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 -:[GRP/]EVENT : Clear uprobe or uretprobe event 32 33 GRP : Group name. If omitted, "uprobes" is the default value. 34 EVENT : Event name. If omitted, the event name is generated based 35 on PATH+OFFSET. 36 PATH : Path to an executable or a library. 37 OFFSET : Offset where the probe is inserted. 38 39 FETCHARGS : Arguments. Each probe can have up to 128 args. 40 %REG : Fetch register REG 41 @ADDR : Fetch memory at ADDR (ADDR should be in userspace) 42 @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) 43 $stackN : Fetch Nth entry of stack (N >= 0) 44 $stack : Fetch stack address. 45 $retval : Fetch return value.(\*1) 46 $comm : Fetch current task comm. 47 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3) 48 \IMM : Store an immediate value to the argument. 49 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 50 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 51 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 52 (x8/x16/x32/x64), "string" and bitfield are supported. 53 54 (\*1) only for return probe. 55 (\*2) this is useful for fetching a field of data structures. 56 (\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe 57 events can access only user-space memory. 58 59Types 60----- 61Several types are supported for fetch-args. Uprobe tracer will access memory 62by given type. Prefix 's' and 'u' means those types are signed and unsigned 63respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 64in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 65or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 66x86-64 uses x64). 67String type is a special type, which fetches a "null-terminated" string from 68user space. 69Bitfield is another special type, which takes 3 parameters, bit-width, bit- 70offset, and container-size (usually 32). The syntax is:: 71 72 b<bit-width>@<bit-offset>/<container-size> 73 74For $comm, the default type is "string"; any other type is invalid. 75 76 77Event Profiling 78--------------- 79You can check the total number of probe hits per event via 80/sys/kernel/debug/tracing/uprobe_profile. The first column is the filename, 81the second is the event name, the third is the number of probe hits. 82 83Usage examples 84-------------- 85 * Add a probe as a new uprobe event, write a new definition to uprobe_events 86 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: 87 88 echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 89 90 * Add a probe as a new uretprobe event:: 91 92 echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 93 94 * Unset registered event:: 95 96 echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 97 98 * Print out the events that are registered:: 99 100 cat /sys/kernel/debug/tracing/uprobe_events 101 102 * Clear all events:: 103 104 echo > /sys/kernel/debug/tracing/uprobe_events 105 106Following example shows how to dump the instruction pointer and %ax register 107at the probed text address. Probe zfree function in /bin/zsh:: 108 109 # cd /sys/kernel/debug/tracing/ 110 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 111 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 112 # objdump -T /bin/zsh | grep -w zfree 113 0000000000446420 g DF .text 0000000000000012 Base zfree 114 1150x46420 is the offset of zfree in object /bin/zsh that is loaded at 1160x00400000. Hence the command to uprobe would be:: 117 118 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 119 120And the same for the uretprobe would be:: 121 122 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 123 124.. note:: User has to explicitly calculate the offset of the probe-point 125 in the object. 126 127We can see the events that are registered by looking at the uprobe_events file. 128:: 129 130 # cat uprobe_events 131 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 132 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 133 134Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. 135:: 136 137 # cat events/uprobes/zfree_entry/format 138 name: zfree_entry 139 ID: 922 140 format: 141 field:unsigned short common_type; offset:0; size:2; signed:0; 142 field:unsigned char common_flags; offset:2; size:1; signed:0; 143 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 144 field:int common_pid; offset:4; size:4; signed:1; 145 field:int common_padding; offset:8; size:4; signed:1; 146 147 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 148 field:u32 arg1; offset:16; size:4; signed:0; 149 field:u32 arg2; offset:20; size:4; signed:0; 150 151 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 152 153Right after definition, each event is disabled by default. For tracing these 154events, you need to enable it by:: 155 156 # echo 1 > events/uprobes/enable 157 158Lets start tracing, sleep for some time and stop tracing. 159:: 160 161 # echo 1 > tracing_on 162 # sleep 20 163 # echo 0 > tracing_on 164 165Also, you can disable the event by:: 166 167 # echo 0 > events/uprobes/enable 168 169And you can see the traced information via /sys/kernel/debug/tracing/trace. 170:: 171 172 # cat trace 173 # tracer: nop 174 # 175 # TASK-PID CPU# TIMESTAMP FUNCTION 176 # | | | | | 177 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 178 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 179 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 180 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 181 182Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 183and contents of ax register being 79. And uretprobe was triggered with ip at 1840x446540 with counterpart function entry at 0x446420. 185