134 lines
5.2 KiB
Plaintext
134 lines
5.2 KiB
Plaintext
Uprobe-tracer: Uprobe-based Event Tracing
|
|
=========================================
|
|
|
|
Documentation written by Srikar Dronamraju
|
|
|
|
|
|
Overview
|
|
--------
|
|
Uprobe based trace events are similar to kprobe based trace events.
|
|
To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
|
|
|
|
Similar to the kprobe-event tracer, this doesn't need to be activated via
|
|
current_tracer. Instead of that, add probe points via
|
|
/sys/kernel/debug/tracing/uprobe_events, and enable it via
|
|
/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
|
|
|
|
However unlike kprobe-event tracer, the uprobe event interface expects the
|
|
user to calculate the offset of the probepoint in the object.
|
|
|
|
Synopsis of uprobe_tracer
|
|
-------------------------
|
|
p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a uprobe
|
|
r[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a return uprobe (uretprobe)
|
|
-:[GRP/]EVENT : Clear uprobe or uretprobe event
|
|
|
|
GRP : Group name. If omitted, "uprobes" is the default value.
|
|
EVENT : Event name. If omitted, the event name is generated based
|
|
on SYMBOL+offs.
|
|
PATH : Path to an executable or a library.
|
|
SYMBOL[+offs] : Symbol+offset where the probe is inserted.
|
|
|
|
FETCHARGS : Arguments. Each probe can have up to 128 args.
|
|
%REG : Fetch register REG
|
|
|
|
Event Profiling
|
|
---------------
|
|
You can check the total number of probe hits and probe miss-hits via
|
|
/sys/kernel/debug/tracing/uprobe_profile.
|
|
The first column is event name, the second is the number of probe hits,
|
|
the third is the number of probe miss-hits.
|
|
|
|
Usage examples
|
|
--------------
|
|
* Add a probe as a new uprobe event, write a new definition to uprobe_events
|
|
as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
|
|
|
|
echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
|
|
|
|
* Add a probe as a new uretprobe event:
|
|
|
|
echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
|
|
|
|
* Unset registered event:
|
|
|
|
echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
|
|
|
|
* Print out the events that are registered:
|
|
|
|
cat /sys/kernel/debug/tracing/uprobe_events
|
|
|
|
* Clear all events:
|
|
|
|
echo > /sys/kernel/debug/tracing/uprobe_events
|
|
|
|
Following example shows how to dump the instruction pointer and %ax register
|
|
at the probed text address. Probe zfree function in /bin/zsh:
|
|
|
|
# cd /sys/kernel/debug/tracing/
|
|
# cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
|
|
00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
|
|
# objdump -T /bin/zsh | grep -w zfree
|
|
0000000000446420 g DF .text 0000000000000012 Base zfree
|
|
|
|
0x46420 is the offset of zfree in object /bin/zsh that is loaded at
|
|
0x00400000. Hence the command to uprobe would be:
|
|
|
|
# echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
|
|
|
|
And the same for the uretprobe would be:
|
|
|
|
# echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
|
|
|
|
Please note: User has to explicitly calculate the offset of the probe-point
|
|
in the object. We can see the events that are registered by looking at the
|
|
uprobe_events file.
|
|
|
|
# cat uprobe_events
|
|
p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
|
|
r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
|
|
|
|
Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
|
|
|
|
# cat events/uprobes/zfree_entry/format
|
|
name: zfree_entry
|
|
ID: 922
|
|
format:
|
|
field:unsigned short common_type; offset:0; size:2; signed:0;
|
|
field:unsigned char common_flags; offset:2; size:1; signed:0;
|
|
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
|
|
field:int common_pid; offset:4; size:4; signed:1;
|
|
field:int common_padding; offset:8; size:4; signed:1;
|
|
|
|
field:unsigned long __probe_ip; offset:12; size:4; signed:0;
|
|
field:u32 arg1; offset:16; size:4; signed:0;
|
|
field:u32 arg2; offset:20; size:4; signed:0;
|
|
|
|
print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
|
|
|
|
Right after definition, each event is disabled by default. For tracing these
|
|
events, you need to enable it by:
|
|
|
|
# echo 1 > events/uprobes/enable
|
|
|
|
Lets disable the event after sleeping for some time.
|
|
|
|
# sleep 20
|
|
# echo 0 > events/uprobes/enable
|
|
|
|
And you can see the traced information via /sys/kernel/debug/tracing/trace.
|
|
|
|
# cat trace
|
|
# tracer: nop
|
|
#
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION
|
|
# | | | | |
|
|
zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
|
|
zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
|
|
zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
|
|
zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
|
|
|
|
Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
|
|
and contents of ax register being 79. And uretprobe was triggered with ip at
|
|
0x446540 with counterpart function entry at 0x446420.
|