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 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 $comm : Fetch current task comm. 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 57 \IMM : Store an immediate value to the argument. 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 61 (x8/x16/x32/x64), "string", "ustring", "symbol", "symstr" 62 and bitfield are supported. 63 64 (\*1) only for the probe on function entry (offs == 0). 65 (\*2) only for return probe. 66 (\*3) this is useful for fetching a field of data structures. 67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 68 69Types 70----- 71Several types are supported for fetch-args. Kprobe tracer will access memory 72by given type. Prefix 's' and 'u' means those types are signed and unsigned 73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 76x86-64 uses x64). 77These value types can be an array. To record array data, you can add '[N]' 78(where N is a fixed number, less than 64) to the base type. 79E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 80Note that the array can be applied to memory type fetchargs, you can not 81apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 82wrong, but '+8($stack):x8[8]' is OK.) 83String type is a special type, which fetches a "null-terminated" string from 84kernel space. This means it will fail and store NULL if the string container 85has been paged out. "ustring" type is an alternative of string for user-space. 86See :ref:`user_mem_access` for more info.. 87The string array type is a bit different from other types. For other base 88types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 89as +0(%di):x32.) But string[1] is not equal to string. The string type itself 90represents "char array", but string array type represents "char * array". 91So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 92Bitfield is another special type, which takes 3 parameters, bit-width, bit- 93offset, and container-size (usually 32). The syntax is:: 94 95 b<bit-width>@<bit-offset>/<container-size> 96 97Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 98which shows given pointer in "symbol+offset" style. 99On the other hand, symbol-string type ('symstr') converts the given address to 100"symbol+offset/symbolsize" style and stores it as a null-terminated string. 101With 'symstr' type, you can filter the event with wildcard pattern of the 102symbols, and you don't need to solve symbol name by yourself. 103For $comm, the default type is "string"; any other type is invalid. 104 105.. _user_mem_access: 106 107User Memory Access 108------------------ 109Kprobe events supports user-space memory access. For that purpose, you can use 110either user-space dereference syntax or 'ustring' type. 111 112The user-space dereference syntax allows you to access a field of a data 113structure in user-space. This is done by adding the "u" prefix to the 114dereference syntax. For example, +u4(%si) means it will read memory from the 115address in the register %si offset by 4, and the memory is expected to be in 116user-space. You can use this for strings too, e.g. +u0(%si):string will read 117a string from the address in the register %si that is expected to be in user- 118space. 'ustring' is a shortcut way of performing the same task. That is, 119+0(%si):ustring is equivalent to +u0(%si):string. 120 121Note that kprobe-event provides the user-memory access syntax but it doesn't 122use it transparently. This means if you use normal dereference or string type 123for user memory, it might fail, and may always fail on some archs. The user 124has to carefully check if the target data is in kernel or user space. 125 126Per-Probe Event Filtering 127------------------------- 128Per-probe event filtering feature allows you to set different filter on each 129probe and gives you what arguments will be shown in trace buffer. If an event 130name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 131under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 132'enable', 'format', 'filter' and 'trigger'. 133 134enable: 135 You can enable/disable the probe by writing 1 or 0 on it. 136 137format: 138 This shows the format of this probe event. 139 140filter: 141 You can write filtering rules of this event. 142 143id: 144 This shows the id of this probe event. 145 146trigger: 147 This allows to install trigger commands which are executed when the event is 148 hit (for details, see Documentation/trace/events.rst, section 6). 149 150Event Profiling 151--------------- 152You can check the total number of probe hits and probe miss-hits via 153/sys/kernel/debug/tracing/kprobe_profile. 154The first column is event name, the second is the number of probe hits, 155the third is the number of probe miss-hits. 156 157Kernel Boot Parameter 158--------------------- 159You can add and enable new kprobe events when booting up the kernel by 160"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 161kprobe events, which format is similar to the kprobe_events. 162The difference is that the probe definition parameters are comma-delimited 163instead of space. For example, adding myprobe event on do_sys_open like below 164 165 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 166 167should be below for kernel boot parameter (just replace spaces with comma) 168 169 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 170 171 172Usage examples 173-------------- 174To add a probe as a new event, write a new definition to kprobe_events 175as below:: 176 177 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 178 179This sets a kprobe on the top of do_sys_open() function with recording 1801st to 4th arguments as "myprobe" event. Note, which register/stack entry is 181assigned to each function argument depends on arch-specific ABI. If you unsure 182the ABI, please try to use probe subcommand of perf-tools (you can find it 183under tools/perf/). 184As this example shows, users can choose more familiar names for each arguments. 185:: 186 187 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 188 189This sets a kretprobe on the return point of do_sys_open() function with 190recording return value as "myretprobe" event. 191You can see the format of these events via 192/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 193:: 194 195 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 196 name: myprobe 197 ID: 780 198 format: 199 field:unsigned short common_type; offset:0; size:2; signed:0; 200 field:unsigned char common_flags; offset:2; size:1; signed:0; 201 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 202 field:int common_pid; offset:4; size:4; signed:1; 203 204 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 205 field:int __probe_nargs; offset:16; size:4; signed:1; 206 field:unsigned long dfd; offset:20; size:4; signed:0; 207 field:unsigned long filename; offset:24; size:4; signed:0; 208 field:unsigned long flags; offset:28; size:4; signed:0; 209 field:unsigned long mode; offset:32; size:4; signed:0; 210 211 212 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 213 REC->dfd, REC->filename, REC->flags, REC->mode 214 215You can see that the event has 4 arguments as in the expressions you specified. 216:: 217 218 echo > /sys/kernel/debug/tracing/kprobe_events 219 220This clears all probe points. 221 222Or, 223:: 224 225 echo -:myprobe >> kprobe_events 226 227This clears probe points selectively. 228 229Right after definition, each event is disabled by default. For tracing these 230events, you need to enable it. 231:: 232 233 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 234 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 235 236Use the following command to start tracing in an interval. 237:: 238 239 # echo 1 > tracing_on 240 Open something... 241 # echo 0 > tracing_on 242 243And you can see the traced information via /sys/kernel/debug/tracing/trace. 244:: 245 246 cat /sys/kernel/debug/tracing/trace 247 # tracer: nop 248 # 249 # TASK-PID CPU# TIMESTAMP FUNCTION 250 # | | | | | 251 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 252 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 253 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 254 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 255 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 256 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 257 258 259Each line shows when the kernel hits an event, and <- SYMBOL means kernel 260returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 261returns from do_sys_open to sys_open+0x1b). 262