Re: [PATCH v2] pahole: Add --kabi_prefix flag

[Shuyi Cheng <chengshuyi@linux.alibaba.com>]

On 5/20/21 8:18 PM, Jiri Olsa wrote:
> On Thu, May 20, 2021 at 08:08:44PM +0800, Shuyi Cheng wrote:
>>
>>
>> On 5/20/21 7:49 PM, Jiri Olsa wrote:
>>> On Thu, May 20, 2021 at 06:27:48PM +0800, Shuyi Cheng wrote:
>>>>
>>>>
>>>> On 5/20/21 4:07 AM, Jiri Olsa wrote:
>>>>> On Wed, May 19, 2021 at 10:44:44AM +0800, Shuyi Cheng wrote:
>>>>>> To solve problems similar to _RH_KABI_REPLACE. The _RH_KABI_REPLACE(_orig,
>>>>>> _new) macros perserve size alignment and kabi agreement between _orig and
>>>>>> _new.Below is the definition of this macro:
>>>>>>
>>>>>> # define _RH_KABI_REPLACE(_orig, _new)            \
>>>>>>        union {                        \
>>>>>>            _new;                    \
>>>>>>            struct {                \
>>>>>>                _orig;                \
>>>>>>            } __UNIQUE_ID(rh_kabi_hide);        \
>>>>>>            __RH_KABI_CHECK_SIZE_ALIGN(_orig, _new);    \
>>>>>>        }
>>>>>
>>>>> hi,
>>>>> that macro sounds familiar ;-) I think this should be already
>>>>> solved directly in the header file by this one:
>>>>>
>>>>>      https://gitlab.com/cki-project/kernel-ark/-/commit/331be9c5a436057ee852075c102d9d90a9046a30
>>>>>
>>>>> jirka
>>>>>
>>>>
>>>> Well, this patch solves this problem very well from the kernel level. But
>>>> there are many mirrors [here](http://debuginfo.centos.org/7/x86_64/) that
>>>> still have this problem. And these mirrors support ebpf, so it is very
>>>> important to effectively extract the btf segment from these mirrors. :-)
>>>
>>> hum, the link shows 3.10.* centos kernel, right?
>>> AFAIK there's no BTF support in those kernels..
>>>
>>> but maybe I'm missing some backporting channel,
>>> could you please point me to related sources?
>>>
>>> jirka
>>>
>>
>> Yes, it is the centos kernel of version 3.10. This version of the centos
>> kernel supports ebpf. As far as I think, BTF features don't depend on the
>> kernel much. Only when using the STRUCT OPS feature does the kernel support
>> BTF, see [here](https://github.com/libbpf/libbpf/blob/57375504c6c9766d23f178c40f71bf5e8df9363d/src/libbpf.c#L2549)
>> and [here](https://www.spinics.net/lists/netdev/msg637060.html). Therefore,
>> in the 3.10 version of the Centos kernel, with the help of the vmlinux btf
>> segment, libbpf CO-RE can be easily supported.
> 
> ok, so you're generating BTF for centos 3.10 kernel?
> 
> I think that fix would be easy kernel backport, but I guess
> we can go with pahole option just as well, if backport is
> not an option for you
> 
> jirka
> 

Yes, I am generating btf segment for centos 3.10 kernel. There are two 
main reasons for adding this patch:
1. There are too many kernel versions in centos 3.10, and a certain 
technical foundation is required to compile the kernel.
2. In my opinion, many people are using pahole to generate vmlinux btf 
segment, and there are still a large number of users on centos.