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 $current : Fetch the address of the current task_struct. 57 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4) 58 this_cpu_read(FETCHARG) : Read the value of the per-CPU variable FETCHARG on the current CPU. 59 this_cpu_ptr(FETCHARG) : Get the address of the per-CPU variable FETCHARG on the current CPU. 60 \IMM : Store an immediate value to the argument. 61 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 62 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 63 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 64 (x8/x16/x32/x64), VFS layer common type(%pd/%pD), "char", 65 "string", "ustring", "symbol", "symstr" and bitfield are 66 supported. 67 (STRUCT[,ASGN])FIELD->MEMBER[->MEMBER] : If BTF is supported, typecast FIELD to 68 a pointer to STRUCT and then derference the pointer defined by 69 ->MEMBER. Note that this is available only when the probe is 70 on function entry. ASGN can be specified optionally. If ASGN 71 is specified, FIELD will be cast to the same offset position 72 as the ASGN member, rather than to the beginning of the STRUCT. 73 (STRUCT[,ASGN])(FETCHARG)->MEMBER[->MEMBER] : typecast can nest, so the above can 74 also be used with another FETCHARG instead of FIELD. 75 76 (\*1) only for the probe on function entry (offs == 0). Note, this argument access 77 is best effort, because depending on the argument type, it may be passed on 78 the stack. But this only support the arguments via registers. 79 (\*2) only for return probe. Note that this is also best effort. Depending on the 80 return value type, it might be passed via a pair of registers. But this only 81 accesses one register. 82 (\*3) this is useful for fetching a field of data structures. 83 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`. 84 85Function arguments at kretprobe 86------------------------------- 87Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This 88is useful to record the function parameter and return value at once, and 89trace the difference of structure fields (for debugging a function whether it 90correctly updates the given data structure or not). 91See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how 92it works. 93 94.. _kprobetrace_types: 95 96Types 97----- 98Several types are supported for fetchargs. Kprobe tracer will access memory 99by given type. Prefix 's' and 'u' means those types are signed and unsigned 100respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 101in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 102or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 103x86-64 uses x64). 104 105These value types can be an array. To record array data, you can add '[N]' 106(where N is a fixed number, less than 64) to the base type. 107E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements. 108Note that the array can be applied to memory type fetchargs, you can not 109apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 110wrong, but '+8($stack):x8[8]' is OK.) 111 112Char type can be used to show the character value of traced arguments. 113 114String type is a special type, which fetches a "null-terminated" string from 115kernel space. This means it will fail and store NULL if the string container 116has been paged out. "ustring" type is an alternative of string for user-space. 117See :ref:`user_mem_access` for more info. 118 119The string array type is a bit different from other types. For other base 120types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 121as +0(%di):x32.) But string[1] is not equal to string. The string type itself 122represents "char array", but string array type represents "char * array". 123So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 124Bitfield is another special type, which takes 3 parameters, bit-width, bit- 125offset, and container-size (usually 32). The syntax is:: 126 127 b<bit-width>@<bit-offset>/<container-size> 128 129Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 130which shows given pointer in "symbol+offset" style. 131On the other hand, symbol-string type ('symstr') converts the given address to 132"symbol+offset/symbolsize" style and stores it as a null-terminated string. 133With 'symstr' type, you can filter the event with wildcard pattern of the 134symbols, and you don't need to solve symbol name by yourself. 135For $comm, the default type is "string"; any other type is invalid. 136 137VFS layer common type(%pd/%pD) is a special type, which fetches dentry's or 138file's name from struct dentry's address or struct file's address. 139 140.. _user_mem_access: 141 142User Memory Access 143------------------ 144Kprobe events supports user-space memory access. For that purpose, you can use 145either user-space dereference syntax or 'ustring' type. 146 147The user-space dereference syntax allows you to access a field of a data 148structure in user-space. This is done by adding the "u" prefix to the 149dereference syntax. For example, +u4(%si) means it will read memory from the 150address in the register %si offset by 4, and the memory is expected to be in 151user-space. You can use this for strings too, e.g. +u0(%si):string will read 152a string from the address in the register %si that is expected to be in user- 153space. 