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Date:   Wed, 16 Sep 2020 11:07:44 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Ian Rogers <irogers@google.com>, Jiri Olsa <jolsa@kernel.org>,
        lkml <linux-kernel@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>,
        Song Liu <songliubraving@fb.com>,
        "Frank Ch. Eigler" <fche@redhat.com>,
        Stephane Eranian <eranian@google.com>,
        Alexey Budankov <alexey.budankov@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>
Subject: Re: [PATCH 16/26] perf tools: Synthesize modules with mmap3
Message-ID: <20200916140744.GR720847@kernel.org>
References: <20200913210313.1985612-1-jolsa@kernel.org>
 <20200913210313.1985612-17-jolsa@kernel.org>
 <20200914160758.GK160517@kernel.org>
 <CAP-5=fV4Agxe3UMpkbWkh9CuG1tYho413w7xehy0SiMX+tPv1Q@mail.gmail.com>
 <20200916082018.GA2301783@krava>
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Em Wed, Sep 16, 2020 at 10:20:18AM +0200, Jiri Olsa escreveu:
> On Tue, Sep 15, 2020 at 01:17:44PM -0700, Ian Rogers wrote:
> 
> SNIP
> 
> > > >       /*
> > > >        * kernel uses 0 for user space maps, see kernel/perf_event.c
> > > >        * __perf_event_mmap
> > > > @@ -631,17 +629,30 @@ int perf_event__synthesize_modules(struct perf_tool *tool, perf_event__handler_t
> > > >                       continue;
> > > >
> > > >               size = PERF_ALIGN(pos->dso->long_name_len + 1, sizeof(u64));
> > > > -             event->mmap.header.type = PERF_RECORD_MMAP;
> > > > -             event->mmap.header.size = (sizeof(event->mmap) -
> > > > -                                     (sizeof(event->mmap.filename) - size));
> > > > -             memset(event->mmap.filename + size, 0, machine->id_hdr_size);
> > > > -             event->mmap.header.size += machine->id_hdr_size;
> > > > -             event->mmap.start = pos->start;
> > > > -             event->mmap.len   = pos->end - pos->start;
> > > > -             event->mmap.pid   = machine->pid;
> > > > -
> > > > -             memcpy(event->mmap.filename, pos->dso->long_name,
> > > > +             event->mmap3.header.type = PERF_RECORD_MMAP3;
> > > > +             event->mmap3.header.size = (sizeof(event->mmap3) -
> > > > +                                        (sizeof(event->mmap3.filename) - size));
> > > > +             memset(event->mmap3.filename + size, 0, machine->id_hdr_size);
> > > > +             event->mmap3.header.size += machine->id_hdr_size;
> > > > +             event->mmap3.start = pos->start;
> > > > +             event->mmap3.len   = pos->end - pos->start;
> > > > +             event->mmap3.pid   = machine->pid;
> > > > +
> > > > +             memcpy(event->mmap3.filename, pos->dso->long_name,
> > > >                      pos->dso->long_name_len + 1);
> > > > +
> > > > +             rc = filename__read_build_id(event->mmap3.filename, event->mmap3.buildid,
> > > > +                                          BUILD_ID_SIZE);
> > > > +             if (rc != BUILD_ID_SIZE) {
> > 
> > IIRC BUILD_ID_SIZE is 20 bytes which is the correct size for SHA-1. A
> > build ID may be 128-bits (16 bytes) if md5 or uuid hashes are used.
> > Should this test just be "> 0" ?
> 
> ah right, will check on that

And how do you deal with this in the kernel? I.e. to inform userspace,
via the PERF_RECORD_MMAP3 (or MMAP2 with that misc bit trick) the size
of the build-id?

- Arnaldo