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

[Jinghao Jia]

On 1/21/25 3:15 AM, Nicolas Schier wrote:
> On Tue, Jan 21, 2025 at 02:48:54AM -0600, Jinghao Jia wrote:
>>
>>
>> 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
> 
> $(srctree) refers to the root of the kernel source tree, while
> $(objtree) refers to the root of the kernel object tree.  Even if both
> variables have the same value for the case you are fixing, it makes
> sense to align to the meaning of the variables as described in
> Documentation/kbuild/makefiles.rst.
> 
> In short: if you want to access file created during the kernel build,
> please always use $(objtree) instead of $(srctree).
> 
> Hope that helps.
> 
> Kind regards,
> Nicolas

I see. Thanks for the clarification! I will add this to v2.

--Jinghao