'ustring' is a shortcut way of performing the same task. That is, 154+0(%si):ustring is equivalent to +u0(%si):string. 155 156Note that kprobe-event provides the user-memory access syntax but it doesn't 157use it transparently. This means if you use normal dereference or string type 158for user memory, it might fail, and may always fail on some architectures. The 159user has to carefully check if the target data is in kernel or user space. 160 161Per-Probe Event Filtering 162------------------------- 163Per-probe event filtering feature allows you to set different filter on each 164probe and gives you what arguments will be shown in trace buffer. If an event 165name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 166under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 167'enable', 'format', 'filter' and 'trigger'. 168 169enable: 170 You can enable/disable the probe by writing 1 or 0 on it. 171 172format: 173 This shows the format of this probe event. 174 175filter: 176 You can write filtering rules of this event. 177 178id: 179 This shows the id of this probe event. 180 181trigger: 182 This allows to install trigger commands which are executed when the event is 183 hit (for details, see Documentation/trace/events.rst, section 6). 184 185Event Profiling 186--------------- 187You can check the total number of probe hits and probe miss-hits via 188/sys/kernel/tracing/kprobe_profile. 189The first column is event name, the second is the number of probe hits, 190the third is the number of probe miss-hits. 191 192Kernel Boot Parameter 193--------------------- 194You can add and enable new kprobe events when booting up the kernel by 195"kprobe_event=" parameter. The parameter accepts a semicolon-delimited 196kprobe events, which format is similar to the kprobe_events. 197The difference is that the probe definition parameters are comma-delimited 198instead of space. For example, adding myprobe event on do_sys_open like below:: 199 200 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack) 201 202should be below for kernel boot parameter (just replace spaces with comma):: 203 204 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack) 205 206 207Usage examples 208-------------- 209To add a probe as a new event, write a new definition to kprobe_events 210as below:: 211 212 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events 213 214This sets a kprobe on the top of do_sys_open() function with recording 2151st to 4th arguments as "myprobe" event. Note, which register/stack entry is 216assigned to each function argument depends on arch-specific ABI. If you unsure 217the ABI, please try to use probe subcommand of perf-tools (you can find it 218under tools/perf/). 219As this example shows, users can choose more familiar names for each arguments. 220:: 221 222 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events 223 224This sets a kretprobe on the return point of do_sys_open() function with 225recording return value as "myretprobe" event. 226You can see the format of these events via 227/sys/kernel/tracing/events/kprobes/<EVENT>/format. 228:: 229 230 cat /sys/kernel/tracing/events/kprobes/myprobe/format 231 name: myprobe 232 ID: 780 233 format: 234 field:unsigned short common_type; offset:0; size:2; signed:0; 235 field:unsigned char common_flags; offset:2; size:1; signed:0; 236 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 237 field:int common_pid; offset:4; size:4; signed:1; 238 239 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 240 field:int __probe_nargs; offset:16; size:4; signed:1; 241 field:unsigned long dfd; offset:20; size:4; signed:0; 242 field:unsigned long filename; offset:24; size:4; signed:0; 243 field:unsigned long flags; offset:28; size:4; signed:0; 244 field:unsigned long mode; offset:32; size:4; signed:0; 245 246 247 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 248 REC->dfd, REC->filename, REC->flags, REC->mode 249 250You can see that the event has 4 arguments as in the expressions you specified. 251:: 252 253 echo > /sys/kernel/tracing/kprobe_events 254 255This clears all probe points. 256 257Or, 258:: 259 260 echo -:myprobe >> kprobe_events 261 262This clears probe points selectively. 263 264Right after definition, each event is disabled by default. For tracing these 265events, you need to enable it. 266:: 267 268 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable 269 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable 270 271Use the following command to start tracing in an interval. 272:: 273 274 # echo 1 > tracing_on 275 Open something... 276 # echo 0 > tracing_on 277 278And you can see the traced information via /sys/kernel/tracing/trace. 279:: 280 281 cat /sys/kernel/tracing/trace 282 # tracer: nop 283 # 284 # TASK-PID CPU# TIMESTAMP FUNCTION 285 # | | | | | 286 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 287 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 288 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 289 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 290 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 291 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 292 293 294Each line shows when the kernel hits an event, and <- SYMBOL means kernel 295returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 296returns from do_sys_open to sys_open+0x1b). 297