netdev - [PATCH bpf v2] bpf: fix build for BPF preload when $(O) points to a relative path

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Message-Id: <20210126161320.24561-1-quentin@isovalent.com>
Date:   Tue, 26 Jan 2021 16:13:20 +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>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: [PATCH bpf v2] 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.

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/#u

v2: Use $(LIBBPF_OUT) instead of $(abspath .), and improve commit log

Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Brendan Higgins <brendanhiggins@...gle.com>
Cc: David Gow <davidgow@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>
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..1951332dd15f 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
-- 
2.25.1