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:OFFSET [FETCHARGS] : Set a uprobe 23 r[:[GRP/]EVENT] PATH:OFFSET [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 PATH+OFFSET. 29 PATH : Path to an executable or a library. 30 OFFSET : Offset where the probe is inserted. 31 32 FETCHARGS : Arguments. Each probe can have up to 128 args. 33 %REG : Fetch register REG 34 @ADDR : Fetch memory at ADDR (ADDR should be in userspace) 35 @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) 36 $stackN : Fetch Nth entry of stack (N >= 0) 37 $stack : Fetch stack address. 38 $retval : Fetch return value.(*) 39 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 40 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 41 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 42 (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield 43 are supported. 44 45 (*) only for return probe. 46 (**) this is useful for fetching a field of data structures. 47 48Types 49----- 50Several types are supported for fetch-args. Uprobe tracer will access memory 51by given type. Prefix 's' and 'u' means those types are signed and unsigned 52respectively. Traced arguments are shown in decimal (signed) or hex (unsigned). 53String type is a special type, which fetches a "null-terminated" string from 54user space. 55Bitfield is another special type, which takes 3 parameters, bit-width, bit- 56offset, and container-size (usually 32). The syntax is; 57 58 b<bit-width>@<bit-offset>/<container-size> 59 60 61Event Profiling 62--------------- 63You can check the total number of probe hits and probe miss-hits via 64/sys/kernel/debug/tracing/uprobe_profile. 65The first column is event name, the second is the number of probe hits, 66the third is the number of probe miss-hits. 67 68Usage examples 69-------------- 70 * Add a probe as a new uprobe event, write a new definition to uprobe_events 71as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash) 72 73 echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 74 75 * Add a probe as a new uretprobe event: 76 77 echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 78 79 * Unset registered event: 80 81 echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 82 83 * Print out the events that are registered: 84 85 cat /sys/kernel/debug/tracing/uprobe_events 86 87 * Clear all events: 88 89 echo > /sys/kernel/debug/tracing/uprobe_events 90 91Following example shows how to dump the instruction pointer and %ax register 92at the probed text address. Probe zfree function in /bin/zsh: 93 94 # cd /sys/kernel/debug/tracing/ 95 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 96 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 97 # objdump -T /bin/zsh | grep -w zfree 98 0000000000446420 g DF .text 0000000000000012 Base zfree 99 100 0x46420 is the offset of zfree in object /bin/zsh that is loaded at 101 0x00400000. Hence the command to uprobe would be: 102 103 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 104 105 And the same for the uretprobe would be: 106 107 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 108 109Please note: User has to explicitly calculate the offset of the probe-point 110in the object. We can see the events that are registered by looking at the 111uprobe_events file. 112 113 # cat uprobe_events 114 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 115 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 116 117Format of events can be seen by viewing the file events/uprobes/zfree_entry/format 118 119 # cat events/uprobes/zfree_entry/format 120 name: zfree_entry 121 ID: 922 122 format: 123 field:unsigned short common_type; offset:0; size:2; signed:0; 124 field:unsigned char common_flags; offset:2; size:1; signed:0; 125 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 126 field:int common_pid; offset:4; size:4; signed:1; 127 field:int common_padding; offset:8; size:4; signed:1; 128 129 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 130 field:u32 arg1; offset:16; size:4; signed:0; 131 field:u32 arg2; offset:20; size:4; signed:0; 132 133 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 134 135Right after definition, each event is disabled by default. For tracing these 136events, you need to enable it by: 137 138 # echo 1 > events/uprobes/enable 139 140Lets disable the event after sleeping for some time. 141 142 # sleep 20 143 # echo 0 > events/uprobes/enable 144 145And you can see the traced information via /sys/kernel/debug/tracing/trace. 146 147 # cat trace 148 # tracer: nop 149 # 150 # TASK-PID CPU# TIMESTAMP FUNCTION 151 # | | | | | 152 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 153 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 154 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 155 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 156 157Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 158and contents of ax register being 79. And uretprobe was triggered with ip at 1590x446540 with counterpart function entry at 0x446420. 160