linux-kernel - RE: Question: perf dso support for /proc/kallsyms

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Date:   Fri, 2 Nov 2018 12:08:04 +0000
From:   Al Grant <Al.Grant@....com>
To:     "leo.yan@...aro.org" <leo.yan@...aro.org>,
        "acme@...hat.com" <acme@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Coresight ML <coresight@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Question: perf dso support for /proc/kallsyms

> Now I still don't understand how the dso/map support "/proc/kallsyms" and
> have no idea to fix this issue, though I spent some time to look into it.

The way this is supported is that at record time, pseudo mmap records
are created for the kernel. But depending on permissions these might
not get created. This isn't just an ETM issue, it can happen on Intel too.

What do you see in "perf report -D", do you see a PERF_RECORD_MMAP
record for "[kernel.kallsyms]_text" and possibly some others for loadable
kernel modules in /lib/modules?

Does it all work if you run perf record as sudo? Or if you do

   sudo sysctl kernel.kptr_restrict=0

before you run perf record?

Al
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