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 23You can also use /sys/kernel/debug/tracing/dynamic_events instead of 24kprobe_events. That interface will provide unified access to other 25dynamic events too. 26 27Synopsis of kprobe_events 28------------------------- 29:: 30 31 p[:[GRP/][EVENT]] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe 32 r[MAXACTIVE][:[GRP/][EVENT]] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe 33 p[:[GRP/][EVENT]] [MOD:]SYM[+0]%return [FETCHARGS] : Set a return probe 34 -:[GRP/][EVENT] : Clear a probe 35 36 GRP : Group name. If omitted, use "kprobes" for it. 37 EVENT : Event name. If omitted, the event name is generated 38 based on SYM+offs or MEMADDR. 39 MOD : Module name which has given SYM. 40 SYM[+offs] : Symbol+offset where the probe is inserted. 41 SYM%return : Return address of the symbol 42 MEMADDR : Address where the probe is inserted. 43 MAXACTIVE : Maximum number of instances of the specified function that 44 can be probed simultaneously, or 0 for the default value 45 as defined in Documentation/trace/kprobes.rst section 1.3.1. 46 47 FETCHARGS : Arguments. Each probe can have up to 128 args. 48 %REG : Fetch register REG 49 @ADDR : Fetch memory at ADDR (ADDR should be in kernel) 50 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) 51 $stackN : Fetch Nth entry of stack (N >= 0) 52 $stack : Fetch stack address. 53 $argN : Fetch the Nth function argument. (N >= 1) (\*1) 54 $retval : Fetch return value.(\*2) 55 $comm : Fetch current task comm. 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 57 \IMM : Store an immediate value to the argument. 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 61 (x8/x16/x32/x64), "string", "ustring" and bitfield 62 are supported. 63 64 (\*1) only for the probe on function entry (offs == 0). 65 (\*2) only for return probe. 66 (\*3) this is useful for fetching a field of data structures. 67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 68 69Types 70----- 71Several types are supported for fetch-args. Kprobe tracer will access memory 72by given type. Prefix 's' and 'u' means those types are signed and unsigned 73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 76x86-64 uses x64). 77These value types can be an array. To record array data, you can add '[N]' 78(where N is a fixed number, less than 64) to the base type. 79E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 80Note that the array can be applied to memory type fetchargs, you can not 81apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 82wrong, but '+8($stack):x8[8]' is OK.) 83String type is a special type, which fetches a "null-terminated" string from 84kernel space. This means it will fail and store NULL if the string container 85has been paged out. "ustring" type is an alternative of string for user-space. 86See :ref:`user_mem_access` for more info.. 87The string array type is a bit different from other types. For other base 88types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 89as +0(%di):x32.) But string[1] is not equal to string. The string type itself 90represents "char array", but string array type represents "char * array". 91So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 92Bitfield is another special type, which takes 3 parameters, bit-width, bit- 93offset, and container-size (usually 32). The syntax is:: 94 95 b<bit-width>@<bit-offset>/<container-size> 96 97Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 98which shows given pointer in "symbol+offset" style. 99For $comm, the default type is "string"; any other type is invalid. 100 101.. _user_mem_access: 102 103User Memory Access 104------------------ 105Kprobe events supports user-space memory access. For that purpose, you can use 106either user-space dereference syntax or 'ustring' type. 107 108The user-space dereference syntax allows you to access a field of a data 109structure in user-space. This is done by adding the "u" prefix to the 110dereference syntax. For example, +u4(%si) means it will read memory from the 111address in the register %si offset by 4, and the memory is expected to be in 112user-space. You can use this for strings too, e.g. +u0(%si):string will read 113a string from the address in the register %si that is expected to be in user- 114space. 'ustring' is a shortcut way of performing the same task. That is, 115+0(%si):ustring is equivalent to +u0(%si):string. 116 117Note that kprobe-event provides the user-memory access syntax but it doesn't 118use it transparently. This means if you use normal dereference or string type 119for user memory, it might fail, and may always fail on some archs. The user 120has to carefully check if the target data is in kernel or user space. 