2020-11-21 17:48 UTC+0800 ~ David Gow [off-list ref]

On Sat, Nov 21, 2020 at 3:38 PM Andrii Nakryiko
[off-list ref] wrote:

quoted

On Thu, Nov 19, 2020 at 12:51 AM David Gow [off-list ref] wrote:

quoted

If BPF_PRELOAD is enabled, and an out-of-tree build is requested with
make O=<path>, compilation seems to fail with:

tools/scripts/Makefile.include:4: *** O=.kunit does not exist.  Stop.
make[4]: *** [../kernel/bpf/preload/Makefile:8: kernel/bpf/preload/libbpf.a] Error 2
make[3]: *** [../scripts/Makefile.build:500: kernel/bpf/preload] Error 2
make[2]: *** [../scripts/Makefile.build:500: kernel/bpf] Error 2
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [.../Makefile:1799: kernel] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:185: __sub-make] Error 2

By the looks of things, this is because the (relative path) O= passed on
the command line is being passed to the libbpf Makefile, which then
can't find the directory. Given OUTPUT= is being passed anyway, we can
work around this by explicitly setting an empty O=, which will be
ignored in favour of OUTPUT= in tools/scripts/Makefile.include.

Strange, but I can't repro it. I use make O=<absolute path> all the
time with no issues. I just tried specifically with a make O=.build,
where .build is inside Linux repo, and it still worked fine. See also
be40920fbf10 ("tools: Let O= makes handle a relative path with -C
option") which was supposed to address such an issue. So I'm wondering
what exactly is causing this problem.

[+ linux-um list]

Hmm... From a quick check, I can't reproduce this on x86, so it's
possibly a UML-specific issue.

The problem here seems to be that $PWD is, for whatever reason, equal
to the srcdir on x86, but not on UML. In general, $PWD behaves pretty
weirdly -- I don't fully understand it -- but if I add a tactical "PWD
:= $(shell pwd)" or use $(CURDIR) instead, the issue shows up on x86
as well. I guess this is because PWD only gets updated when set by a
shell or something, and UML does this somewhere?

Thoughts?

Cheers,
-- David

Hi David, Andrii,

David, did you use a different command for building for UML and x86? I'm
asking because I reproduce on x86, but only for some targets, in
particular when I tried bindeb-pkg.

With "make O=.build vmlinux", I have:
- $(O) for "dummy" check in tools/scripts/Makefile.include set to
/linux/.build
- $(PWD) for same check set to /linux/tools
- Since $(O) is an absolute path, the "dummy" check passes

With "make O=.build bindeb-pkg", I have instead:
- $(O) set to .build (relative path)
- $(PWD) set to /linux/.build
- "dummy" check changes to /linux/.build and searches for .build in it,
which fails and aborts the build

(tools/scripts/Makefile.include is included from libbpf's Makefile,
called from kernel/bpf/preload/Makefile.)

I'm not sure how exactly the bindeb-pkg target ends up passing these values.

For what it's worth, I have been solving this (before finding this
thread) with a fix close to yours, I pass "O=$(abspath .)" on the
command line for building libbpf in kernel/bpf/preload/Makefile. It
looked consistent to me with the "tools/:" target from the main
Makefile, where "O=$(abspath $(objtree))" is passed (and $(objtree) is ".").

I hope this helps,
Quentin