From: Quentin Monnet quentin@isovalent.com
[ Upstream commit 150a27328b681425c8cab239894a48f2aeb870e9 ]
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.
Note that the error does not occur for all make targets and architectures combinations. This was observed on x86 for "bindeb-pkg", or for a regular build for UML [0].
Here are some details. The root Makefile recursively calls itself once, after changing directory to $(O). The content for the variable $(PWD) is preserved across recursive calls to make, so it is unchanged at this step. For "bindeb-pkg", $(PWD) is eventually updated because the target writes a new Makefile (as debian/rules) and calls it indirectly through dpkg-buildpackage. This script does not preserve $(PWD), which is reset to the current working directory when the target in debian/rules is called.
Although not investigated, it seems likely that something similar causes UML to change its value for $(PWD).
Non-trivial fixes could be to remove the use of $(PWD) from the "dummy" check, or to make sure that $(PWD) and $(O) are preserved or updated to always play well and form a valid $(PWD)/$(O) path across the different targets and architectures. Instead, we take a simpler approach and just update $(O) when calling libbpf's Makefile, so it points to an absolute path which should always resolve for the "dummy" check run (through includes) by that Makefile.
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@google.com/t/#...
Fixes: d71fa5c9763c ("bpf: Add kernel module with user mode driver that populates bpffs.") Reported-by: David Gow davidgow@google.com Signed-off-by: Quentin Monnet quentin@isovalent.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Andrii Nakryiko andrii@kernel.org Cc: Brendan Higgins brendanhiggins@google.com Cc: Masahiro Yamada masahiroy@kernel.org Link: https://lore.kernel.org/bpf/20210126161320.24561-1-quentin@isovalent.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 23ee310b6eb49..1951332dd15f5 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))
+# Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test +# in tools/scripts/Makefile.include always 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 $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \ -I $(srctree)/tools/lib/ -Wno-unused-result