2021-01-25 16:32 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> > On Mon, Jan 25, 2021 at 7:49 AM Quentin Monnet <quentin@xxxxxxxxxxxxx> wrote: >> >> 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@xxxxxxxxxx/t/#u >> >> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> >> Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx> >> Cc: David Gow <davidgow@xxxxxxxxxx> >> Reported-by: David Gow <davidgow@xxxxxxxxxx> >> Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx> >> --- > > I still think it would benefit everyone to figure out where this is > breaking (given Linux Makefile explicitly tries to handle such > relative path situation for O=, I believe), but this is trivial > enough, so: > > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx> Agreed, I'll try to spend a bit more time on this when I can. But it would be nice to have the fix in the meantime. Thanks for the review and ack. > > BTW, you haven't specified which tree you intended it for. Oops! I _knew_ I was missing something, sorry. This build issue is here since eBPF preload was introduced, so I meant to send to the *bpf* tree. Because it does not concern the major build targets, I was not sure if a "Fixes:" tag would be appropriate. If we want one, it should be for d71fa5c9763c ("bpf: Add kernel module with user mode driver that populates bpffs.") > >> 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)/ $(LIBBPF_OUT)/libbpf.a > > why not O=$(LIBBPF_OUT), btw? $(LIBBPF_OUT) points to /.../linux/ZZ_BUILD/build/kernel/bpf/preload. This is an absolute path so the "dummy" check should work, too. I preferred to align the value on the root Makefile, which has "O=$(abspath $(objtree))" for target "tools/%", but no strong opinion here. David would simply empty the variable in his patch. I'm fine with any of the three versions. Would you prefer me to resend with $(LIBBPF_OUT)? Thanks, Quentin
