1 Kprobe-based Event Tracing 2 ========================== 3 4 Documentation is written by Masami Hiramatsu 5 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 body except for __kprobes 12functions). Unlike the Tracepoint based event, this can be added and removed 13dynamically, on the fly. 14 15To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y. 16 17Similar to the events tracer, this doesn't need to be activated via 18current_tracer. Instead of that, add probe points via 19/sys/kernel/debug/tracing/kprobe_events, and enable it via 20/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled. 21 22 23Synopsis of kprobe_events 24------------------------- 25 p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe 26 r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe 27 -:[GRP/]EVENT : Clear a probe 28 29 GRP : Group name. If omitted, use "kprobes" for it. 30 EVENT : Event name. If omitted, the event name is generated 31 based on SYM+offs or MEMADDR. 32 MOD : Module name which has given SYM. 33 SYM[+offs] : Symbol+offset where the probe is inserted. 34 MEMADDR : Address where the probe is inserted. 35 36 FETCHARGS : Arguments. Each probe can have up to 128 args. 37 %REG : Fetch register REG 38 @ADDR : Fetch memory at ADDR (ADDR should be in kernel) 39 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) 40 $stackN : Fetch Nth entry of stack (N >= 0) 41 $stack : Fetch stack address. 42 $retval : Fetch return value.(*) 43 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) 44 NAME=FETCHARG : Set NAME as the argument name of FETCHARG. 45 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types 46 (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield 47 are supported. 48 49 (*) only for return probe. 50 (**) this is useful for fetching a field of data structures. 51 52Types 53----- 54Several types are supported for fetch-args. Kprobe tracer will access memory 55by given type. Prefix 's' and 'u' means those types are signed and unsigned 56respectively. Traced arguments are shown in decimal (signed) or hex (unsigned). 57String type is a special type, which fetches a "null-terminated" string from 58kernel space. This means it will fail and store NULL if the string container 59has been paged out. 60Bitfield is another special type, which takes 3 parameters, bit-width, bit- 61offset, and container-size (usually 32). The syntax is; 62 63 b<bit-width>@<bit-offset>/<container-size> 64 65 66Per-Probe Event Filtering 67------------------------- 68 Per-probe event filtering feature allows you to set different filter on each 69probe and gives you what arguments will be shown in trace buffer. If an event 70name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event 71under tracing/events/kprobes/<EVENT>, at the directory you can see 'id', 72'enabled', 'format' and 'filter'. 73 74enabled: 75 You can enable/disable the probe by writing 1 or 0 on it. 76 77format: 78 This shows the format of this probe event. 79 80filter: 81 You can write filtering rules of this event. 82 83id: 84 This shows the id of this probe event. 85 86 87Event Profiling 88--------------- 89 You can check the total number of probe hits and probe miss-hits via 90/sys/kernel/debug/tracing/kprobe_profile. 91 The first column is event name, the second is the number of probe hits, 92the third is the number of probe miss-hits. 93 94 95Usage examples 96-------------- 97To add a probe as a new event, write a new definition to kprobe_events 98as below. 99 100 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events 101 102 This sets a kprobe on the top of do_sys_open() function with recording 1031st to 4th arguments as "myprobe" event. Note, which register/stack entry is 104assigned to each function argument depends on arch-specific ABI. If you unsure 105the ABI, please try to use probe subcommand of perf-tools (you can find it 106under tools/perf/). 107As this example shows, users can choose more familiar names for each arguments. 108 109 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events 110 111 This sets a kretprobe on the return point of do_sys_open() function with 112recording return value as "myretprobe" event. 113 You can see the format of these events via 114/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. 115 116 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format 117name: myprobe 118ID: 780 119format: 120 field:unsigned short common_type; offset:0; size:2; signed:0; 121 field:unsigned char common_flags; offset:2; size:1; signed:0; 122 field:unsigned char common_preempt_count; offset:3; size:1;signed:0; 123 field:int common_pid; offset:4; size:4; signed:1; 124 125 field:unsigned long __probe_ip; offset:12; size:4; signed:0; 126 field:int __probe_nargs; offset:16; size:4; signed:1; 127 field:unsigned long dfd; offset:20; size:4; signed:0; 128 field:unsigned long filename; offset:24; size:4; signed:0; 129 field:unsigned long flags; offset:28; size:4; signed:0; 130 field:unsigned long mode; offset:32; size:4; signed:0; 131 132 133print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, 134REC->dfd, REC->filename, REC->flags, REC->mode 135 136 You can see that the event has 4 arguments as in the expressions you specified. 137 138 echo > /sys/kernel/debug/tracing/kprobe_events 139 140 This clears all probe points. 141 142 Or, 143 144 echo -:myprobe >> kprobe_events 145 146 This clears probe points selectively. 147 148 Right after definition, each event is disabled by default. For tracing these 149events, you need to enable it. 150 151 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable 152 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable 153 154 And you can see the traced information via /sys/kernel/debug/tracing/trace. 155 156 cat /sys/kernel/debug/tracing/trace 157# tracer: nop 158# 159# TASK-PID CPU# TIMESTAMP FUNCTION 160# | | | | | 161 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 162 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe 163 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 164 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 165 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 166 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 167 168 169 Each line shows when the kernel hits an event, and <- SYMBOL means kernel 170returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel 171returns from do_sys_open to sys_open+0x1b). 172 173