this is to take the existing config file and run make listnewconfig. A side effect of the dynamic nature of CONFIG_GCC_PLUGINS is that if you take a .config generated on a system that doesn’t have the plugin devel package installed and then use it to run listnewconfig on a system that does have the package installed it will show up as a new symbol:

$ make defconfig
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
*** Default configuration is based on 'x86_64_defconfig'
# configuration written to .config
$ make listnewconfig
scripts/kconfig/conf  --listnewconfig Kconfig
$ sudo dnf install gcc-plugin-devel

..various installation messages..

$ make listnewconfig
scripts/kconfig/conf  --listnewconfig Kconfig

We have some scripts that expect a clean listnewconfig (i.e. we’ve set values for all configuraiton options) so this led to some confusion about why things were not working on my system which had gcc-plugin-devel installed (“But this worked when we built it last week!”)

The other case of dynamic configuration options happened with powerpc. Distros support multiple architectures but almost all development is done on our x86 workstations (until the year arm64 on the desktop happens of course). We can generate configuration files by setting the ARCH=blah option on the command line. Today I discovered that there’s a dynamic config option in powerpc:

    depends on PPC64 && CPU_LITTLE_ENDIAN
    def_bool $(success,$(srctree)/arch/powerpc/tools/gcc-check-mprofile-kernel.sh $(CC) -I$(srctree)/include -D__KERNEL__)

This results in CONFIG_MPROFILE_KERNEL not being visible unless you have the appropriate compiler installed which means, again, different behavior for listnewconfig.

Obviously the system environment makes a difference when building the kernel but it’s a bit surprising where this shows up. The moral I have learned is to think a little bit more closely about how exactly kernel configuration files are generated.