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/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 $comm : Fetch current task comm. 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 57 \IMM : Store an immediate value to the argument. 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 61 (x8/x16/x32/x64), "string", "ustring", "symbol", "symstr" 62 and bitfield are supported. 63 64 (\*1) only for the probe on function entry (offs == 0). 65 (\*2) only for return probe. 66 (\*3) this is useful for fetching a field of data structures. 67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 68 69Types 70----- 71Several types are supported for fetchargs. Kprobe tracer will access memory 72by given type. Prefix 's' and 'u' means those types are signed and unsigned 73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 76x86-64 uses x64). 77 78These value types can be an array. To record array data, you can add '[N]' 79(where N is a fixed number, less than 64) to the base type. 80E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 81Note that the array can be applied to memory type fetchargs, you can not 82apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 83wrong, but '+8($stack):x8[8]' is OK.) 84 85String type is a special type, which fetches a "null-terminated" string from 86kernel space. This means it will fail and store NULL if the string container 87has been paged out. "ustring" type is an alternative of string for user-space. 88See :ref:`user_mem_access` for more info. 89 90The string array type is a bit different from other types. For other base 91types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 92as +0(%di):x32.) But string[1] is not equal to string. The string type itself 93represents "char array", but string array type represents "char * array". 94So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 95Bitfield is another special type, which takes 3 parameters, bit-width, bit- 96offset, and container-size (usually 32). The syntax is:: 97 98 b<bit-width>@<bit-offset>/<container-size> 99 100Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 101which shows given pointer in "symbol+offset" style. 102On the other hand, symbol-string type ('symstr') converts the given address to 103"symbol+offset/symbolsize" style and stores it as a null-terminated string. 104With 'symstr' type, you can filter the event with wildcard pattern of the 105symbols, and you don't need to solve symbol name by yourself. 106For $comm, the default type is "string"; any other type is invalid. 107 108.. _user_mem_access: 109 110User Memory Access 111------------------ 112Kprobe events supports user-space memory access. For that purpose, you can use 113either user-space dereference syntax or 'ustring' type. 114 115The user-space dereference syntax allows you to access a field of a data 116structure in user-space. This is done by adding the "u" prefix to the 117dereference syntax. For example, +u4(%si) means it will read memory from the 118address in the register %si offset by 4, and the memory is expected to be in 119user-space. You can use this for strings too, e.g. +u0(%si):string will read 120a string from the address in the register %si that is expected to be in user- 121space. 'ustring' is a shortcut way of performing the same task. That is, 122+0(%si):ustring is equivalent to +u0(%si):string. 123 124Note that kprobe-event provides the user-memory access syntax but it doesn't 125use it transparently. This means if you use normal dereference or string type 126for user memory, it might fail, and may always fail on some architectures. The 127user has to carefully check if the target data is in kernel or user space. 128 129Per-Probe Event Filtering 130------------------------- 131Per-probe event filtering feature allows you to set different filter on each 132probe and gives you what arguments will be shown in trace buffer. If an event 133name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 134under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 135'enable', 'format', 'filter' and 'trigger'. 136 137enable: 138 You can enable/disable the probe by writing 1 or 0 on it. 139 140format: 141 This shows the format of this probe event. 142 143filter: 144 You can write filtering rules of this event. 145 146id: 147 This shows the id of this probe event. 148 149trigger: 150 This allows to install trigger commands which are executed when the event is 151 hit (for details, see Documentation/trace/events.rst, section 6). 152 153Event Profiling 154--------------- 155You can check the total number of probe hits and probe miss-hits via 156/sys/kernel/debug/tracing/kprobe_profile. 157The first column is event name, the second is the number of probe hits, 158the third is the number of probe miss-hits. 159 160Kernel Boot Parameter 161--------------------- 162You can add and enable new kprobe events when booting up the kernel by 163"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 164kprobe events, which format is similar to the kprobe_events. 165The difference is that the probe definition parameters are comma-delimited 166instead of space. For example, adding myprobe event on do_sys_open like below:: 167 168 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 169 170should be below for kernel boot parameter (just replace spaces with comma):: 171 172 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 173 174 175Usage examples 176-------------- 177To add a probe as a new event, write a new definition to kprobe_events 178as below:: 179 180 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 181 182This sets a kprobe on the top of do_sys_open() function with recording 1831st to 4th arguments as "myprobe" event. Note, which register/stack entry is 184assigned to each function argument depends on arch-specific ABI. If you unsure 185the ABI, please try to use probe subcommand of perf-tools (you can find it 186under tools/perf/). 187As this example shows, users can choose more familiar names for each arguments. 188:: 189 190 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 191 192This sets a kretprobe on the return point of do_sys_open() function with 193recording return value as "myretprobe" event. 194You can see the format of these events via 195/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 196:: 197 198 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 199 name: myprobe 200 ID: 780 201 format: 202 field:unsigned short common_type; offset:0; size:2; signed:0; 203 field:unsigned char common_flags; offset:2; size:1; signed:0; 204 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 205 field:int common_pid; offset:4; size:4; signed:1; 206 207 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 208 field:int __probe_nargs; offset:16; size:4; signed:1; 209 field:unsigned long dfd; offset:20; size:4; signed:0; 210 field:unsigned long filename; offset:24; size:4; signed:0; 211 field:unsigned long flags; offset:28; size:4; signed:0; 212 field:unsigned long mode; offset:32; size:4; signed:0; 213 214 215 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 216 REC->dfd, REC->filename, REC->flags, REC->mode 217 218You can see that the event has 4 arguments as in the expressions you specified. 219:: 220 221 echo > /sys/kernel/debug/tracing/kprobe_events 222 223This clears all probe points. 224 225Or, 226:: 227 228 echo -:myprobe >> kprobe_events 229 230This clears probe points selectively. 231 232Right after definition, each event is disabled by default. For tracing these 233events, you need to enable it. 234:: 235 236 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 237 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 238 239Use the following command to start tracing in an interval. 240:: 241 242 # echo 1 > tracing_on 243 Open something... 244 # echo 0 > tracing_on 245 246And you can see the traced information via /sys/kernel/debug/tracing/trace. 247:: 248 249 cat /sys/kernel/debug/tracing/trace 250 # tracer: nop 251 # 252 # TASK-PID CPU# TIMESTAMP FUNCTION 253 # | | | | | 254 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 255 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 256 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 257 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 258 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 259 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 260 261 262Each line shows when the kernel hits an event, and <- SYMBOL means kernel 263returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 264returns from do_sys_open to sys_open+0x1b). 265