KBUILD_EXTRA_SYMBOLS is needed when module dependencies exist

[william-ye999]

Is this a bug? Assume that there are two kernel modules A and B. When module A uses the symbols exported by module B, kernel module of livepatch can be created for module A because KBUILD_EXTRA_SYMBOLS has been added to the Makefile of module A.

    1. SymB is defined in module B and is exported.
    2. Module A uses SymB.
    3. KBUILD_EXTRA_SYMBOLS is added in Makefile of module A.
    4. Make a livepatch to module A.

However, if the symbol exported by module B is not used in module A but the symbol is used in the livepatch, the livepatch fails to be created even if KBUILD_EXTRA_SYMBOLS is added to the Makefile of module A.

    1. SymB is defined in module B and is exported.
    2. Module A does not use SymB.
    3. KBUILD_EXTRA_SYMBOLS is added in Makefile of module A.
    4. Make a livepatch with SymB to module A.

This is because KBUILD_EXTRA_SYMBOLS needs to be added to kmod/patch/Makefile of kpatch too. If there are many livepatches for similar situations that need to be made, the contents of the kmod/patch/Makefile need to be constantly modified. How do we solve this problem?

Note: The dependency symbols are removed from Module.symvers in commit 52c3416db00d970c91a6992ab6e5ff48e077ad29 (modpost: track if the symbol origin is a dump file or ELF object) in the Linux community.

[sm00th]

We actually already relay KBUILD_EXTRA_SYMBOLS via an env var here. The problem is we currently unset it by default. Could you try removing that line and running kpatch-build as KBUILD_EXTRA_SYMBOLS=... kpatch-build... to see if it works properly with your usecase?

[william-ye999]

Thank you for your reply.

As you said, I removed the KBUILD_EXTRA_SYMBOLS initialization part and tested it, but the result still failed.

After I reverted the patch from the community and added KBUILD_EXTRA_SYMBOLS to the Makefile in module A and kmod/patch, the test was successful.

[sm00th]

As you said, I removed the KBUILD_EXTRA_SYMBOLS initialization part and tested it, but the result still failed.

Hm. At what step does it fail? I've tried reproducing it with sample modules and it built successfully.

First without adding KBUILD_EXTRA_SYMBOLS:

$ kpatch-build -d -e ktest_module_b.ko -s moduleb/ -t all test.patch
...
ERROR: kpatch build failed. Check ~/.kpatch/build.log for more details.
$ grep modpost ~/.kpatch/build.log
ERROR: modpost: "modulea_exported_func" [~/.kpatch/tmp/patch/livepatch-test.ko] undefined!
...

Then with:

$ KBUILD_EXTRA_SYMBOLS=~/oot/modulea/Module.symvers kpatch-build -d -e ktest_module_b.ko -s moduleb/ -t all test.patch
...
+ export KPATCH_BUILD=/lib/modules/5.15.6-100.fc34.x86_64/build KPATCH_NAME=livepatch-test KBUILD_EXTRA_SYMBOLS=~/oot/modulea/Module.symvers KPATCH_LDFLAGS=
+ KPATCH_BUILD=/lib/modules/5.15.6-100.fc34.x86_64/build
+ KPATCH_NAME=livepatch-test
+ KBUILD_EXTRA_SYMBOLS=~/oot/modulea/Module.symvers
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ - we can see in debug output that it properly applied
+ KPATCH_LDFLAGS=
...
SUCCESS

[william-ye999]

Thanks a lot.

I tested the patch by referring to the instructions you gave, and the livepatch was successfully compiled.

By the way, is there a way to directly set the value of KBUILD_EXTRA_SYMBOLS by referring to the Makefile of module A instead of explicitly assigning it?

[sm00th]

Thanks a lot.
I tested the patch by referring to the instructions you gave, and the livepatch was successfully compiled.

Glad it helped. I've added a commit removing that KBUILD_EXTRA_SYMBOLS initialization line in #1242.

By the way, is there a way to directly set the value of KBUILD_EXTRA_SYMBOLS by referring to the Makefile of module A instead of explicitly assigning it?

According to kernel's documentation you can use a common kbuild/makefile for both modules in one pass instead, but if you can't do that you are stuck with providing Module.symvers paths in KBUILD_EXTRA_SYMBOLS.

[william-ye999]

According to kernel's documentation you can use a common kbuild/makefile for both modules in one pass instead, but if you can't do that you are stuck with providing Module.symvers paths in KBUILD_EXTRA_SYMBOLS.

It can not work in my scene. I will use the former method. Thanks a lot. Can I close this issue?

[sm00th]

The issue will be automatically closed once we merge the fix.

[william-ye999]

One more thing, if I have a lot of similar livepatches to make, do I have to change the KBUILD_EXTRA_SYMBOLS every time?

[sm00th]

One more thing, if I have a lot of similar livepatches to make, do I have to change the KBUILD_EXTRA_SYMBOLS every time?

Yes. You can use export to define it once if they all use the same parent module(s) though if they all use the same parent module.