1 Uprobe-tracer: Uprobe-based Event Tracing 2 ========================================= 3 4 Documentation written by Srikar Dronamraju 5 6 7Overview 8-------- 9Uprobe based trace events are similar to kprobe based trace events. 10To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y. 11 12Similar to the kprobe-event tracer, this doesn't need to be activated via 13current_tracer. Instead of that, add probe points via 14/sys/kernel/debug/tracing/uprobe_events, and enable it via 15/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled. 16 17However unlike kprobe-event tracer, the uprobe event interface expects the 18user to calculate the offset of the probepoint in the object. 19 20Synopsis of uprobe_tracer 21------------------------- 22 p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a uprobe 23 r[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a return uprobe (uretprobe) 24 -:[GRP/]EVENT : Clear uprobe or uretprobe event 25 26 GRP : Group name. If omitted, "uprobes" is the default value. 27 EVENT : Event name. If omitted, the event name is generated based 28 on SYMBOL+offs. 29 PATH : Path to an executable or a library. 30 SYMBOL[+offs] : Symbol+offset where the probe is inserted. 31 32 FETCHARGS : Arguments. Each probe can have up to 128 args. 33 %REG : Fetch register REG 34 35Event Profiling 36--------------- 37You can check the total number of probe hits and probe miss-hits via 38/sys/kernel/debug/tracing/uprobe_profile. 39The first column is event name, the second is the number of probe hits, 40the third is the number of probe miss-hits. 41 42Usage examples 43-------------- 44 * Add a probe as a new uprobe event, write a new definition to uprobe_events 45as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash) 46 47 echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 48 49 * Add a probe as a new uretprobe event: 50 51 echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 52 53 * Unset registered event: 54 55 echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 56 57 * Print out the events that are registered: 58 59 cat /sys/kernel/debug/tracing/uprobe_events 60 61 * Clear all events: 62 63 echo > /sys/kernel/debug/tracing/uprobe_events 64 65Following example shows how to dump the instruction pointer and %ax register 66at the probed text address. Probe zfree function in /bin/zsh: 67 68 # cd /sys/kernel/debug/tracing/ 69 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 70 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 71 # objdump -T /bin/zsh | grep -w zfree 72 0000000000446420 g DF .text 0000000000000012 Base zfree 73 74 0x46420 is the offset of zfree in object /bin/zsh that is loaded at 75 0x00400000. Hence the command to uprobe would be: 76 77 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 78 79 And the same for the uretprobe would be: 80 81 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 82 83Please note: User has to explicitly calculate the offset of the probe-point 84in the object. We can see the events that are registered by looking at the 85uprobe_events file. 86 87 # cat uprobe_events 88 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 89 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 90 91Format of events can be seen by viewing the file events/uprobes/zfree_entry/format 92 93 # cat events/uprobes/zfree_entry/format 94 name: zfree_entry 95 ID: 922 96 format: 97 field:unsigned short common_type; offset:0; size:2; signed:0; 98 field:unsigned char common_flags; offset:2; size:1; signed:0; 99 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 100 field:int common_pid; offset:4; size:4; signed:1; 101 field:int common_padding; offset:8; size:4; signed:1; 102 103 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 104 field:u32 arg1; offset:16; size:4; signed:0; 105 field:u32 arg2; offset:20; size:4; signed:0; 106 107 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 108 109Right after definition, each event is disabled by default. For tracing these 110events, you need to enable it by: 111 112 # echo 1 > events/uprobes/enable 113 114Lets disable the event after sleeping for some time. 115 116 # sleep 20 117 # echo 0 > events/uprobes/enable 118 119And you can see the traced information via /sys/kernel/debug/tracing/trace. 120 121 # cat trace 122 # tracer: nop 123 # 124 # TASK-PID CPU# TIMESTAMP FUNCTION 125 # | | | | | 126 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 127 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 128 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 129 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 130 131Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 132and contents of ax register being 79. And uretprobe was triggered with ip at 1330x446540 with counterpart function entry at 0x446420. 134