ICP build error with --enable-linux-builtin and separate kernel source and build directories

[nivedita76]

System information

Type	Version/Name
Distribution Name	Gentoo
Distribution Version	NA
Linux Kernel	5.5
Architecture	x86_64
ZFS Version	git
SPL Version	NA

Describe the problem you're observing

When building zfs builtin to the kernel, using
configure --with-config=kernel --enable-linux-builtin --with-linux=${KSRC} --with-linux-obj=${KOBJ}
the icp subdirectory fails because it can't find include files.

Describe how to reproduce the problem

Include any warning/errors/backtraces from the system logs

[nivedita76]

The error is because module/icp/Makefile.in adds -I options using $(src), and copy-builtin deletes the src/obj definitions from the Makefile. This results in the options being added as -Ifs/zfs/icp/include. This does not work with a separate kernel build directory, as the compiler is run from the build directory.

The attached patch fixes the issue for me.
icp.txt

[nivedita76]

Are the src/obj defines needed for a modular build? Or are they always unnecessary in the module/ subdir?

[sempervictus]

There arent buildbots for the in-tree stuff, so breakage in this functionality of usually detected by someone trying to actually do it :). Part of the reason my shop has been staying on modified versions of stable tags is that there's a bunch of OS-related rework going on, which tends to hit the build system - we've found it easier to divine and backport the requirements for some feature/commit to the stable branches than trying to keep up with the OS code churn.
Thanks for the patch.

[behlendorf]

@nivedita76 thanks for the patch. The obj and src defines are needed to support VPATH in the out-of-tree modular builds, but your fix looks like a reasonable way to address the in-kernel issue. Would you mind opening a PR with the proposed change?

The CI does test in-kernel builds but it doesn't happen to cover this case. It would be nice to update the test scripts to provide coverage for this. @nivedita76 would you mind posting the steps you used to hit this issue. For reference, this is the script used by CI to test in-kernel builds.

[nivedita76]

Attached script can be used to repro.

test-zfs.txt