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 $comm : Fetch current task comm. 40 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 41 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 42 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 43 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 44 (x8/x16/x32/x64), "string" and bitfield are supported. 45 46 (*) only for return probe. 47 (**) this is useful for fetching a field of data structures. 48 49Types 50----- 51Several types are supported for fetch-args. Uprobe tracer will access memory 52by given type. Prefix 's' and 'u' means those types are signed and unsigned 53respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 54in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 55or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 56x86-64 uses x64). 57String type is a special type, which fetches a "null-terminated" string from 58user space. 59Bitfield is another special type, which takes 3 parameters, bit-width, bit- 60offset, and container-size (usually 32). The syntax is; 61 62 b<bit-width>@<bit-offset>/<container-size> 63 64For $comm, the default type is "string"; any other type is invalid. 65 66 67Event Profiling 68--------------- 69You can check the total number of probe hits and probe miss-hits via 70/sys/kernel/debug/tracing/uprobe_profile. 71The first column is event name, the second is the number of probe hits, 72the third is the number of probe miss-hits. 73 74Usage examples 75-------------- 76 * Add a probe as a new uprobe event, write a new definition to uprobe_events 77as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash) 78 79 echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 80 81 * Add a probe as a new uretprobe event: 82 83 echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 84 85 * Unset registered event: 86 87 echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events 88 89 * Print out the events that are registered: 90 91 cat /sys/kernel/debug/tracing/uprobe_events 92 93 * Clear all events: 94 95 echo > /sys/kernel/debug/tracing/uprobe_events 96 97Following example shows how to dump the instruction pointer and %ax register 98at the probed text address. Probe zfree function in /bin/zsh: 99 100 # cd /sys/kernel/debug/tracing/ 101 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp 102 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh 103 # objdump -T /bin/zsh | grep -w zfree 104 0000000000446420 g DF .text 0000000000000012 Base zfree 105 106 0x46420 is the offset of zfree in object /bin/zsh that is loaded at 107 0x00400000. Hence the command to uprobe would be: 108 109 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events 110 111 And the same for the uretprobe would be: 112 113 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events 114 115Please note: User has to explicitly calculate the offset of the probe-point 116in the object. We can see the events that are registered by looking at the 117uprobe_events file. 118 119 # cat uprobe_events 120 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax 121 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax 122 123Format of events can be seen by viewing the file events/uprobes/zfree_entry/format 124 125 # cat events/uprobes/zfree_entry/format 126 name: zfree_entry 127 ID: 922 128 format: 129 field:unsigned short common_type; offset:0; size:2; signed:0; 130 field:unsigned char common_flags; offset:2; size:1; signed:0; 131 field:unsigned char common_preempt_count; offset:3; size:1; signed:0; 132 field:int common_pid; offset:4; size:4; signed:1; 133 field:int common_padding; offset:8; size:4; signed:1; 134 135 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 136 field:u32 arg1; offset:16; size:4; signed:0; 137 field:u32 arg2; offset:20; size:4; signed:0; 138 139 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 140 141Right after definition, each event is disabled by default. For tracing these 142events, you need to enable it by: 143 144 # echo 1 > events/uprobes/enable 145 146Lets disable the event after sleeping for some time. 147 148 # sleep 20 149 # echo 0 > events/uprobes/enable 150 151And you can see the traced information via /sys/kernel/debug/tracing/trace. 152 153 # cat trace 154 # tracer: nop 155 # 156 # TASK-PID CPU# TIMESTAMP FUNCTION 157 # | | | | | 158 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 159 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 160 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 161 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 162 163Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 164and contents of ax register being 79. And uretprobe was triggered with ip at 1650x446540 with counterpart function entry at 0x446420. 166