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 $argN : Fetch the Nth function argument. (N >= 1) (\*1) 49 $retval : Fetch return value.(\*2) 50 $comm : Fetch current task comm. 51 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(\*3) 52 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 53 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 54 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 55 (x8/x16/x32/x64), "string" and bitfield are supported. 56 57 (\*1) only for the probe on function entry (offs == 0). 58 (\*2) only for return probe. 59 (\*3) this is useful for fetching a field of data structures. 60 61Types 62----- 63Several types are supported for fetch-args. Kprobe tracer will access memory 64by given type. Prefix 's' and 'u' means those types are signed and unsigned 65respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 66in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 67or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 68x86-64 uses x64). 69These value types can be an array. To record array data, you can add '[N]' 70(where N is a fixed number, less than 64) to the base type. 71E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 72Note that the array can be applied to memory type fetchargs, you can not 73apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 74wrong, but '+8($stack):x8[8]' is OK.) 75String type is a special type, which fetches a "null-terminated" string from 76kernel space. This means it will fail and store NULL if the string container 77has been paged out. 78The string array type is a bit different from other types. For other base 79types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 80as +0(%di):x32.) But string[1] is not equal to string. The string type itself 81represents "char array", but string array type represents "char * array". 82So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 83Bitfield is another special type, which takes 3 parameters, bit-width, bit- 84offset, and container-size (usually 32). The syntax is:: 85 86 b<bit-width>@<bit-offset>/<container-size> 87 88Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 89which shows given pointer in "symbol+offset" style. 90For $comm, the default type is "string"; any other type is invalid. 91 92 93Per-Probe Event Filtering 94------------------------- 95Per-probe event filtering feature allows you to set different filter on each 96probe and gives you what arguments will be shown in trace buffer. If an event 97name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 98under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 99'enable', 'format', 'filter' and 'trigger'. 100 101enable: 102 You can enable/disable the probe by writing 1 or 0 on it. 103 104format: 105 This shows the format of this probe event. 106 107filter: 108 You can write filtering rules of this event. 109 110id: 111 This shows the id of this probe event. 112 113trigger: 114 This allows to install trigger commands which are executed when the event is 115 hit (for details, see Documentation/trace/events.rst, section 6). 116 117Event Profiling 118--------------- 119You can check the total number of probe hits and probe miss-hits via 120/sys/kernel/debug/tracing/kprobe_profile. 121The first column is event name, the second is the number of probe hits, 122the third is the number of probe miss-hits. 123 124 125Usage examples 126-------------- 127To add a probe as a new event, write a new definition to kprobe_events 128as below:: 129 130 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 131 132This sets a kprobe on the top of do_sys_open() function with recording 1331st to 4th arguments as "myprobe" event. Note, which register/stack entry is 134assigned to each function argument depends on arch-specific ABI. If you unsure 135the ABI, please try to use probe subcommand of perf-tools (you can find it 136under tools/perf/). 137As this example shows, users can choose more familiar names for each arguments. 138:: 139 140 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 141 142This sets a kretprobe on the return point of do_sys_open() function with 143recording return value as "myretprobe" event. 144You can see the format of these events via 145/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 146:: 147 148 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 149 name: myprobe 150 ID: 780 151 format: 152 field:unsigned short common_type; offset:0; size:2; signed:0; 153 field:unsigned char common_flags; offset:2; size:1; signed:0; 154 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 155 field:int common_pid; offset:4; size:4; signed:1; 156 157 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 158 field:int __probe_nargs; offset:16; size:4; signed:1; 159 field:unsigned long dfd; offset:20; size:4; signed:0; 160 field:unsigned long filename; offset:24; size:4; signed:0; 161 field:unsigned long flags; offset:28; size:4; signed:0; 162 field:unsigned long mode; offset:32; size:4; signed:0; 163 164 165 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 166 REC->dfd, REC->filename, REC->flags, REC->mode 167 168You can see that the event has 4 arguments as in the expressions you specified. 169:: 170 171 echo > /sys/kernel/debug/tracing/kprobe_events 172 173This clears all probe points. 174 175Or, 176:: 177 178 echo -:myprobe >> kprobe_events 179 180This clears probe points selectively. 181 182Right after definition, each event is disabled by default. For tracing these 183events, you need to enable it. 184:: 185 186 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 187 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 188 189And you can see the traced information via /sys/kernel/debug/tracing/trace. 190:: 191 192 cat /sys/kernel/debug/tracing/trace 193 # tracer: nop 194 # 195 # TASK-PID CPU# TIMESTAMP FUNCTION 196 # | | | | | 197 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 198 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 199 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 200 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 201 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 202 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 203 204 205Each line shows when the kernel hits an event, and <- SYMBOL means kernel 206returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 207returns from do_sys_open to sys_open+0x1b). 208 209