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//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 probe GRP : Group name. If omitted, use "uprobes" for it. 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 -------------- To add a probe as a new event, write a new definition to uprobe_events as below. echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash echo > /sys/kernel/debug/tracing/uprobe_events This clears all probe points. The following example shows how to dump the instruction pointer and %ax a register at the probed text address. Here we are trying to probe function zfree 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 probe would be : # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events Please note: User has to explicitly calculate the offset of the probepoint in the object. We can see the events that are registered by looking at the uprobe_events file. # cat uprobe_events p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format # cat events/uprobes/p_zsh_0x46420/format name: p_zsh_0x46420 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: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 Each line shows us probes were triggered for a pid 24842 with ip being 0x446421 and contents of ax register being 79.