I was working on fixing the cross-compilation for the selftests/vm tests. Currently, there are two issues in my testing:

1) problem: required library missing from some cross-compile environments: tools/testing/selftests/vm/mlock-random-test.c requires libcap be installed. The target line for mlock-random-test in tools/testing/selftests/vm/Makefile looks like this:

$(OUTPUT)/mlock-random-test: LDLIBS += -lcap

and mlock-random-test.c has this include line: #include <sys/capability.h>

this is confusing, since this is different from the header file linux/capability.h. It is associated with the capability library (libcap) and not the kernel. In any event, on some distros and in some cross-compile SDKs the package containing these files is not installed by default.

Once this library is installed, things progress farther. Using an Ubuntu system, you can install the cross version of this library (for arm64) by doing: $ sudo apt install libcap-dev:arm64

1) solution: I would like to add some meta-data about this build dependency, by putting something in the settings file as a hint to CI build systems. Specifically, I'd like to create the file 'tools/testing/selftests/vm/settings', with the content: NEED_LIB=cap

We already use settings for other meta-data about a test (right now, just a non-default timeout value), but I don't want to create a new file or syntax for this build dependency data.

Let me know what you think.

I may follow up with some script in the kernel source tree to check these dependencies, independent of any CI system. I have such a script in Fuego that I could submit, but it would need some work to fit into the kernel build flow for kselftest. The goal would be to provide a nicely formatted warning, with a recommendation for a package install. But that's more work than I think is needed right now just to let developers know there's a build dependency here.

2) problem: reference to source-relative header file the Makefile for vm uses a relative path for include directories. Specifically, it has the line: CFLAGS = -Wall -I ../../../../../usr/include $(EXTRA_CFLAGS)

I believe this needs to reference kernel include files from the output directory, not the source directory.

With the relative include directory path, the program userfaultfd.c gets compilation error like this:

userfaultfd.c:267:21: error: 'UFFD_API_RANGE_IOCTLS_BASIC' undeclared here (not in a function) .expected_ioctls = UFFD_API_RANGE_IOCTLS_BASIC, ^ userfaultfd.c: In function 'uffd_poll_thread': userfaultfd.c:529:8: error: 'UFFD_EVENT_FORK' undeclared (first use in this function) case UFFD_EVENT_FORK: ^ userfaultfd.c:529:8: note: each undeclared identifier is reported only once for each function it appears in userfaultfd.c:531:18: error: 'union <anonymous>' has no member named 'fork' uffd = msg.arg.fork.ufd; ^

2) incomplete solution: I originally changed this line to read: CFLAGS = -Wall -I $(KBUILD_OUTPUT)/usr/include $(EXTRA_CFLAGS)

This works when the output directory is specified using KBUILD_OUTPUT, but not when the output directory is specified using O= I'm not sure what happens when the output directory is specified with a non-source-tree current working directory.

In any event, while researching a proper solution to this, I found the following in tools/testing/selftests/Makefile:

If compiling with ifneq ($(O),) BUILD := $(O) else ifneq ($(KBUILD_OUTPUT),) BUILD := $(KBUILD_OUTPUT)/kselftest else BUILD := $(shell pwd) DEFAULT_INSTALL_HDR_PATH := 1 endif endif

This doesn't seem right. It looks like the selftests Makefile treats a directory passed in using O= different from one specified using KBUILD_OUTPUT or the current working directory. In the KBUILD_OUTPUT case, you get an extra 'kselftest' directory layer that you don't get for the other two.

In contrast, the kernel top-level Makefile has this: ifeq ("$(origin O)", "command line") KBUILD_OUTPUT := $(O) endif (and from then on, the top-level Makefile appears to only use KBUILD_OUTPUT)

This makes it look like the rest of the kernel build system treats O= and KBUILD_OUTPUT identically.

Am I missing something, or is there a flaw in the O=/KBUILD_OUTPUT handling in kselftest? Please let me know and I'll try to work out an appropriate fix for cross-compiling the vm tests. -- Tim