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From: Yonghong Song <yhs@fb.com>
To: Jiri Olsa <jolsa@redhat.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	lkml <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ingo Molnar <mingo@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Michael Petlan <mpetlan@redhat.com>,
	Ian Rogers <irogers@google.com>,
	Stephane Eranian <eranian@google.com>,
	Alexei Budankov <abudankov@huawei.com>,
	Andi Kleen <ak@linux.intel.com>,
	Adrian Hunter <adrian.hunter@intel.com>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function
Date: Tue, 26 Jan 2021 13:00:06 -0800	[thread overview]
Message-ID: <9490aa32-ad01-4a55-c2a0-e688226ecc71@fb.com> (raw)
In-Reply-To: <20210126205236.GD120879@krava>

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

  reply	other threads:[~2021-01-27 12:27 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-14 13:40 [PATCHv7 bpf-next 0/3] perf: Add mmap2 build id support Jiri Olsa
2021-01-14 13:40 ` [PATCH bpf-next 1/3] bpf: Move stack_map_get_build_id into lib Jiri Olsa
2021-01-14 13:40 ` [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function Jiri Olsa
2021-01-14 18:56   ` Yonghong Song
2021-01-14 20:01     ` Jiri Olsa
2021-01-14 21:05       ` Yonghong Song
2021-01-14 22:02         ` Jiri Olsa
2021-01-14 23:43           ` Yonghong Song
2021-01-15  3:47             ` Alexei Starovoitov
2021-01-26 20:52               ` Jiri Olsa
2021-01-26 21:00                 ` Yonghong Song [this message]
2021-01-14 13:40 ` [PATCH bpf-next 3/3] perf: Add build id data in mmap2 event Jiri Olsa
2021-01-15  3:50 ` [PATCHv7 bpf-next 0/3] perf: Add mmap2 build id support patchwork-bot+netdevbpf

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