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> --- kernel/bpf/preload/Makefile | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/bpf/preload/Makefile b/kernel/bpf/preload/Makefile index 23ee310b6eb4..11b9896424c0 100644 --- a/kernel/bpf/preload/Makefile +++ b/kernel/bpf/preload/Makefile @@ -4,8 +4,11 @@ LIBBPF_SRCS = $(srctree)/tools/lib/bpf/ LIBBPF_A = $(obj)/libbpf.a LIBBPF_OUT = $(abspath $(obj)) +# Set $(O) so that the "dummy" test in tools/scripts/Makefile.include, called +# by libbpf's Makefile, succeeds when building the kernel with $(O) pointing to +# a relative path, as in "make O=build bindeb-pkg". $(LIBBPF_A): - $(Q)$(MAKE) -C $(LIBBPF_SRCS) OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a + $(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(abspath .) OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \ -I $(srctree)/tools/lib/ -Wno-unused-result -- 2.25.1
