1========================================= 2Uprobe-tracer: Uprobe-based Event Tracing 3========================================= 4 5:Author: Srikar Dronamraju 6 7 8Overview 9-------- 10Uprobe based trace events are similar to kprobe based trace events. 11To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y. 12 13Similar to the kprobe-event tracer, this doesn't need to be activated via 14current_tracer. Instead of that, add probe points via 15/sys/kernel/debug/tracing/uprobe_events, and enable it via 16/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled. 17 18However unlike kprobe-event tracer, the uprobe event interface expects the 19user to calculate the offset of the probepoint in the object. 20 21Synopsis of uprobe_tracer 22------------------------- 23:: 24 25 p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe 26 r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe) 27 -:[GRP/]EVENT : Clear uprobe or uretprobe event 28 29 GRP : Group name. If omitted, "uprobes" is the default value. 30 EVENT : Event name. If omitted, the event name is generated based 31 on PATH+OFFSET. 32 PATH : Path to an executable or a library. 33 OFFSET : Offset where the probe is inserted. 34 35 FETCHARGS : Arguments. Each probe can have up to 128 args. 36 %REG : Fetch register REG 37 @ADDR : Fetch memory at ADDR (ADDR should be in userspace) 38 @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) 39 $stackN : Fetch Nth entry of stack (N >= 0) 40 $stack : Fetch stack address. 41 $retval : Fetch return value.(*) 42 $comm : Fetch current task comm. 43 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 44 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 45 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 46 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 47 (x8/x16/x32/x64), "string" and bitfield are supported. 48 49 (*) only for return probe. 50 (**) this is useful for fetching a field of data structures. 51 52Types 53----- 54Several types are supported for fetch-args. Uprobe tracer will access memory 55by given type. Prefix 's' and 'u' means those types are signed and unsigned 56respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 57in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 58or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 59x86-64 uses x64). 60String type is a special type, which fetches a "null-terminated" string from 61user space. 62Bitfield is another special type, which takes 3 parameters, bit-width, bit- 63offset, and container-size (usually 32). The syntax is:: 64 65 b<bit-width>@<bit-offset>/<container-size> 66 67For $comm, the default type is "string"; any other type is invalid. 68 69 70Event Profiling 71--------------- 72You can check the total number of probe hits and probe miss-hits via 73/sys/kernel/debug/tracing/uprobe_profile. 74The first column is event name, the second is the number of probe hits, 75the third is the number of probe miss-hits. 76 77Usage examples 78-------------- 79 * Add a probe as a new uprobe event, write a new definition to uprobe_events 80 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: 81 82 echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 83 84 * Add a probe as a new uretprobe event:: 85 86 echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 87 88 * Unset registered event:: 89 90 echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 91 92 * Print out the events that are registered:: 93 94 cat /sys/kernel/debug/tracing/uprobe_events 95 96 * Clear all events:: 97 98 echo > /sys/kernel/debug/tracing/uprobe_events 99 100Following example shows how to dump the instruction pointer and %ax register 101at the probed text address. Probe zfree function in /bin/zsh:: 102 103 # cd /sys/kernel/debug/tracing/ 104 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 105 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 106 # objdump -T /bin/zsh | grep -w zfree 107 0000000000446420 g DF .text 0000000000000012 Base zfree 108 1090x46420 is the offset of zfree in object /bin/zsh that is loaded at 1100x00400000. Hence the command to uprobe would be:: 111 112 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 113 114And the same for the uretprobe would be:: 115 116 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 117 118.. note:: User has to explicitly calculate the offset of the probe-point 119 in the object. 120 121We can see the events that are registered by looking at the uprobe_events file. 122:: 123 124 # cat uprobe_events 125 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 126 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 127 128Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. 129:: 130 131 # cat events/uprobes/zfree_entry/format 132 name: zfree_entry 133 ID: 922 134 format: 135 field:unsigned short common_type; offset:0; size:2; signed:0; 136 field:unsigned char common_flags; offset:2; size:1; signed:0; 137 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 138 field:int common_pid; offset:4; size:4; signed:1; 139 field:int common_padding; offset:8; size:4; signed:1; 140 141 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 142 field:u32 arg1; offset:16; size:4; signed:0; 143 field:u32 arg2; offset:20; size:4; signed:0; 144 145 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 146 147Right after definition, each event is disabled by default. For tracing these 148events, you need to enable it by:: 149 150 # echo 1 > events/uprobes/enable 151 152Lets disable the event after sleeping for some time. 153:: 154 155 # sleep 20 156 # echo 0 > events/uprobes/enable 157 158And you can see the traced information via /sys/kernel/debug/tracing/trace. 159:: 160 161 # cat trace 162 # tracer: nop 163 # 164 # TASK-PID CPU# TIMESTAMP FUNCTION 165 # | | | | | 166 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 167 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 168 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 169 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 170 171Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 172and contents of ax register being 79. And uretprobe was triggered with ip at 1730x446540 with counterpart function entry at 0x446420. 174