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 $comm : Fetch current task comm. 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 57 \IMM : Store an immediate value to the argument. 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 61 (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr" 62 and bitfield are supported. 63 64 (\*1) only for the probe on function entry (offs == 0). Note, this argument access 65 is best effort, because depending on the argument type, it may be passed on 66 the stack. But this only support the arguments via registers. 67 (\*2) only for return probe. Note that this is also best effort. Depending on the 68 return value type, it might be passed via a pair of registers. But this only 69 accesses one register. 70 (\*3) this is useful for fetching a field of data structures. 71 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 72 73.. _kprobetrace_types: 74 75Types 76----- 77Several types are supported for fetchargs. Kprobe tracer will access memory 78by given type. Prefix 's' and 'u' means those types are signed and unsigned 79respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 80in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 81or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 82x86-64 uses x64). 83 84These value types can be an array. To record array data, you can add '[N]' 85(where N is a fixed number, less than 64) to the base type. 86E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 87Note that the array can be applied to memory type fetchargs, you can not 88apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 89wrong, but '+8($stack):x8[8]' is OK.) 90 91Char type can be used to show the character value of traced arguments. 92 93String type is a special type, which fetches a "null-terminated" string from 94kernel space. This means it will fail and store NULL if the string container 95has been paged out. "ustring" type is an alternative of string for user-space. 96See :ref:`user_mem_access` for more info. 97 98The string array type is a bit different from other types. For other base 99types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 100as +0(%di):x32.) But string[1] is not equal to string. The string type itself 101represents "char array", but string array type represents "char * array". 102So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 103Bitfield is another special type, which takes 3 parameters, bit-width, bit- 104offset, and container-size (usually 32). The syntax is:: 105 106 b<bit-width>@<bit-offset>/<container-size> 107 108Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 109which shows given pointer in "symbol+offset" style. 110On the other hand, symbol-string type ('symstr') converts the given address to 111"symbol+offset/symbolsize" style and stores it as a null-terminated string. 112With 'symstr' type, you can filter the event with wildcard pattern of the 113symbols, and you don't need to solve symbol name by yourself. 114For $comm, the default type is "string"; any other type is invalid. 115 116.. _user_mem_access: 117 118User Memory Access 119------------------ 120Kprobe events supports user-space memory access. For that purpose, you can use 121either user-space dereference syntax or 'ustring' type. 122 123The user-space dereference syntax allows you to access a field of a data 124structure in user-space. This is done by adding the "u" prefix to the 125dereference syntax. For example, +u4(%si) means it will read memory from the 126address in the register %si offset by 4, and the memory is expected to be in 127user-space. You can use this for strings too, e.g. +u0(%si):string will read 128a string from the address in the register %si that is expected to be in user- 129space. 'ustring' is a shortcut way of performing the same task. That is, 130+0(%si):ustring is equivalent to +u0(%si):string. 131 132Note that kprobe-event provides the user-memory access syntax but it doesn't 133use it transparently. This means if you use normal dereference or string type 134for user memory, it might fail, and may always fail on some architectures. The 135user has to carefully check if the target data is in kernel or user space. 136 137Per-Probe Event Filtering 138------------------------- 139Per-probe event filtering feature allows you to set different filter on each 140probe and gives you what arguments will be shown in trace buffer. If an event 141name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 142under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 143'enable', 'format', 'filter' and 'trigger'. 144 145enable: 146 You can enable/disable the probe by writing 1 or 0 on it. 147 148format: 149 This shows the format of this probe event. 150 151filter: 152 You can write filtering rules of this event. 153 154id: 155 This shows the id of this probe event. 156 157trigger: 158 This allows to install trigger commands which are executed when the event is 159 hit (for details, see Documentation/trace/events.rst, section 6). 160 161Event Profiling 162--------------- 163You can check the total number of probe hits and probe miss-hits via 164/sys/kernel/tracing/kprobe_profile. 165The first column is event name, the second is the number of probe hits, 166the third is the number of probe miss-hits. 167 168Kernel Boot Parameter 169--------------------- 170You can add and enable new kprobe events when booting up the kernel by 171"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 172kprobe events, which format is similar to the kprobe_events. 173The difference is that the probe definition parameters are comma-delimited 174instead of space. For example, adding myprobe event on do_sys_open like below:: 175 176 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 177 178should be below for kernel boot parameter (just replace spaces with comma):: 179 180 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 181 182 183Usage examples 184-------------- 185To add a probe as a new event, write a new definition to kprobe_events 186as below:: 187 188 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events 189 190This sets a kprobe on the top of do_sys_open() function with recording 1911st to 4th arguments as "myprobe" event. Note, which register/stack entry is 192assigned to each function argument depends on arch-specific ABI. If you unsure 193the ABI, please try to use probe subcommand of perf-tools (you can find it 194under tools/perf/). 195As this example shows, users can choose more familiar names for each arguments. 196:: 197 198 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events 199 200This sets a kretprobe on the return point of do_sys_open() function with 201recording return value as "myretprobe" event. 202You can see the format of these events via 203/sys/kernel/tracing/events/kprobes/<EVENT>/format. 204:: 205 206 cat /sys/kernel/tracing/events/kprobes/myprobe/format 207 name: myprobe 208 ID: 780 209 format: 210 field:unsigned short common_type; offset:0; size:2; signed:0; 211 field:unsigned char common_flags; offset:2; size:1; signed:0; 212 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 213 field:int common_pid; offset:4; size:4; signed:1; 214 215 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 216 field:int __probe_nargs; offset:16; size:4; signed:1; 217 field:unsigned long dfd; offset:20; size:4; signed:0; 218 field:unsigned long filename; offset:24; size:4; signed:0; 219 field:unsigned long flags; offset:28; size:4; signed:0; 220 field:unsigned long mode; offset:32; size:4; signed:0; 221 222 223 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 224 REC->dfd, REC->filename, REC->flags, REC->mode 225 226You can see that the event has 4 arguments as in the expressions you specified. 227:: 228 229 echo > /sys/kernel/tracing/kprobe_events 230 231This clears all probe points. 232 233Or, 234:: 235 236 echo -:myprobe >> kprobe_events 237 238This clears probe points selectively. 239 240Right after definition, each event is disabled by default. For tracing these 241events, you need to enable it. 242:: 243 244 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable 245 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable 246 247Use the following command to start tracing in an interval. 248:: 249 250 # echo 1 > tracing_on 251 Open something... 252 # echo 0 > tracing_on 253 254And you can see the traced information via /sys/kernel/tracing/trace. 255:: 256 257 cat /sys/kernel/tracing/trace 258 # tracer: nop 259 # 260 # TASK-PID CPU# TIMESTAMP FUNCTION 261 # | | | | | 262 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 263 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 264 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 265 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 266 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 267 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 268 269 270Each line shows when the kernel hits an event, and <- SYMBOL means kernel 271returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 272returns from do_sys_open to sys_open+0x1b). 273