On Mon, Jan 25, 2021 at 11:49 PM Quentin Monnet <quentin@xxxxxxxxxxxxx> wrote: > > Building the kernel with CONFIG_BPF_PRELOAD, and by providing a relative > path for the output directory, may fail with the following error: > > $ make O=build bindeb-pkg > ... > /.../linux/tools/scripts/Makefile.include:5: *** O=build does not exist. Stop. > make[7]: *** [/.../linux/kernel/bpf/preload/Makefile:9: kernel/bpf/preload/libbpf.a] Error 2 > make[6]: *** [/.../linux/scripts/Makefile.build:500: kernel/bpf/preload] Error 2 > make[5]: *** [/.../linux/scripts/Makefile.build:500: kernel/bpf] Error 2 > make[4]: *** [/.../linux/Makefile:1799: kernel] Error 2 > make[4]: *** Waiting for unfinished jobs.... > > In the case above, for the "bindeb-pkg" target, the error is produced by > the "dummy" check in Makefile.include, called from libbpf's Makefile. > This check changes directory to $(PWD) before checking for the existence > of $(O). But at this step we have $(PWD) pointing to "/.../linux/build", > and $(O) pointing to "build". So the Makefile.include tries in fact to > assert the existence of a directory named "/.../linux/build/build", > which does not exist. > > By contrast, other tools called from the main Linux Makefile get the > variable set to $(abspath $(objtree)), where $(objtree) is ".". We can > update the Makefile for kernel/bpf/preload to set $(O) to the same > value, to permit compiling with a relative path for output. Note that > apart from the Makefile.include, the variable $(O) is not used in > libbpf's build system. > > Note that the error does not occur for all make targets and > architectures combinations. > > - On x86, "make O=build vmlinux" appears to work fine. > $(PWD) points to "/.../linux/tools", but $(O) points to the absolute > path "/.../linux/build" and the test succeeds. > - On UML, it has been reported to fail with a message similar to the > above (see [0]). > - On x86, "make O=build bindeb-pkg" fails, as described above. > > It is unsure where the different values for $(O) and $(PWD) come from > (likely some recursive make with different arguments at some point), and > because several targets are broken, it feels safer to fix the $(O) value > passed to libbpf rather than to hunt down all changes to the variable. > > David Gow previously posted a slightly different version of this patch > as a RFC [0], two months ago or so. > > [0] https://lore.kernel.org/bpf/20201119085022.3606135-1-davidgow@xxxxxxxxxx/t/#u > > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> > Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx> > Cc: David Gow <davidgow@xxxxxxxxxx> > Reported-by: David Gow <davidgow@xxxxxxxxxx> > Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx> > --- Thanks for following up on this. I've tested it against my usecase (allyesconfig with ARCH=um), and it fixes the issue nicely. Tested-by: David Gow <davidgow@xxxxxxxxxx> While I tend to agree it'd be nicer to track down the place $(O) and $(PWD) are broken, too, given that there seem to be a number of different ways to trigger this (bindeb-pkg and ARCH=um), I'm not sure there's only one breakage to find. Cheers, -- David
