Re: [PATCH bpf] samples/bpf: fix broken vmlinux path for VMLINUX_BTF

[Jinghao Jia]

On 1/20/25 6:47 PM, Jinghao Jia wrote:
> 
> 
> On 1/20/25 6:42 PM, Masahiro Yamada wrote:
>> On Mon, Jan 20, 2025 at 11:30 AM Jinghao Jia <jinghao7@xxxxxxxxxxxx> wrote:
>>>
>>> Commit 13b25489b6f8 ("kbuild: change working directory to external
>>> module directory with M=") changed kbuild working directory of bpf
>>> samples to $(srctree)/samples/bpf, which broke the vmlinux path for
>>> VMLINUX_BTF, as the Makefile assumes the current work directory to be
>>> $(srctree):
>>>
>>>   Makefile:316: *** Cannot find a vmlinux for VMLINUX_BTF at any of "  /path/to/linux/samples/bpf/vmlinux", build the kernel or set VMLINUX_BTF like "VMLINUX_BTF=/sys/kernel/btf/vmlinux" or VMLINUX_H variable.  Stop.
>>>
>>> Correctly refer to the kernel source directory using $(srctree).
>>>
>>> Fixes: 13b25489b6f8 ("kbuild: change working directory to external module directory with M=")
>>> Tested-by: Ruowen Qin <ruqin@xxxxxxxxxx>
>>> Signed-off-by: Jinghao Jia <jinghao7@xxxxxxxxxxxx>
>>> ---
>>>  samples/bpf/Makefile | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>>> index 96a05e70ace3..f97295724a14 100644
>>> --- a/samples/bpf/Makefile
>>> +++ b/samples/bpf/Makefile
>>> @@ -307,7 +307,7 @@ $(obj)/$(TRACE_HELPERS): TPROGS_CFLAGS := $(TPROGS_CFLAGS) -D__must_check=
>>>
>>>  VMLINUX_BTF_PATHS ?= $(abspath $(if $(O),$(O)/vmlinux))                                \
>>>                      $(abspath $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)) \
>>> -                    $(abspath ./vmlinux)
>>> +                    $(abspath $(srctree)/vmlinux)
>>
>> This is wrong and will not work for O= build.
>>
>> The prefix should be $(objtree)/
>>
>>
> 
> I thought the $(srctree)/vmlinux is a fallback when $(O) is not defined, am I
> understanding it wrong here?
> 
> --Jinghao
> 

I played with kbuild a little bit more. It seems that the way the Makefiles are
set up does not allow a sample/bpf build w/o O= when the kernel itself is built
w/ O=, as it directly invokes the top-level Makefile. If O= is passed to the
sample/bpf build, the $(abspath $(if $(O),$(O)/vmlinux)) part should take care
of that. With that said, please do correct me if I am wrong.

--Jinghao

>>
>>
>>
>>>  VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
>>>
>>>  $(obj)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
>>> --
>>> 2.48.1
>>>
>>
>>
>