121 122Per-Probe Event Filtering 123------------------------- 124Per-probe event filtering feature allows you to set different filter on each 125probe and gives you what arguments will be shown in trace buffer. If an event 126name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 127under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 128'enable', 'format', 'filter' and 'trigger'. 129 130enable: 131 You can enable/disable the probe by writing 1 or 0 on it. 132 133format: 134 This shows the format of this probe event. 135 136filter: 137 You can write filtering rules of this event. 138 139id: 140 This shows the id of this probe event. 141 142trigger: 143 This allows to install trigger commands which are executed when the event is 144 hit (for details, see Documentation/trace/events.rst, section 6). 145 146Event Profiling 147--------------- 148You can check the total number of probe hits and probe miss-hits via 149/sys/kernel/debug/tracing/kprobe_profile. 150The first column is event name, the second is the number of probe hits, 151the third is the number of probe miss-hits. 152 153Kernel Boot Parameter 154--------------------- 155You can add and enable new kprobe events when booting up the kernel by 156"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 157kprobe events, which format is similar to the kprobe_events. 158The difference is that the probe definition parameters are comma-delimited 159instead of space. For example, adding myprobe event on do_sys_open like below 160 161 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 162 163should be below for kernel boot parameter (just replace spaces with comma) 164 165 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 166 167 168Usage examples 169-------------- 170To add a probe as a new event, write a new definition to kprobe_events 171as below:: 172 173 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 174 175This sets a kprobe on the top of do_sys_open() function with recording 1761st to 4th arguments as "myprobe" event. Note, which register/stack entry is 177assigned to each function argument depends on arch-specific ABI. If you unsure 178the ABI, please try to use probe subcommand of perf-tools (you can find it 179under tools/perf/). 180As this example shows, users can choose more familiar names for each arguments. 181:: 182 183 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 184 185This sets a kretprobe on the return point of do_sys_open() function with 186recording return value as "myretprobe" event. 187You can see the format of these events via 188/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 189:: 190 191 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 192 name: myprobe 193 ID: 780 194 format: 195 field:unsigned short common_type; offset:0; size:2; signed:0; 196 field:unsigned char common_flags; offset:2; size:1; signed:0; 197 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 198 field:int common_pid; offset:4; size:4; signed:1; 199 200 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 201 field:int __probe_nargs; offset:16; size:4; signed:1; 202 field:unsigned long dfd; offset:20; size:4; signed:0; 203 field:unsigned long filename; offset:24; size:4; signed:0; 204 field:unsigned long flags; offset:28; size:4; signed:0; 205 field:unsigned long mode; offset:32; size:4; signed:0; 206 207 208 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 209 REC->dfd, REC->filename, REC->flags, REC->mode 210 211You can see that the event has 4 arguments as in the expressions you specified. 212:: 213 214 echo > /sys/kernel/debug/tracing/kprobe_events 215 216This clears all probe points. 217 218Or, 219:: 220 221 echo -:myprobe >> kprobe_events 222 223This clears probe points selectively. 224 225Right after definition, each event is disabled by default. For tracing these 226events, you need to enable it. 227:: 228 229 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 230 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 231 232Use the following command to start tracing in an interval. 233:: 234 235 # echo 1 > tracing_on 236 Open something... 237 # echo 0 > tracing_on 238 239And you can see the traced information via /sys/kernel/debug/tracing/trace. 240:: 241 242 cat /sys/kernel/debug/tracing/trace 243 # tracer: nop 244 # 245 # TASK-PID CPU# TIMESTAMP FUNCTION 246 # | | | | | 247 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 248 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 249 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 250 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 251 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 252 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 253 254 255Each line shows when the kernel hits an event, and <- SYMBOL means kernel 256returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 257returns from do_sys_open to sys_open+0x1b). 258