linux-kernel - [PATCH] trace/uprobes: fix output issue with address randomization

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Date:   Fri, 15 Dec 2017 16:36:25 +0100
From:   Thomas Richter <tmricht@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org, peterz@...radead.org
Cc:     heiko.carstens@...ibm.com, schwidefsky@...ibm.com,
        brueckner@...ux.vnet.ibm.com,
        Thomas Richter <tmricht@...ux.vnet.ibm.com>
Subject: [PATCH] trace/uprobes: fix output issue with address randomization

Commit ad67b74d2469 ("printk: hash addresses printed with %p")
changed %p to hash pointers in order to avoid leaking
kernel addresses.

This breaks the tool perf probe.

To set a uprobe on a function named inet_pton in libc library,
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

However the output shows a randomized address:
 # cat /sys/kernel/debug/tracing/uprobe_events
 p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x000000002d0f8952

The displayed address 0x000000002d0f8952 is incorrect and breaks
tools post processing it:

 # 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)

Using the %px printk format string fixes this issue for root
and shows the correct address allowing the perf probe tool
to resolve the address to the symbol:

 # echo "p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x142060"
	> /sys/kernel/debug/tracing/uprobe_events
 # cat /sys/kernel/debug/tracing/uprobe_events
 p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x0000000000142060
 # linux/tools/perf/perf probe -l
  probe_libc:inet_pton (on __inet_pton@...olv/inet_pton.c
					in /usr/lib64/libc-2.26.so)

Signed-off-by: Thomas Richter <tmricht@...ux.vnet.ibm.com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
---
 kernel/trace/trace_uprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index 40592e7b3568..268029ae1be6 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -608,7 +608,7 @@ static int probes_seq_show(struct seq_file *m, void *v)
 
 	/* Don't print "0x  (null)" when offset is 0 */
 	if (tu->offset) {
-		seq_printf(m, "0x%p", (void *)tu->offset);
+		seq_printf(m, "0x%px", (void *)tu->offset);
 	} else {
 		switch (sizeof(void *)) {
 		case 4:
-- 
2.13.4