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1=========================================2Uprobe-tracer: Uprobe-based Event Tracing3=========================================4 5:Author: Srikar Dronamraju6 7 8Overview9--------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 via14current_tracer. Instead of that, add probe points via15/sys/kernel/tracing/uprobe_events, and enable it via16/sys/kernel/tracing/events/uprobes/<EVENT>/enable.17 18However unlike kprobe-event tracer, the uprobe event interface expects the19user to calculate the offset of the probepoint in the object.20 21You can also use /sys/kernel/tracing/dynamic_events instead of22uprobe_events. That interface will provide unified access to other23dynamic events too.24 25Synopsis of uprobe_tracer26-------------------------27::28 29 p[:[GRP/][EVENT]] PATH:OFFSET [FETCHARGS] : Set a uprobe30 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 event33 34 GRP : Group name. If omitted, "uprobes" is the default value.35 EVENT : Event name. If omitted, the event name is generated based36 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 REG43 @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 types53 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types54 (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, because uprobe59 events can access only user-space memory.60 61Types62-----63Several types are supported for fetch-args. Uprobe tracer will access memory64by given type. Prefix 's' and 'u' means those types are signed and unsigned65respectively. 'x' prefix implies it is unsigned. Traced arguments are shown66in 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, and68x86-64 uses x64).69String type is a special type, which fetches a "null-terminated" string from70user 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 Profiling80---------------81You can check the total number of probe hits per event via82/sys/kernel/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 examples86--------------87 * Add a probe as a new uprobe event, write a new definition to uprobe_events88 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)::89 90 echo 'p /bin/bash:0x4245c0' > /sys/kernel/tracing/uprobe_events91 92 * Add a probe as a new uretprobe event::93 94 echo 'r /bin/bash:0x4245c0' > /sys/kernel/tracing/uprobe_events95 96 * Unset registered event::97 98 echo '-:p_bash_0x4245c0' >> /sys/kernel/tracing/uprobe_events99 100 * Print out the events that are registered::101 102 cat /sys/kernel/tracing/uprobe_events103 104 * Clear all events::105 106 echo > /sys/kernel/tracing/uprobe_events107 108Following example shows how to dump the instruction pointer and %ax register109at the probed text address. Probe zfree function in /bin/zsh::110 111 # cd /sys/kernel/tracing/112 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp113 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh114 # objdump -T /bin/zsh | grep -w zfree115 0000000000446420 g DF .text 0000000000000012 Base zfree116 1170x46420 is the offset of zfree in object /bin/zsh that is loaded at1180x00400000. Hence the command to uprobe would be::119 120 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events121 122And the same for the uretprobe would be::123 124 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events125 126.. note:: User has to explicitly calculate the offset of the probe-point127 in the object.128 129We can see the events that are registered by looking at the uprobe_events file.130::131 132 # cat uprobe_events133 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax134 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax135 136Format of events can be seen by viewing the file events/uprobes/zfree_entry/format.137::138 139 # cat events/uprobes/zfree_entry/format140 name: zfree_entry141 ID: 922142 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->arg2154 155Right after definition, each event is disabled by default. For tracing these156events, you need to enable it by::157 158 # echo 1 > events/uprobes/enable159 160Lets start tracing, sleep for some time and stop tracing.161::162 163 # echo 1 > tracing_on164 # sleep 20165 # echo 0 > tracing_on166 167Also, you can disable the event by::168 169 # echo 0 > events/uprobes/enable170 171And you can see the traced information via /sys/kernel/tracing/trace.172::173 174 # cat trace175 # tracer: nop176 #177 # TASK-PID CPU# TIMESTAMP FUNCTION178 # | | | | |179 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79180 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0181 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79182 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0183 184Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420185and contents of ax register being 79. And uretprobe was triggered with ip at1860x446540 with counterpart function entry at 0x446420.187