On Tue, Oct 10, 2023 at 5:43 AM Marcos Paulo de Souza <mpdesouza@xxxxxxx> wrote: > > Hi all, > > I found an issue while moving the livepatch kselftest modules to be built on the > fly, instead of building them on kernel building. > > If, for some reason, there is a recursive make invocation that starts from the > top level Makefile and in the leaf Makefile it tries to build a module (using M= > in the make invocation), it doesn't produce the module. This happens because the > toplevel Makefile checks for M= only once. This is controlled by the > sub_make_done variable, which is exported after checking the command line > options are passed to the top level Makefile. Once this variable is set it's > the M= setting is never checked again on the recursive call. > > This can be observed when cleaning the bpf kselftest dir. When calling > > $ make TARGETS="bpf" SKIP_TARGETS="" kselftest-clean > > What happens: > > 1. It checks for some command line settings (like M=) was passed (it wasn't), > set some definitions and exports sub_make_done. > > 2. Jump into tools/testing/selftests/bpf, and calls the clean target. > > 3. The clean target is overwritten to remove some files and then jump to > bpf_testmod dir and call clean there > > 4. On bpf_testmod/Makefile, the clean target will execute > $(Q)make -C $(KDIR) M=$(BPF_TESTMOD_DIR) clean > > 5. The KDIR is to toplevel dir. The top Makefile will check that sub_make_done was > already set, ignoring the M= setting. > > 6. As M= wasn't checked, KBUILD_EXTMOD isn't set, and the clean target applies > to the kernel as a whole, making it clean all generated code/objects and > everything. > > One way to avoid it is to call "unexport sub_make_done" on > tools/testing/selftests/bpf/bpf_testmod/Makefile before processing the all > target, forcing the toplevel Makefile to process the M=, producing the module > file correctly. > > If the M=dir points to /lib/modules/.../build, then it fails with "m2c: No such > file", which I already reported here[1]. At the time this problem was treated > like a problem with kselftest infrastructure. > > Important: The process works fine if the initial make invocation is targeted to a > different directory (using -C), since it doesn't goes through the toplevel > Makefile, and sub_make_done variable is not set. > > I attached a minimal reproducer, that can be used to better understand the > problem. The "make testmod" and "make testmod-clean" have the same effect that > can be seem with the bpf kselftests. There is a unexport call commented on > test-mods/Makefile, and once that is called the process works as expected. > > Is there a better way to fix this? Is this really a problem, or am I missing > something? Or, using KBUILD_EXTMOD will work too. -- Best Regards Masahiro Yamada
