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), VFS layer common type(%pd/%pD), "char", 62 "string", "ustring", "symbol", "symstr" and bitfield are 63 supported. 64 65 (\*1) only for the probe on function entry (offs == 0). Note, this argument access 66 is best effort, because depending on the argument type, it may be passed on 67 the stack. But this only support the arguments via registers. 68 (\*2) only for return probe. Note that this is also best effort. Depending on the 69 return value type, it might be passed via a pair of registers. But this only 70 accesses one register. 71 (\*3) this is useful for fetching a field of data structures. 72 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 73 74Function arguments at kretprobe 75------------------------------- 76Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This 77is useful to record the function parameter and return value at once, and 78trace the difference of structure fields (for debugging a function whether it 79correctly updates the given data structure or not). 80See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how 81it works. 82 83.. _kprobetrace_types: 84 85Types 86----- 87Several types are supported for fetchargs. Kprobe tracer will access memory 88by given type. Prefix 's' and 'u' means those types are signed and unsigned 89respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 90in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 91or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 92x86-64 uses x64). 93 94These value types can be an array. To record array data, you can add '[N]' 95(where N is a fixed number, less than 64) to the base type. 96E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 97Note that the array can be applied to memory type fetchargs, you can not 98apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 99wrong, but '+8($stack):x8[8]' is OK.) 100 101Char type can be used to show the character value of traced arguments. 102 103String type is a special type, which fetches a "null-terminated" string from 104kernel space. This means it will fail and store NULL if the string container 105has been paged out. "ustring" type is an alternative of string for user-space. 106See :ref:`user_mem_access` for more info. 107 108The string array type is a bit different from other types. For other base 109types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 110as +0(%di):x32.) But string[1] is not equal to string. The string type itself 111represents "char array", but string array type represents "char * array". 112So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 113Bitfield is another special type, which takes 3 parameters, bit-width, bit- 114offset, and container-size (usually 32). The syntax is:: 115 116 b<bit-width>@<bit-offset>/<container-size> 117 118Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 119which shows given pointer in "symbol+offset" style. 120On the other hand, symbol-string type ('symstr') converts the given address to 121"symbol+offset/symbolsize" style and stores it as a null-terminated string. 122With 'symstr' type, you can filter the event with wildcard pattern of the 123symbols, and you don't need to solve symbol name by yourself. 124For $comm, the default type is "string"; any other type is invalid. 125 126VFS layer common type(%pd/%pD) is a special type, which fetches dentry's or 127file's name from struct dentry's address or struct file's address. 128 129.. _user_mem_access: 130 131User Memory Access 132------------------ 133Kprobe events supports user-space memory access. For that purpose, you can use 134either user-space dereference syntax or 'ustring' type. 135 136The user-space dereference syntax allows you to access a field of a data 137structure in user-space. This is done by adding the "u" prefix to the 138dereference syntax. For example, +u4(%si) means it will read memory from the 139address in the register %si offset by 4, and the memory is expected to be in 140user-space. You can use this for strings too, e.g. +u0(%si):string will read 141a string from the address in the register %si that is expected to be in user- 142space. 'ustring' is a shortcut way of performing the same task. That is, 143+0(%si):ustring is equivalent to +u0(%si):string. 144 145Note that kprobe-event provides the user-memory access syntax but it doesn't 146use it transparently. This means if you use normal dereference or string type 147for user memory, it might fail, and may always fail on some architectures. The 148user has to carefully check if the target data is in kernel or user space. 149 150Per-Probe Event Filtering 151------------------------- 152Per-probe event filtering feature allows you to set different filter on each 153probe and gives you what arguments will be shown in trace buffer. If an event 154name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 155under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 156'enable', 'format', 'filter' and 'trigger'. 157 158enable: 159 You can enable/disable the probe by writing 1 or 0 on it. 160 161format: 162 This shows the format of this probe event. 163 164filter: 165 You can write filtering rules of this event. 166 167id: 168 This shows the id of this probe event. 169 170trigger: 171 This allows to install trigger commands which are executed when the event is 172 hit (for details, see Documentation/trace/events.rst, section 6). 173 174Event Profiling 175--------------- 176You can check the total number of probe hits and probe miss-hits via 177/sys/kernel/tracing/kprobe_profile. 178The first column is event name, the second is the number of probe hits, 179the third is the number of probe miss-hits. 180 181Kernel Boot Parameter 182--------------------- 183You can add and enable new kprobe events when booting up the kernel by 184"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 185kprobe events, which format is similar to the kprobe_events. 186The difference is that the probe definition parameters are comma-delimited 187instead of space. For example, adding myprobe event on do_sys_open like below:: 188 189 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 190 191should be below for kernel boot parameter (just replace spaces with comma):: 192 193 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 194 195 196Usage examples 197-------------- 198To add a probe as a new event, write a new definition to kprobe_events 199as below:: 200 201 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events 202 203This sets a kprobe on the top of do_sys_open() function with recording 2041st to 4th arguments as "myprobe" event. Note, which register/stack entry is 205assigned to each function argument depends on arch-specific ABI. If you unsure 206the ABI, please try to use probe subcommand of perf-tools (you can find it 207under tools/perf/). 208As this example shows, users can choose more familiar names for each arguments. 209:: 210 211 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events 212 213This sets a kretprobe on the return point of do_sys_open() function with 214recording return value as "myretprobe" event. 215You can see the format of these events via 216/sys/kernel/tracing/events/kprobes/<EVENT>/format. 217:: 218 219 cat /sys/kernel/tracing/events/kprobes/myprobe/format 220 name: myprobe 221 ID: 780 222 format: 223 field:unsigned short common_type; offset:0; size:2; signed:0; 224 field:unsigned char common_flags; offset:2; size:1; signed:0; 225 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 226 field:int common_pid; offset:4; size:4; signed:1; 227 228 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 229 field:int __probe_nargs; offset:16; size:4; signed:1; 230 field:unsigned long dfd; offset:20; size:4; signed:0; 231 field:unsigned long filename; offset:24; size:4; signed:0; 232 field:unsigned long flags; offset:28; size:4; signed:0; 233 field:unsigned long mode; offset:32; size:4; signed:0; 234 235 236 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 237 REC->dfd, REC->filename, REC->flags, REC->mode 238 239You can see that the event has 4 arguments as in the expressions you specified. 240:: 241 242 echo > /sys/kernel/tracing/kprobe_events 243 244This clears all probe points. 245 246Or, 247:: 248 249 echo -:myprobe >> kprobe_events 250 251This clears probe points selectively. 252 253Right after definition, each event is disabled by default. For tracing these 254events, you need to enable it. 255:: 256 257 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable 258 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable 259 260Use the following command to start tracing in an interval. 261:: 262 263 # echo 1 > tracing_on 264 Open something... 265 # echo 0 > tracing_on 266 267And you can see the traced information via /sys/kernel/tracing/trace. 268:: 269 270 cat /sys/kernel/tracing/trace 271 # tracer: nop 272 # 273 # TASK-PID CPU# TIMESTAMP FUNCTION 274 # | | | | | 275 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 276 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 277 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 278 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 279 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 280 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 281 282 283Each line shows when the kernel hits an event, and <- SYMBOL means kernel 284returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 285returns from do_sys_open to sys_open+0x1b). 286