Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function

Yonghong Song
2021-01-27T03:27:29Z

On 1/26/21 12:52 PM, Jiri Olsa wrote:
> On Thu, Jan 14, 2021 at 07:47:20PM -0800, Alexei Starovoitov wrote:
>> On Thu, Jan 14, 2021 at 3:44 PM Yonghong Song <yhs@fb.com> wrote:
>>>
>>>
>>>
>>> On 1/14/21 2:02 PM, Jiri Olsa wrote:
>>>> On Thu, Jan 14, 2021 at 01:05:33PM -0800, Yonghong Song wrote:
>>>>>
>>>>>
>>>>> On 1/14/21 12:01 PM, Jiri Olsa wrote:
>>>>>> On Thu, Jan 14, 2021 at 10:56:33AM -0800, Yonghong Song wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 1/14/21 5:40 AM, Jiri Olsa wrote:
>>>>>>>> It's possible to have other build id types (other than default SHA1).
>>>>>>>> Currently there's also ld support for MD5 build id.
>>>>>>>
>>>>>>> Currently, bpf build_id based stackmap does not returns the size of
>>>>>>> the build_id. Did you see an issue here? I guess user space can check
>>>>>>> the length of non-zero bits of the build id to decide what kind of
>>>>>>> type it is, right?
>>>>>>
>>>>>> you can have zero bytes in the build id hash, so you need to get the size
>>>>>>
>>>>>> I never saw MD5 being used in practise just SHA1, but we added the
>>>>>> size to be complete and make sure we'll fit with build id, because
>>>>>> there's only limited space in mmap2 event
>>>>>
>>>>> I am asking to check whether we should extend uapi struct
>>>>> bpf_stack_build_id to include build_id_size as well. I guess
>>>>> we can delay this until a real use case.
>>>>
>>>> right, we can try make some MD5 build id binaries and check if it
>>>> explodes with some bcc tools, but I don't expect that.. I'll try
>>>> to find some time for that
>>>
>>> Thanks. We may have issues on bcc side. For build_id collected in
>>> kernel, bcc always generates a length-20 string. But for user
>>> binaries, the build_id string length is equal to actual size of
>>> the build_id. They may not match (MD5 length is 16).
>>> The fix is probably to append '0's (up to length 20) for user
>>> binary build_id's.
>>>
>>> I guess MD5 is very seldom used. I will wait if you can reproduce
>>> the issue and then we might fix it.
>>
>> Indeed.
>> Jiri, please check whether md5 is really an issue.
>> Sounds like we have to do something on the kernel side.
>> Hopefully zero padding will be enough.
>> I would prefer to avoid extending uapi struct to cover rare case.
>
> build_id_parse is already doing the zero padding, so we are ok
>
> I tried several bcc tools over perf bench with md5 buildid and
> the results looked ok

Great. Thanks for confirmation!

>
> jirka
>