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 -:[GRP/]EVENT : Clear a probe 34 35 GRP : Group name. If omitted, use "kprobes" for it. 36 EVENT : Event name. If omitted, the event name is generated 37 based on SYM+offs or MEMADDR. 38 MOD : Module name which has given SYM. 39 SYM[+offs] : Symbol+offset where the probe is inserted. 40 MEMADDR : Address where the probe is inserted. 41 MAXACTIVE : Maximum number of instances of the specified function that 42 can be probed simultaneously, or 0 for the default value 43 as defined in Documentation/kprobes.txt section 1.3.1. 44 45 FETCHARGS : Arguments. Each probe can have up to 128 args. 46 %REG : Fetch register REG 47 @ADDR : Fetch memory at ADDR (ADDR should be in kernel) 48 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) 49 $stackN : Fetch Nth entry of stack (N >= 0) 50 $stack : Fetch stack address. 51 $argN : Fetch the Nth function argument. (N >= 1) (\*1) 52 $retval : Fetch return value.(\*2) 53 $comm : Fetch current task comm. 54 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 55 \IMM : Store an immediate value to the argument. 56 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 57 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 58 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 59 (x8/x16/x32/x64), "string", "ustring" and bitfield 60 are supported. 61 62 (\*1) only for the probe on function entry (offs == 0). 63 (\*2) only for return probe. 64 (\*3) this is useful for fetching a field of data structures. 65 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 66 67Types 68----- 69Several types are supported for fetch-args. Kprobe tracer will access memory 70by given type. Prefix 's' and 'u' means those types are signed and unsigned 71respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 72in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 73or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 74x86-64 uses x64). 75These value types can be an array. To record array data, you can add '[N]' 76(where N is a fixed number, less than 64) to the base type. 77E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 78Note that the array can be applied to memory type fetchargs, you can not 79apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 80wrong, but '+8($stack):x8[8]' is OK.) 81String type is a special type, which fetches a "null-terminated" string from 82kernel space. This means it will fail and store NULL if the string container 83has been paged out. "ustring" type is an alternative of string for user-space. 84See :ref:`user_mem_access` for more info.. 85The string array type is a bit different from other types. For other base 86types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 87as +0(%di):x32.) But string[1] is not equal to string. The string type itself 88represents "char array", but string array type represents "char * array". 89So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 90Bitfield is another special type, which takes 3 parameters, bit-width, bit- 91offset, and container-size (usually 32). The syntax is:: 92 93 b<bit-width>@<bit-offset>/<container-size> 94 95Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 96which shows given pointer in "symbol+offset" style. 97For $comm, the default type is "string"; any other type is invalid. 98 99.. _user_mem_access: 100User Memory Access 101------------------ 102Kprobe events supports user-space memory access. For that purpose, you can use 103either user-space dereference syntax or 'ustring' type. 104 105The user-space dereference syntax allows you to access a field of a data 106structure in user-space. This is done by adding the "u" prefix to the 107dereference syntax. For example, +u4(%si) means it will read memory from the 108address in the register %si offset by 4, and the memory is expected to be in 109user-space. You can use this for strings too, e.g. +u0(%si):string will read 110a string from the address in the register %si that is expected to be in user- 111space. 'ustring' is a shortcut way of performing the same task. That is, 112+0(%si):ustring is equivalent to +u0(%si):string. 113 114Note that kprobe-event provides the user-memory access syntax but it doesn't 115use it transparently. This means if you use normal dereference or string type 116for user memory, it might fail, and may always fail on some archs. The user 117has to carefully check if the target data is in kernel or user space. 118 119Per-Probe Event Filtering 120------------------------- 121Per-probe event filtering feature allows you to set different filter on each 122probe and gives you what arguments will be shown in trace buffer. If an event 123name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 124under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 125'enable', 'format', 'filter' and 'trigger'. 126 127enable: 128 You can enable/disable the probe by writing 1 or 0 on it. 129 130format: 131 This shows the format of this probe event. 132 133filter: 134 You can write filtering rules of this event. 135 136id: 137 This shows the id of this probe event. 138 139trigger: 140 This allows to install trigger commands which are executed when the event is 141 hit (for details, see Documentation/trace/events.rst, section 6). 142 143Event Profiling 144--------------- 145You can check the total number of probe hits and probe miss-hits via 146/sys/kernel/debug/tracing/kprobe_profile. 147The first column is event name, the second is the number of probe hits, 148the third is the number of probe miss-hits. 149 150Kernel Boot Parameter 151--------------------- 152You can add and enable new kprobe events when booting up the kernel by 153"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 154kprobe events, which format is similar to the kprobe_events. 155The difference is that the probe definition parameters are comma-delimited 156instead of space. For example, adding myprobe event on do_sys_open like below 157 158 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 159 160should be below for kernel boot parameter (just replace spaces with comma) 161 162 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 163 164 165Usage examples 166-------------- 167To add a probe as a new event, write a new definition to kprobe_events 168as below:: 169 170 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 171 172This sets a kprobe on the top of do_sys_open() function with recording 1731st to 4th arguments as "myprobe" event. Note, which register/stack entry is 174assigned to each function argument depends on arch-specific ABI. If you unsure 175the ABI, please try to use probe subcommand of perf-tools (you can find it 176under tools/perf/). 177As this example shows, users can choose more familiar names for each arguments. 178:: 179 180 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 181 182This sets a kretprobe on the return point of do_sys_open() function with 183recording return value as "myretprobe" event. 184You can see the format of these events via 185/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 186:: 187 188 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 189 name: myprobe 190 ID: 780 191 format: 192 field:unsigned short common_type; offset:0; size:2; signed:0; 193 field:unsigned char common_flags; offset:2; size:1; signed:0; 194 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 195 field:int common_pid; offset:4; size:4; signed:1; 196 197 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 198 field:int __probe_nargs; offset:16; size:4; signed:1; 199 field:unsigned long dfd; offset:20; size:4; signed:0; 200 field:unsigned long filename; offset:24; size:4; signed:0; 201 field:unsigned long flags; offset:28; size:4; signed:0; 202 field:unsigned long mode; offset:32; size:4; signed:0; 203 204 205 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 206 REC->dfd, REC->filename, REC->flags, REC->mode 207 208You can see that the event has 4 arguments as in the expressions you specified. 209:: 210 211 echo > /sys/kernel/debug/tracing/kprobe_events 212 213This clears all probe points. 214 215Or, 216:: 217 218 echo -:myprobe >> kprobe_events 219 220This clears probe points selectively. 221 222Right after definition, each event is disabled by default. For tracing these 223events, you need to enable it. 224:: 225 226 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 227 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 228 229Use the following command to start tracing in an interval. 230:: 231 232 # echo 1 > tracing_on 233 Open something... 234 # echo 0 > tracing_on 235 236And you can see the traced information via /sys/kernel/debug/tracing/trace. 237:: 238 239 cat /sys/kernel/debug/tracing/trace 240 # tracer: nop 241 # 242 # TASK-PID CPU# TIMESTAMP FUNCTION 243 # | | | | | 244 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 245 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 246 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 247 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 248 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 249 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 250 251 252Each line shows when the kernel hits an event, and <- SYMBOL means kernel 253returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 254returns from do_sys_open to sys_open+0x1b). 255 256