linux-kernel - Re: [PATCH 03/24] perf: Add build id data in mmap2 event

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Message-ID: <20201110122232.GH2594@hirez.programming.kicks-ass.net>
Date:   Tue, 10 Nov 2020 13:22:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        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>,
        Song Liu <songliubraving@...com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 03/24] perf: Add build id data in mmap2 event

On Tue, Nov 10, 2020 at 08:54:26AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Nov 10, 2020 at 09:07:16AM +0100, Peter Zijlstra escreveu:
> > On Mon, Nov 09, 2020 at 10:53:54PM +0100, Jiri Olsa wrote:
> > > Adding support to carry build id data in mmap2 event.
> > > 
> > > The build id data replaces maj/min/ino/ino_generation
> > > fields, whichc are also used to identify map's binary,
> > > so it's ok to replace them with build id data:
> > > 
> > >   union {
> > >           struct {
> > >                   u32       maj;
> > >                   u32       min;
> > >                   u64       ino;
> > >                   u64       ino_generation;
> > >           };
> > >           struct {
> > >                   u8        build_id[20];
> > >                   u8        build_id_size;
> > 
> > What's the purpose of a size field for a fixed size array? Also, I'd
> > flip the order of these fields, first have the size and then the array.
> 
> There can be different types of build-ids, with different sizes,
> flipping the order of the fields is indeed sensible, as we could then
> support even larger build_ids if the need arises :)

3 whole bytes.. whooo!