114 lines
4.2 KiB
Plaintext
114 lines
4.2 KiB
Plaintext
Uprobe-tracer: Uprobe-based Event Tracing
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=========================================
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Documentation written by Srikar Dronamraju
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Overview
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--------
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Uprobe based trace events are similar to kprobe based trace events.
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To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
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Similar to the kprobe-event tracer, this doesn't need to be activated via
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current_tracer. Instead of that, add probe points via
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/sys/kernel/debug/tracing/uprobe_events, and enable it via
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/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
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However unlike kprobe-event tracer, the uprobe event interface expects the
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user to calculate the offset of the probepoint in the object
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Synopsis of uprobe_tracer
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-------------------------
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p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a probe
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GRP : Group name. If omitted, use "uprobes" for it.
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EVENT : Event name. If omitted, the event name is generated
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based on SYMBOL+offs.
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PATH : path to an executable or a library.
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SYMBOL[+offs] : Symbol+offset where the probe is inserted.
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FETCHARGS : Arguments. Each probe can have up to 128 args.
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%REG : Fetch register REG
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Event Profiling
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---------------
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You can check the total number of probe hits and probe miss-hits via
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/sys/kernel/debug/tracing/uprobe_profile.
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The first column is event name, the second is the number of probe hits,
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the third is the number of probe miss-hits.
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Usage examples
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--------------
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To add a probe as a new event, write a new definition to uprobe_events
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as below.
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echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
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This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash
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echo > /sys/kernel/debug/tracing/uprobe_events
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This clears all probe points.
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The following example shows how to dump the instruction pointer and %ax
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a register at the probed text address. Here we are trying to probe
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function zfree in /bin/zsh
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# cd /sys/kernel/debug/tracing/
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# cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
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00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
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# objdump -T /bin/zsh | grep -w zfree
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0000000000446420 g DF .text 0000000000000012 Base zfree
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0x46420 is the offset of zfree in object /bin/zsh that is loaded at
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0x00400000. Hence the command to probe would be :
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# echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
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Please note: User has to explicitly calculate the offset of the probepoint
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in the object. We can see the events that are registered by looking at the
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uprobe_events file.
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# cat uprobe_events
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p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax
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The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format
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# cat events/uprobes/p_zsh_0x46420/format
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name: p_zsh_0x46420
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ID: 922
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format:
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field:unsigned short common_type; offset:0; size:2; signed:0;
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field:unsigned char common_flags; offset:2; size:1; signed:0;
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field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
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field:int common_pid; offset:4; size:4; signed:1;
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field:int common_padding; offset:8; size:4; signed:1;
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field:unsigned long __probe_ip; offset:12; size:4; signed:0;
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field:u32 arg1; offset:16; size:4; signed:0;
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field:u32 arg2; offset:20; size:4; signed:0;
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print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
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Right after definition, each event is disabled by default. For tracing these
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events, you need to enable it by:
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# echo 1 > events/uprobes/enable
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Lets disable the event after sleeping for some time.
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# sleep 20
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# echo 0 > events/uprobes/enable
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And you can see the traced information via /sys/kernel/debug/tracing/trace.
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# cat trace
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# tracer: nop
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
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zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
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zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
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zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
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Each line shows us probes were triggered for a pid 24842 with ip being
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0x446421 and contents of ax register being 79.
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