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 73Function arguments at kretprobe 74------------------------------- 75Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This 76is useful to record the function parameter and return value at once, and 77trace the difference of structure fields (for debuging a function whether it 78correctly updates the given data structure or not). 79See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how 80it works. 81 82.. _kprobetrace_types: 83 84Types 85----- 86Several types are supported for fetchargs. Kprobe tracer will access memory 87by given type. Prefix 's' and 'u' means those types are signed and unsigned 88respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 89in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 90or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 91x86-64 uses x64). 92 93These value types can be an array. To record array data, you can add '[N]' 94(where N is a fixed number, less than 64) to the base type. 95E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 96Note that the array can be applied to memory type fetchargs, you can not 97apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 98wrong, but '+8($stack):x8[8]' is OK.) 99 100Char type can be used to show the character value of traced arguments. 101 102String type is a special type, which fetches a "null-terminated" string from 103kernel space. This means it will fail and store NULL if the string container 104has been paged out. "ustring" type is an alternative of string for user-space. 105See :ref:`user_mem_access` for more info. 106 107The string array type is a bit different from other types. For other base 108types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 109as +0(%di):x32.) But string[1] is not equal to string. The string type itself 110represents "char array", but string array type represents "char * array". 111So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 112Bitfield is another special type, which takes 3 parameters, bit-width, bit- 113offset, and container-size (usually 32). The syntax is:: 114 115 b<bit-width>@<bit-offset>/<container-size> 116 117Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 118which shows given pointer in "symbol+offset" style. 119On the other hand, symbol-string type ('symstr') converts the given address to 120"symbol+offset/symbolsize" style and stores it as a null-terminated string. 121With 'symstr' type, you can filter the event with wildcard pattern of the 122symbols, and you don't need to solve symbol name by yourself. 123For $comm, the default type is "string"; any other type is invalid. 124 125.. _user_mem_access: 126 127User Memory Access 128------------------ 129Kprobe events supports user-space memory access. For that purpose, you can use 130either user-space dereference syntax or 'ustring' type. 131 132The user-space dereference syntax allows you to access a field of a data 133structure in user-space. This is done by adding the "u" prefix to the 134dereference syntax. For example, +u4(%si) means it will read memory from the 135address in the register %si offset by 4, and the memory is expected to be in 136user-space. You can use this for strings too, e.g. +u0(%si):string will read 137a string from the address in the register %si that is expected to be in user- 138space. 'ustring' is a shortcut way of performing the same task. That is, 139+0(%si):ustring is equivalent to +u0(%si):string. 140 141Note that kprobe-event provides the user-memory access syntax but it doesn't 142use it transparently. This means if you use normal dereference or string type 143for user memory, it might fail, and may always fail on some architectures. The 144user has to carefully check if the target data is in kernel or user space. 145 146Per-Probe Event Filtering 147------------------------- 148Per-probe event filtering feature allows you to set different filter on each 149probe and gives you what arguments will be shown in trace buffer. If an event 150name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 151under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 152'enable', 'format', 'filter' and 'trigger'. 153 154enable: 155 You can enable/disable the probe by writing 1 or 0 on it. 156 157format: 158 This shows the format of this probe event. 159 160filter: 161 You can write filtering rules of this event. 162 163id: 164 This shows the id of this probe event. 165 166trigger: 167 This allows to install trigger commands which are executed when the event is 168 hit (for details, see Documentation/trace/events.rst, section 6). 169 170Event Profiling 171--------------- 172You can check the total number of probe hits and probe miss-hits via 173/sys/kernel/tracing/kprobe_profile. 174The first column is event name, the second is the number of probe hits, 175the third is the number of probe miss-hits. 176 177Kernel Boot Parameter 178--------------------- 179You can add and enable new kprobe events when booting up the kernel by 180"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 181kprobe events, which format is similar to the kprobe_events. 182The difference is that the probe definition parameters are comma-delimited 183instead of space. For example, adding myprobe event on do_sys_open like below:: 184 185 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 186 187should be below for kernel boot parameter (just replace spaces with comma):: 188 189 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 190 191 192Usage examples 193-------------- 194To add a probe as a new event, write a new definition to kprobe_events 195as below:: 196 197 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events 198 199This sets a kprobe on the top of do_sys_open() function with recording 2001st to 4th arguments as "myprobe" event. Note, which register/stack entry is 201assigned to each function argument depends on arch-specific ABI. If you unsure 202the ABI, please try to use probe subcommand of perf-tools (you can find it 203under tools/perf/). 204As this example shows, users can choose more familiar names for each arguments. 205:: 206 207 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events 208 209This sets a kretprobe on the return point of do_sys_open() function with 210recording return value as "myretprobe" event. 211You can see the format of these events via 212/sys/kernel/tracing/events/kprobes/<EVENT>/format. 213:: 214 215 cat /sys/kernel/tracing/events/kprobes/myprobe/format 216 name: myprobe 217 ID: 780 218 format: 219 field:unsigned short common_type; offset:0; size:2; signed:0; 220 field:unsigned char common_flags; offset:2; size:1; signed:0; 221 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 222 field:int common_pid; offset:4; size:4; signed:1; 223 224 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 225 field:int __probe_nargs; offset:16; size:4; signed:1; 226 field:unsigned long dfd; offset:20; size:4; signed:0; 227 field:unsigned long filename; offset:24; size:4; signed:0; 228 field:unsigned long flags; offset:28; size:4; signed:0; 229 field:unsigned long mode; offset:32; size:4; signed:0; 230 231 232 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 233 REC->dfd, REC->filename, REC->flags, REC->mode 234 235You can see that the event has 4 arguments as in the expressions you specified. 236:: 237 238 echo > /sys/kernel/tracing/kprobe_events 239 240This clears all probe points. 241 242Or, 243:: 244 245 echo -:myprobe >> kprobe_events 246 247This clears probe points selectively. 248 249Right after definition, each event is disabled by default. For tracing these 250events, you need to enable it. 251:: 252 253 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable 254 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable 255 256Use the following command to start tracing in an interval. 257:: 258 259 # echo 1 > tracing_on 260 Open something... 261 # echo 0 > tracing_on 262 263And you can see the traced information via /sys/kernel/tracing/trace. 264:: 265 266 cat /sys/kernel/tracing/trace 267 # tracer: nop 268 # 269 # TASK-PID CPU# TIMESTAMP FUNCTION 270 # | | | | | 271 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 272 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 273 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 274 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 275 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 276 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 277 278 279Each line shows when the kernel hits an event, and <- SYMBOL means kernel 280returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 281returns from do_sys_open to sys_open+0x1b). 282