Date:   Mon, 25 Jan 2021 15:49:38 +0000
From:   Quentin Monnet <quentin@...valent.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Quentin Monnet <quentin@...valent.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        David Gow <davidgow@...gle.com>
Subject: [PATCH] bpf: fix build for BPF preload when $(O) points to a relative path

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@google.com/t/#u

Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Brendan Higgins <brendanhiggins@...gle.com>
Cc: David Gow <davidgow@...gle.com>
Reported-by: David Gow <davidgow@...gle.com>
Signed-off-by: Quentin Monnet <quentin@...valent.com>
---
 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

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