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.(*) 46 $comm : Fetch current task comm. 47 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 48 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 49 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 50 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 51 (x8/x16/x32/x64), "string" and bitfield are supported. 52 53 (*) only for return probe. 54 (**) this is useful for fetching a field of data structures. 55 56Types 57----- 58Several types are supported for fetch-args. Uprobe tracer will access memory 59by given type. Prefix 's' and 'u' means those types are signed and unsigned 60respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 61in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 62or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 63x86-64 uses x64). 64String type is a special type, which fetches a "null-terminated" string from 65user space. 66Bitfield is another special type, which takes 3 parameters, bit-width, bit- 67offset, and container-size (usually 32). The syntax is:: 68 69 b<bit-width>@<bit-offset>/<container-size> 70 71For $comm, the default type is "string"; any other type is invalid. 72 73 74Event Profiling 75--------------- 76You can check the total number of probe hits per event via 77/sys/kernel/debug/tracing/uprobe_profile. The first column is the filename, 78the second is the event name, the third is the number of probe hits. 79 80Usage examples 81-------------- 82 * Add a probe as a new uprobe event, write a new definition to uprobe_events 83 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: 84 85 echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 86 87 * Add a probe as a new uretprobe event:: 88 89 echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 90 91 * Unset registered event:: 92 93 echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 94 95 * Print out the events that are registered:: 96 97 cat /sys/kernel/debug/tracing/uprobe_events 98 99 * Clear all events:: 100 101 echo > /sys/kernel/debug/tracing/uprobe_events 102 103Following example shows how to dump the instruction pointer and %ax register 104at the probed text address. Probe zfree function in /bin/zsh:: 105 106 # cd /sys/kernel/debug/tracing/ 107 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 108 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 109 # objdump -T /bin/zsh | grep -w zfree 110 0000000000446420 g DF .text 0000000000000012 Base zfree 111 1120x46420 is the offset of zfree in object /bin/zsh that is loaded at 1130x00400000. Hence the command to uprobe would be:: 114 115 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 116 117And the same for the uretprobe would be:: 118 119 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 120 121.. note:: User has to explicitly calculate the offset of the probe-point 122 in the object. 123 124We can see the events that are registered by looking at the uprobe_events file. 125:: 126 127 # cat uprobe_events 128 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 129 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 130 131Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. 132:: 133 134 # cat events/uprobes/zfree_entry/format 135 name: zfree_entry 136 ID: 922 137 format: 138 field:unsigned short common_type; offset:0; size:2; signed:0; 139 field:unsigned char common_flags; offset:2; size:1; signed:0; 140 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 141 field:int common_pid; offset:4; size:4; signed:1; 142 field:int common_padding; offset:8; size:4; signed:1; 143 144 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 145 field:u32 arg1; offset:16; size:4; signed:0; 146 field:u32 arg2; offset:20; size:4; signed:0; 147 148 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 149 150Right after definition, each event is disabled by default. For tracing these 151events, you need to enable it by:: 152 153 # echo 1 > events/uprobes/enable 154 155Lets disable the event after sleeping for some time. 156:: 157 158 # sleep 20 159 # echo 0 > events/uprobes/enable 160 161And you can see the traced information via /sys/kernel/debug/tracing/trace. 162:: 163 164 # cat trace 165 # tracer: nop 166 # 167 # TASK-PID CPU# TIMESTAMP FUNCTION 168 # | | | | | 169 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 170 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 171 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 172 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 173 174Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 175and contents of ax register being 79. And uretprobe was triggered with ip at 1760x446540 with counterpart function entry at 0x446420. 177