Marking loadable kernel module as in-tree

Question

This question is about linux kernel 4.10.

Loading an out-of-tree LKM causes kernel to print a warning:

module: loading out-of-tree module taints kernel.

This raises from this check in module.c: if (!get_modinfo(info, "intree")) {

Reading get_modinfo it seams that "intree" is just a a magic-string livnig inside the .ko file.

Running readelf on a random LKM I found in my system shows this:

readelf -a imon.ko | grep intree 161: 00000000000006c0 9 OBJECT LOCAL DEFAULT 13 __UNIQUE_ID_intree1

While looking for intree in a simple, custom hello_world LKM returns no results.

Is this actually the case?

How are some modules marked as being in-tree? Is it done by adding a macro to the module (like MODULE_LICENCE), or by building the module in a specific way or something else?

Solution

In short, the build system contrives to add the line MODULE_INFO(intree, "Y"); to the "modulename.mod.c" file if and only if the module is being built intree.

There is an obvious way to fool the system by adding that line to one of your module's regular ".c" files, but I'm not sure why you'd want to.

Longer version....

External modules are normally built with a command similar to this:

$ make M=`pwd` modules

or the old syntax:

$ make SUBDIRS=`pwd` modules

The presence of a non-empty M or SUBDIRS causes the kernel's top-level "Makefile" to set the KBUILD_EXTMOD variable. It won't be set for a normal kernel build.

For stage 2 of module building (when the message "Building modules, stage 2" is output), make runs the "scripts/Makefile.modpost" makefile. That runs scripts/mod/modpost with different options when KBUILD_EXTMOD is set. In particular, the -I option is used when KBUILD_EXTMOD is set.

Looking at the source for modpost in "scripts/mod/modpost.c", the external_module variable has an initial value of 0, but the -I option sets it to 1. The function add_intree_flag() is called with the second parameter is_intree set to !external_module. The add_intree_flag() function writes MODULE_INFO(intree, "Y"); to the "modulename.mod.c" file if and only if its is_intree parameter is true.

So the difference between intree modules and external modules is the presence of the MODULE_INFO(intree, "Y"); macro call in the "modulename.mod.c" file. This gets compiled to "modulename.mod.o" and linked with the module's other object files to form the "modulename.ko" file.

Comments

[frr]

At a first look, this looks a little unfair to people writing perfectly open-source kernel code, but having to manage their drivers/modules out of tree. Only a few elite individuals enjoy the luxury of having their code actually in the vanilla tree. So as an "outsider", you can fudge that macro while staying out of tree, or you need to keep patching your local copy of the upstream vanilla tree. AFAIR, a tainted kernel is deprived of some debugging features...

[frr]

OR, THEN AGAIN, there is another way of looking at this: maybe the Linux leaders do want you to get used to the GIT. To maintain your own local branch, and instead of publishing a tarball, you ask an upstream maintainer to include your code, by sending a pull request...