linux-kernel - Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id

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Date:   Thu, 14 Jan 2021 23:02:34 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Yonghong Song <yhs@...com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Song Liu <songliubraving@...com>,
        lkml <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Alexei Budankov <abudankov@...wei.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function

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

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

jirka