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>/enable. 17 18However unlike kprobe-event tracer, the uprobe event interface expects the 19user to calculate the offset of the probepoint in the object. 20 21You can also use /sys/kernel/debug/tracing/dynamic_events instead of 22uprobe_events. That interface will provide unified access to other 23dynamic events too. 24 25Synopsis of uprobe_tracer 26------------------------- 27:: 28 29 p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe 30 r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe) 31 p[:[GRP/]EVENT] PATH:OFFSET%return [FETCHARGS] : Set a return uprobe (uretprobe) 32 -:[GRP/]EVENT : Clear uprobe or uretprobe event 33 34 GRP : Group name. If omitted, "uprobes" is the default value. 35 EVENT : Event name. If omitted, the event name is generated based 36 on PATH+OFFSET. 37 PATH : Path to an executable or a library. 38 OFFSET : Offset where the probe is inserted. 39 OFFSET%return : Offset where the return probe is inserted. 40 41 FETCHARGS : Arguments. Each probe can have up to 128 args. 42 %REG : Fetch register REG 43 @ADDR : Fetch memory at ADDR (ADDR should be in userspace) 44 @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) 45 $stackN : Fetch Nth entry of stack (N >= 0) 46 $stack : Fetch stack address. 47 $retval : Fetch return value.(\*1) 48 $comm : Fetch current task comm. 49 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3) 50 \IMM : Store an immediate value to the argument. 51 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 52 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 53 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 54 (x8/x16/x32/x64), "string" and bitfield are supported. 55 56 (\*1) only for return probe. 57 (\*2) this is useful for fetching a field of data structures. 58 (\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe 59 events can access only user-space memory. 60 61Types 62----- 63Several types are supported for fetch-args. Uprobe tracer will access memory 64by given type. Prefix 's' and 'u' means those types are signed and unsigned 65respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 66in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 67or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 68x86-64 uses x64). 69String type is a special type, which fetches a "null-terminated" string from 70user space. 71Bitfield is another special type, which takes 3 parameters, bit-width, bit- 72offset, and container-size (usually 32). The syntax is:: 73 74 b<bit-width>@<bit-offset>/<container-size> 75 76For $comm, the default type is "string"; any other type is invalid. 77 78 79Event Profiling 80--------------- 81You can check the total number of probe hits per event via 82/sys/kernel/debug/tracing/uprobe_profile. The first column is the filename, 83the second is the event name, the third is the number of probe hits. 84 85Usage examples 86-------------- 87 * Add a probe as a new uprobe event, write a new definition to uprobe_events 88 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: 89 90 echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 91 92 * Add a probe as a new uretprobe event:: 93 94 echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 95 96 * Unset registered event:: 97 98 echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 99 100 * Print out the events that are registered:: 101 102 cat /sys/kernel/debug/tracing/uprobe_events 103 104 * Clear all events:: 105 106 echo > /sys/kernel/debug/tracing/uprobe_events 107 108Following example shows how to dump the instruction pointer and %ax register 109at the probed text address. Probe zfree function in /bin/zsh:: 110 111 # cd /sys/kernel/debug/tracing/ 112 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 113 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 114 # objdump -T /bin/zsh | grep -w zfree 115 0000000000446420 g DF .text 0000000000000012 Base zfree 116 1170x46420 is the offset of zfree in object /bin/zsh that is loaded at 1180x00400000. Hence the command to uprobe would be:: 119 120 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 121 122And the same for the uretprobe would be:: 123 124 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 125 126.. note:: User has to explicitly calculate the offset of the probe-point 127 in the object. 128 129We can see the events that are registered by looking at the uprobe_events file. 130:: 131 132 # cat uprobe_events 133 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 134 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 135 136Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. 137:: 138 139 # cat events/uprobes/zfree_entry/format 140 name: zfree_entry 141 ID: 922 142 format: 143 field:unsigned short common_type; offset:0; size:2; signed:0; 144 field:unsigned char common_flags; offset:2; size:1; signed:0; 145 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 146 field:int common_pid; offset:4; size:4; signed:1; 147 field:int common_padding; offset:8; size:4; signed:1; 148 149 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 150 field:u32 arg1; offset:16; size:4; signed:0; 151 field:u32 arg2; offset:20; size:4; signed:0; 152 153 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 154 155Right after definition, each event is disabled by default. For tracing these 156events, you need to enable it by:: 157 158 # echo 1 > events/uprobes/enable 159 160Lets start tracing, sleep for some time and stop tracing. 161:: 162 163 # echo 1 > tracing_on 164 # sleep 20 165 # echo 0 > tracing_on 166 167Also, you can disable the event by:: 168 169 # echo 0 > events/uprobes/enable 170 171And you can see the traced information via /sys/kernel/debug/tracing/trace. 172:: 173 174 # cat trace 175 # tracer: nop 176 # 177 # TASK-PID CPU# TIMESTAMP FUNCTION 178 # | | | | | 179 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 180 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 181 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 182 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 183 184Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 185and contents of ax register being 79. And uretprobe was triggered with ip at 1860x446540 with counterpart function entry at 0x446420. 187