TIL, Today I Learned, is more of a "I just figured this out: here are my notes, you may find them useful too" rather than a full blog post

Some applications in a Linux system depends on certain kernel features to work properly. I'm currently working on adding support for CRIU [1] in Buildroot [2] which has such requirements.

That's when I stumble upon the LIBFOO_LINUX_CONFIG_FIXUP variable.


As the documentation [3] says:

* +LIBFOO_LINUX_CONFIG_FIXUPS+ lists the Linux kernel configuration
  options that are needed to build and use this package, and without
  which the package is fundamentally broken. This shall be a set of
  calls to one of the kconfig tweaking option: `KCONFIG_ENABLE_OPT`,
  This is seldom used, as package usually have no strict requirements on
  the kernel options.

This is a variable to tweak the kernel configuration from the package itself. In my case I had to:


To enable CONFIG_CHECKPOINT_RESTORE upon package selection.



TIL, Today I Learned, is more of a "I just figured this out: here are my notes, you may find them useful too" rather than a full blog post

In Buildroot [1], the integrity of (allmost) all downloaded packages is verified against a hash. Even packages that are fetched from a git repository is verified this way.

This is a good thing that no one really should work around.

Today I had a debug-session for one package which I locally cloned and frequently made changes to. I told Buildroot to fetch the source code for the package locally to speed up my iterations between new code changes and testing on target.

The hash did of course change after each code change, which became quite annoying.


The BR_NO_CHECK_HASH_FOR was something I found by a coincidence when looking into the support/download/check-hash file.

No wonder why I have missed this one, it is not mention in the documentation at all:

[22:58:16]marcus@goliat:~/git/buildroot$ git grep BR_NO_CHECK_HASH_FOR docs/

It does what it says - do not check hash for a certain list of files.

Here is an example on how the linux package make use of BR_NO_CHECK_HASH_FOR:



# We have no way to know the hashes for user-supplied patches.


I found this useful for my bug-hunting, but as a general rule, this is something that you probably should not use.

Buildroot: out-of-tree builds

Buildroot: out-of-tree builds

Like the Linux kernel, Buildroot supports building out-of-tree in a very similar way. This could be very useful when you have multiple configurations in the same Buildroot repository that you want to build without interference.

Consider the following example:

cd buildroot/
mkdir -p ../outputs/device{1,2}

make O=../outputs/device1 menuconfig
make O=../outputs/device1

make O=../outputs/device2 menuconfig
make O=../outputs/device2

Each output has its own .config so you may change the build configurations independently.

The big benefit compared with git worktree [1] or other multiple-instances-of-the-same-repository-approaches is that the dl directory is shared (without need to specify BR2_DL_DIR [2]) among all build directories.

The feature is documented in the Buildroot manual [3].

config utility for Buildroot

config utility for Buildroot

I'm using the ./scripts/config script in the Linux kernel tree a lot. The script is used to manipulate a .config file from the command line which is quite nice to be able to do.

I use it mostly to enable configurations from a script or as a part of automated tests.

Buildroot is also using KBuild as its configuration system so I adapted this script and submitted a patch. It is now part of the mainline.

commit f2c5c7a263d9d61faba80544e0f6ac6da03716ee
Author: Marcus Folkesson <marcus.folkesson@gmail.com>
Date:   Tue Sep 26 23:15:02 2017 +0200

utils/config: new script to manipulate .config files on the command line

Default prefix is set to `BR2_` but may be overridden by setting

Example usage:

./utils/config --package --enable GNUPG

Check state of config option `BR2_PACKAGE_GNUPG`:
./utils/config --package --state GNUPG

./utils/config --package --disable GNUPG

Set `BR2_TARGET_GENERIC_ISSUE` to "Welcome":
./utils/config --set-str  TARGET_GENERIC_ISSUE "Welcome"

Copied from the Linux kernel (4.13-rc6) source code and adapted to
Thanks to Andi Kleen who is the original author of this script.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
[Arnout: merge the two tr invications]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

In addition to all commands found in he Linux kernel version, I also added the --package config as a shortcut to operate on packages.