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150a27328b
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
Fixes: d71fa5c976 ("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
29 lines
1014 B
Makefile
29 lines
1014 B
Makefile
# SPDX-License-Identifier: GPL-2.0
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LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
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LIBBPF_A = $(obj)/libbpf.a
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LIBBPF_OUT = $(abspath $(obj))
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# Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test
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# in tools/scripts/Makefile.include always succeeds when building the kernel
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# with $(O) pointing to a relative path, as in "make O=build bindeb-pkg".
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$(LIBBPF_A):
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$(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
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userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
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-I $(srctree)/tools/lib/ -Wno-unused-result
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userprogs := bpf_preload_umd
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clean-files := $(userprogs) bpf_helper_defs.h FEATURE-DUMP.libbpf staticobjs/ feature/
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bpf_preload_umd-objs := iterators/iterators.o
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bpf_preload_umd-userldlibs := $(LIBBPF_A) -lelf -lz
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$(obj)/bpf_preload_umd: $(LIBBPF_A)
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$(obj)/bpf_preload_umd_blob.o: $(obj)/bpf_preload_umd
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obj-$(CONFIG_BPF_PRELOAD_UMD) += bpf_preload.o
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bpf_preload-objs += bpf_preload_kern.o bpf_preload_umd_blob.o
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