linux-kernel - Linux 4.15.0-rc3 perf probe/uprobe issue with address randomization

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Message-Id: <8402bed0-bb40-53dc-faf8-bd26f6997de5@linux.vnet.ibm.com>
Date:   Fri, 15 Dec 2017 15:14:37 +0100
From:   Thomas-Mich Richter <tmricht@...ux.vnet.ibm.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Linux 4.15.0-rc3 perf probe/uprobe issue with address randomization

During debugging of perf probe tool I discovered an issue with
uprobes and address randomization.

To set a uprobe on a function named inet_pton in libc library, you
obtain the address of the symbol inet_pton using command nm and
then use the following command to set the uprobe:

# echo "p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x142060"
	> /sys/kernel/debug/tracing/uprobe_events

0x142060 is the address of inet_pton on my system.
This works nicely and the uprobe is usable.

The issue is with the output:
# cat /sys/kernel/debug/tracing/uprobe_events
p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x000000002d0f8952
#

The displayed address 0x000000002d0f8952 is wrong, probably
randomized and post processing of this output with the perf
probe tool fails due to this random address:

# linux/tools/perf/perf probe -l
Failed to find debug information for address 2d0f8952
  probe_libc:inet_pton (on 0x2d0f8952 in /usr/lib64/libc-2.26.so)
# 

So how to fix this (if at all)?
Is replacing %p by %llx in line 612 of file kernel/trace/trace_uprobe.c
   seq_printf(m, "0x%p", (void *)tu->offset)
an option?
Or is this broken by design and intention?

Thanks for hints and suggestions.
-- 
Thomas Richter, Dept 3303, IBM LTC Boeblingen Germany
--
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