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 $comm : Fetch current task comm. 54 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 55 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 56 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 57 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 58 (x8/x16/x32/x64), "string", "ustring" and bitfield 59 are supported. 60 61 (\*1) only for the probe on function entry (offs == 0). 62 (\*2) only for return probe. 63 (\*3) this is useful for fetching a field of data structures. 64 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 65 66Types 67----- 68Several types are supported for fetch-args. Kprobe tracer will access memory 69by given type. Prefix 's' and 'u' means those types are signed and unsigned 70respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 71in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 72or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 73x86-64 uses x64). 74These value types can be an array. To record array data, you can add '[N]' 75(where N is a fixed number, less than 64) to the base type. 76E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 77Note that the array can be applied to memory type fetchargs, you can not 78apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 79wrong, but '+8($stack):x8[8]' is OK.) 80String type is a special type, which fetches a "null-terminated" string from 81kernel space. This means it will fail and store NULL if the string container 82has been paged out. "ustring" type is an alternative of string for user-space. 83See :ref:`user_mem_access` for more info.. 84The string array type is a bit different from other types. For other base 85types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 86as +0(%di):x32.) But string[1] is not equal to string. The string type itself 87represents "char array", but string array type represents "char * array". 88So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 89Bitfield is another special type, which takes 3 parameters, bit-width, bit- 90offset, and container-size (usually 32). The syntax is:: 91 92 b<bit-width>@<bit-offset>/<container-size> 93 94Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 95which shows given pointer in "symbol+offset" style. 96For $comm, the default type is "string"; any other type is invalid. 97 98.. _user_mem_access: 99User Memory Access 100------------------ 101Kprobe events supports user-space memory access. For that purpose, you can use 102either user-space dereference syntax or 'ustring' type. 103 104The user-space dereference syntax allows you to access a field of a data 105structure in user-space. This is done by adding the "u" prefix to the 106dereference syntax. For example, +u4(%si) means it will read memory from the 107address in the register %si offset by 4, and the memory is expected to be in 108user-space. You can use this for strings too, e.g. +u0(%si):string will read 109a string from the address in the register %si that is expected to be in user- 110space. 'ustring' is a shortcut way of performing the same task. That is, 111+0(%si):ustring is equivalent to +u0(%si):string. 112 113Note that kprobe-event provides the user-memory access syntax but it doesn't 114use it transparently. This means if you use normal dereference or string type 115for user memory, it might fail, and may always fail on some archs. The user 116has to carefully check if the target data is in kernel or user space. 117 118Per-Probe Event Filtering 119------------------------- 120Per-probe event filtering feature allows you to set different filter on each 121probe and gives you what arguments will be shown in trace buffer. If an event 122name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 123under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 124'enable', 'format', 'filter' and 'trigger'. 125 126enable: 127 You can enable/disable the probe by writing 1 or 0 on it. 128 129format: 130 This shows the format of this probe event. 131 132filter: 133 You can write filtering rules of this event. 134 135id: 136 This shows the id of this probe event. 137 138trigger: 139 This allows to install trigger commands which are executed when the event is 140 hit (for details, see Documentation/trace/events.rst, section 6). 141 142Event Profiling 143--------------- 144You can check the total number of probe hits and probe miss-hits via 145/sys/kernel/debug/tracing/kprobe_profile. 146The first column is event name, the second is the number of probe hits, 147the third is the number of probe miss-hits. 148 149Kernel Boot Parameter 150--------------------- 151You can add and enable new kprobe events when booting up the kernel by 152"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 153kprobe events, which format is similar to the kprobe_events. 154The difference is that the probe definition parameters are comma-delimited 155instead of space. For example, adding myprobe event on do_sys_open like below 156 157 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 158 159should be below for kernel boot parameter (just replace spaces with comma) 160 161 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 162 163 164Usage examples 165-------------- 166To add a probe as a new event, write a new definition to kprobe_events 167as below:: 168 169 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 170 171This sets a kprobe on the top of do_sys_open() function with recording 1721st to 4th arguments as "myprobe" event. Note, which register/stack entry is 173assigned to each function argument depends on arch-specific ABI. If you unsure 174the ABI, please try to use probe subcommand of perf-tools (you can find it 175under tools/perf/). 176As this example shows, users can choose more familiar names for each arguments. 177:: 178 179 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 180 181This sets a kretprobe on the return point of do_sys_open() function with 182recording return value as "myretprobe" event. 183You can see the format of these events via 184/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 185:: 186 187 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 188 name: myprobe 189 ID: 780 190 format: 191 field:unsigned short common_type; offset:0; size:2; signed:0; 192 field:unsigned char common_flags; offset:2; size:1; signed:0; 193 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 194 field:int common_pid; offset:4; size:4; signed:1; 195 196 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 197 field:int __probe_nargs; offset:16; size:4; signed:1; 198 field:unsigned long dfd; offset:20; size:4; signed:0; 199 field:unsigned long filename; offset:24; size:4; signed:0; 200 field:unsigned long flags; offset:28; size:4; signed:0; 201 field:unsigned long mode; offset:32; size:4; signed:0; 202 203 204 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 205 REC->dfd, REC->filename, REC->flags, REC->mode 206 207You can see that the event has 4 arguments as in the expressions you specified. 208:: 209 210 echo > /sys/kernel/debug/tracing/kprobe_events 211 212This clears all probe points. 213 214Or, 215:: 216 217 echo -:myprobe >> kprobe_events 218 219This clears probe points selectively. 220 221Right after definition, each event is disabled by default. For tracing these 222events, you need to enable it. 223:: 224 225 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 226 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 227 228Use the following command to start tracing in an interval. 229:: 230 231 # echo 1 > tracing_on 232 Open something... 233 # echo 0 > tracing_on 234 235And you can see the traced information via /sys/kernel/debug/tracing/trace. 236:: 237 238 cat /sys/kernel/debug/tracing/trace 239 # tracer: nop 240 # 241 # TASK-PID CPU# TIMESTAMP FUNCTION 242 # | | | | | 243 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 244 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 245 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 246 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 247 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 248 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 249 250 251Each line shows when the kernel hits an event, and <- SYMBOL means kernel 252returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 253returns from do_sys_open to sys_open+0x1b). 254 255