1========================== 2Kprobe-based Event Tracing 3========================== 4 5:Author: Masami Hiramatsu 6 7Overview 8-------- 9These events are similar to tracepoint-based events. Instead of tracepoints, 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 event tracer, this doesn't need to be activated via 19current_tracer. Instead of that, add probe points via 20/sys/kernel/tracing/kprobe_events, and enable it via 21/sys/kernel/tracing/events/kprobes/<EVENT>/enable. 22 23You can also use /sys/kernel/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 $comm : Fetch current task comm. 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 57 \IMM : Store an immediate value to the argument. 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 61 (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr" 62 and bitfield are supported. 63 64 (\*1) only for the probe on function entry (offs == 0). 65 (\*2) only for return probe. 66 (\*3) this is useful for fetching a field of data structures. 67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 68 69.. _kprobetrace_types: 70 71Types 72----- 73Several types are supported for fetchargs. Kprobe tracer will access memory 74by given type. Prefix 's' and 'u' means those types are signed and unsigned 75respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 76in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 77or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 78x86-64 uses x64). 79 80These value types can be an array. To record array data, you can add '[N]' 81(where N is a fixed number, less than 64) to the base type. 82E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 83Note that the array can be applied to memory type fetchargs, you can not 84apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 85wrong, but '+8($stack):x8[8]' is OK.) 86 87Char type can be used to show the character value of traced arguments. 88 89String type is a special type, which fetches a "null-terminated" string from 90kernel space. This means it will fail and store NULL if the string container 91has been paged out. "ustring" type is an alternative of string for user-space. 92See :ref:`user_mem_access` for more info. 93 94The string array type is a bit different from other types. For other base 95types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 96as +0(%di):x32.) But string[1] is not equal to string. The string type itself 97represents "char array", but string array type represents "char * array". 98So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 99Bitfield is another special type, which takes 3 parameters, bit-width, bit- 100offset, and container-size (usually 32). The syntax is:: 101 102 b<bit-width>@<bit-offset>/<container-size> 103 104Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 105which shows given pointer in "symbol+offset" style. 106On the other hand, symbol-string type ('symstr') converts the given address to 107"symbol+offset/symbolsize" style and stores it as a null-terminated string. 108With 'symstr' type, you can filter the event with wildcard pattern of the 109symbols, and you don't need to solve symbol name by yourself. 110For $comm, the default type is "string"; any other type is invalid. 111 112.. _user_mem_access: 113 114User Memory Access 115------------------ 116Kprobe events supports user-space memory access. For that purpose, you can use 117either user-space dereference syntax or 'ustring' type. 118 119The user-space dereference syntax allows you to access a field of a data 120structure in user-space. This is done by adding the "u" prefix to the 121dereference syntax. For example, +u4(%si) means it will read memory from the 122address in the register %si offset by 4, and the memory is expected to be in 123user-space. You can use this for strings too, e.g. +u0(%si):string will read 124a string from the address in the register %si that is expected to be in user- 125space. 'ustring' is a shortcut way of performing the same task. That is, 126+0(%si):ustring is equivalent to +u0(%si):string. 127 128Note that kprobe-event provides the user-memory access syntax but it doesn't 129use it transparently. This means if you use normal dereference or string type 130for user memory, it might fail, and may always fail on some architectures. The 131user has to carefully check if the target data is in kernel or user space. 132 133Per-Probe Event Filtering 134------------------------- 135Per-probe event filtering feature allows you to set different filter on each 136probe and gives you what arguments will be shown in trace buffer. If an event 137name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 138under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 139'enable', 'format', 'filter' and 'trigger'. 140 141enable: 142 You can enable/disable the probe by writing 1 or 0 on it. 143 144format: 145 This shows the format of this probe event. 146 147filter: 148 You can write filtering rules of this event. 149 150id: 151 This shows the id of this probe event. 152 153trigger: 154 This allows to install trigger commands which are executed when the event is 155 hit (for details, see Documentation/trace/events.rst, section 6). 156 157Event Profiling 158--------------- 159You can check the total number of probe hits and probe miss-hits via 160/sys/kernel/tracing/kprobe_profile. 161The first column is event name, the second is the number of probe hits, 162the third is the number of probe miss-hits. 163 164Kernel Boot Parameter 165--------------------- 166You can add and enable new kprobe events when booting up the kernel by 167"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 168kprobe events, which format is similar to the kprobe_events. 169The difference is that the probe definition parameters are comma-delimited 170instead of space. For example, adding myprobe event on do_sys_open like below:: 171 172 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 173 174should be below for kernel boot parameter (just replace spaces with comma):: 175 176 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 177 178 179Usage examples 180-------------- 181To add a probe as a new event, write a new definition to kprobe_events 182as below:: 183 184 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events 185 186This sets a kprobe on the top of do_sys_open() function with recording 1871st to 4th arguments as "myprobe" event. Note, which register/stack entry is 188assigned to each function argument depends on arch-specific ABI. If you unsure 189the ABI, please try to use probe subcommand of perf-tools (you can find it 190under tools/perf/). 191As this example shows, users can choose more familiar names for each arguments. 192:: 193 194 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events 195 196This sets a kretprobe on the return point of do_sys_open() function with 197recording return value as "myretprobe" event. 198You can see the format of these events via 199/sys/kernel/tracing/events/kprobes/<EVENT>/format. 200:: 201 202 cat /sys/kernel/tracing/events/kprobes/myprobe/format 203 name: myprobe 204 ID: 780 205 format: 206 field:unsigned short common_type; offset:0; size:2; signed:0; 207 field:unsigned char common_flags; offset:2; size:1; signed:0; 208 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 209 field:int common_pid; offset:4; size:4; signed:1; 210 211 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 212 field:int __probe_nargs; offset:16; size:4; signed:1; 213 field:unsigned long dfd; offset:20; size:4; signed:0; 214 field:unsigned long filename; offset:24; size:4; signed:0; 215 field:unsigned long flags; offset:28; size:4; signed:0; 216 field:unsigned long mode; offset:32; size:4; signed:0; 217 218 219 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 220 REC->dfd, REC->filename, REC->flags, REC->mode 221 222You can see that the event has 4 arguments as in the expressions you specified. 223:: 224 225 echo > /sys/kernel/tracing/kprobe_events 226 227This clears all probe points. 228 229Or, 230:: 231 232 echo -:myprobe >> kprobe_events 233 234This clears probe points selectively. 235 236Right after definition, each event is disabled by default. For tracing these 237events, you need to enable it. 238:: 239 240 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable 241 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable 242 243Use the following command to start tracing in an interval. 244:: 245 246 # echo 1 > tracing_on 247 Open something... 248 # echo 0 > tracing_on 249 250And you can see the traced information via /sys/kernel/tracing/trace. 251:: 252 253 cat /sys/kernel/tracing/trace 254 # tracer: nop 255 # 256 # TASK-PID CPU# TIMESTAMP FUNCTION 257 # | | | | | 258 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 259 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 260 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 261 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 262 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 263 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 264 265 266Each line shows when the kernel hits an event, and <- SYMBOL means kernel 267returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 268returns from do_sys_open to sys_open+0x1b). 269