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

[Jinghao Jia]

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

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