1========================== 2Kprobe-based Event Tracing 3========================== 4 5:Author: Masami Hiramatsu 6 7Overview 8-------- 9These events are similar to tracepoint based events. Instead of Tracepoint, 10this is based on kprobes (kprobe and kretprobe). So it can probe wherever 11kprobes can probe (this means, all functions except those with 12__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL). 13Unlike the Tracepoint based event, this can be added and removed 14dynamically, on the fly. 15 16To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y. 17 18Similar to the events tracer, this doesn't need to be activated via 19current_tracer. Instead of that, add probe points via 20/sys/kernel/debug/tracing/kprobe_events, and enable it via 21/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enable. 22 23 24Synopsis of kprobe_events 25------------------------- 26:: 27 28 p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe 29 r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe 30 -:[GRP/]EVENT : Clear a probe 31 32 GRP : Group name. If omitted, use "kprobes" for it. 33 EVENT : Event name. If omitted, the event name is generated 34 based on SYM+offs or MEMADDR. 35 MOD : Module name which has given SYM. 36 SYM[+offs] : Symbol+offset where the probe is inserted. 37 MEMADDR : Address where the probe is inserted. 38 MAXACTIVE : Maximum number of instances of the specified function that 39 can be probed simultaneously, or 0 for the default value 40 as defined in Documentation/kprobes.txt section 1.3.1. 41 42 FETCHARGS : Arguments. Each probe can have up to 128 args. 43 %REG : Fetch register REG 44 @ADDR : Fetch memory at ADDR (ADDR should be in kernel) 45 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) 46 $stackN : Fetch Nth entry of stack (N >= 0) 47 $stack : Fetch stack address. 48 $retval : Fetch return value.(*) 49 $comm : Fetch current task comm. 50 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 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 (*) only for return probe. 57 (**) this is useful for fetching a field of data structures. 58 59Types 60----- 61Several types are supported for fetch-args. Kprobe 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 68kernel space. This means it will fail and store NULL if the string container 69has been paged out. 70Bitfield is another special type, which takes 3 parameters, bit-width, bit- 71offset, and container-size (usually 32). The syntax is:: 72 73 b<bit-width>@<bit-offset>/<container-size> 74 75For $comm, the default type is "string"; any other type is invalid. 76 77 78Per-Probe Event Filtering 79------------------------- 80Per-probe event filtering feature allows you to set different filter on each 81probe and gives you what arguments will be shown in trace buffer. If an event 82name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 83under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 84'enable', 'format', 'filter' and 'trigger'. 85 86enable: 87 You can enable/disable the probe by writing 1 or 0 on it. 88 89format: 90 This shows the format of this probe event. 91 92filter: 93 You can write filtering rules of this event. 94 95id: 96 This shows the id of this probe event. 97 98trigger: 99 This allows to install trigger commands which are executed when the event is 100 hit (for details, see Documentation/trace/events.rst, section 6). 101 102Event Profiling 103--------------- 104You can check the total number of probe hits and probe miss-hits via 105/sys/kernel/debug/tracing/kprobe_profile. 106The first column is event name, the second is the number of probe hits, 107the third is the number of probe miss-hits. 108 109 110Usage examples 111-------------- 112To add a probe as a new event, write a new definition to kprobe_events 113as below:: 114 115 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 116 117This sets a kprobe on the top of do_sys_open() function with recording 1181st to 4th arguments as "myprobe" event. Note, which register/stack entry is 119assigned to each function argument depends on arch-specific ABI. If you unsure 120the ABI, please try to use probe subcommand of perf-tools (you can find it 121under tools/perf/). 122As this example shows, users can choose more familiar names for each arguments. 123:: 124 125 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 126 127This sets a kretprobe on the return point of do_sys_open() function with 128recording return value as "myretprobe" event. 129You can see the format of these events via 130/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 131:: 132 133 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 134 name: myprobe 135 ID: 780 136 format: 137 field:unsigned short common_type; offset:0; size:2; signed:0; 138 field:unsigned char common_flags; offset:2; size:1; signed:0; 139 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 140 field:int common_pid; offset:4; size:4; signed:1; 141 142 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 143 field:int __probe_nargs; offset:16; size:4; signed:1; 144 field:unsigned long dfd; offset:20; size:4; signed:0; 145 field:unsigned long filename; offset:24; size:4; signed:0; 146 field:unsigned long flags; offset:28; size:4; signed:0; 147 field:unsigned long mode; offset:32; size:4; signed:0; 148 149 150 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 151 REC->dfd, REC->filename, REC->flags, REC->mode 152 153You can see that the event has 4 arguments as in the expressions you specified. 154:: 155 156 echo > /sys/kernel/debug/tracing/kprobe_events 157 158This clears all probe points. 159 160Or, 161:: 162 163 echo -:myprobe >> kprobe_events 164 165This clears probe points selectively. 166 167Right after definition, each event is disabled by default. For tracing these 168events, you need to enable it. 169:: 170 171 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 172 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 173 174And you can see the traced information via /sys/kernel/debug/tracing/trace. 175:: 176 177 cat /sys/kernel/debug/tracing/trace 178 # tracer: nop 179 # 180 # TASK-PID CPU# TIMESTAMP FUNCTION 181 # | | | | | 182 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 183 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 184 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 185 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 186 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 187 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 188 189 190Each line shows when the kernel hits an event, and <- SYMBOL means kernel 191returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 192returns from do_sys_open to sys_open+0x1b). 193 194