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 -:[GRP/]EVENT : Clear a probe 34 35 GRP : Group name. If omitted, use "kprobes" for it. 36 EVENT : Event name. If omitted, the event name is generated 37 based on SYM+offs or MEMADDR. 38 MOD : Module name which has given SYM. 39 SYM[+offs] : Symbol+offset where the probe is inserted. 40 MEMADDR : Address where the probe is inserted. 41 MAXACTIVE : Maximum number of instances of the specified function that 42 can be probed simultaneously, or 0 for the default value 43 as defined in Documentation/kprobes.txt section 1.3.1. 44 45 FETCHARGS : Arguments. Each probe can have up to 128 args. 46 %REG : Fetch register REG 47 @ADDR : Fetch memory at ADDR (ADDR should be in kernel) 48 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) 49 $stackN : Fetch Nth entry of stack (N >= 0) 50 $stack : Fetch stack address. 51 $argN : Fetch the Nth function argument. (N >= 1) (\*1) 52 $retval : Fetch return value.(\*2) 53 $comm : Fetch current task comm. 54 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(\*3) 55 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 56 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 57 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types 58 (x8/x16/x32/x64), "string" and bitfield are supported. 59 60 (\*1) only for the probe on function entry (offs == 0). 61 (\*2) only for return probe. 62 (\*3) this is useful for fetching a field of data structures. 63 64Types 65----- 66Several types are supported for fetch-args. Kprobe tracer will access memory 67by given type. Prefix 's' and 'u' means those types are signed and unsigned 68respectively. 'x' prefix implies it is unsigned. Traced arguments are shown 69in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' 70or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and 71x86-64 uses x64). 72These value types can be an array. To record array data, you can add '[N]' 73(where N is a fixed number, less than 64) to the base type. 74E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements. 75Note that the array can be applied to memory type fetchargs, you can not 76apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is 77wrong, but '+8($stack):x8[8]' is OK.) 78String type is a special type, which fetches a "null-terminated" string from 79kernel space. This means it will fail and store NULL if the string container 80has been paged out. 81The string array type is a bit different from other types. For other base 82types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same 83as +0(%di):x32.) But string[1] is not equal to string. The string type itself 84represents "char array", but string array type represents "char * array". 85So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string. 86Bitfield is another special type, which takes 3 parameters, bit-width, bit- 87offset, and container-size (usually 32). The syntax is:: 88 89 b<bit-width>@<bit-offset>/<container-size> 90 91Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 92which shows given pointer in "symbol+offset" style. 93For $comm, the default type is "string"; any other type is invalid. 94 95 96Per-Probe Event Filtering 97------------------------- 98Per-probe event filtering feature allows you to set different filter on each 99probe and gives you what arguments will be shown in trace buffer. If an event 100name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 101under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 102'enable', 'format', 'filter' and 'trigger'. 103 104enable: 105 You can enable/disable the probe by writing 1 or 0 on it. 106 107format: 108 This shows the format of this probe event. 109 110filter: 111 You can write filtering rules of this event. 112 113id: 114 This shows the id of this probe event. 115 116trigger: 117 This allows to install trigger commands which are executed when the event is 118 hit (for details, see Documentation/trace/events.rst, section 6). 119 120Event Profiling 121--------------- 122You can check the total number of probe hits and probe miss-hits via 123/sys/kernel/debug/tracing/kprobe_profile. 124The first column is event name, the second is the number of probe hits, 125the third is the number of probe miss-hits. 126 127 128Usage examples 129-------------- 130To add a probe as a new event, write a new definition to kprobe_events 131as below:: 132 133 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 134 135This sets a kprobe on the top of do_sys_open() function with recording 1361st to 4th arguments as "myprobe" event. Note, which register/stack entry is 137assigned to each function argument depends on arch-specific ABI. If you unsure 138the ABI, please try to use probe subcommand of perf-tools (you can find it 139under tools/perf/). 140As this example shows, users can choose more familiar names for each arguments. 141:: 142 143 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 144 145This sets a kretprobe on the return point of do_sys_open() function with 146recording return value as "myretprobe" event. 147You can see the format of these events via 148/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 149:: 150 151 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 152 name: myprobe 153 ID: 780 154 format: 155 field:unsigned short common_type; offset:0; size:2; signed:0; 156 field:unsigned char common_flags; offset:2; size:1; signed:0; 157 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 158 field:int common_pid; offset:4; size:4; signed:1; 159 160 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 161 field:int __probe_nargs; offset:16; size:4; signed:1; 162 field:unsigned long dfd; offset:20; size:4; signed:0; 163 field:unsigned long filename; offset:24; size:4; signed:0; 164 field:unsigned long flags; offset:28; size:4; signed:0; 165 field:unsigned long mode; offset:32; size:4; signed:0; 166 167 168 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 169 REC->dfd, REC->filename, REC->flags, REC->mode 170 171You can see that the event has 4 arguments as in the expressions you specified. 172:: 173 174 echo > /sys/kernel/debug/tracing/kprobe_events 175 176This clears all probe points. 177 178Or, 179:: 180 181 echo -:myprobe >> kprobe_events 182 183This clears probe points selectively. 184 185Right after definition, each event is disabled by default. For tracing these 186events, you need to enable it. 187:: 188 189 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 190 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 191 192And you can see the traced information via /sys/kernel/debug/tracing/trace. 193:: 194 195 cat /sys/kernel/debug/tracing/trace 196 # tracer: nop 197 # 198 # TASK-PID CPU# TIMESTAMP FUNCTION 199 # | | | | | 200 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 201 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 202 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 203 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 204 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 205 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 206 207 208Each line shows when the kernel hits an event, and <- SYMBOL means kernel 209returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 210returns from do_sys_open to sys_open+0x1b). 211 212