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

From: Yonghong Song <yhs@fb.com>
To: Jiri Olsa <jolsa@redhat.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@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: Thu, 14 Jan 2021 15:43:56 -0800	[thread overview]
Message-ID: <5043cef5-eda7-4373-dcb5-546f6192e1a9@fb.com> (raw)
In-Reply-To: <20210114220234.GA1456269@krava>

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.

> 
> perf tool uses build ids in .debug cache as file links, and we had
> few isues there
> 
> jirka
> 