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| -rw-r--r-- | Documentation/trace/kprobetrace.txt | 139 |
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diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt new file mode 100644 index 000000000000..efff6eb1b3db --- /dev/null +++ b/Documentation/trace/kprobetrace.txt @@ -0,0 +1,139 @@ + Kprobe-based Event Tracer + ========================= + + Documentation is written by Masami Hiramatsu + + +Overview +-------- +This tracer is similar to the events tracer which is based on Tracepoint +infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe +and kretprobe). It probes anywhere where kprobes can probe(this means, all +functions body except for __kprobes functions). + +Unlike the function tracer, this tracer can probe instructions inside of +kernel functions. It allows you to check which instruction has been executed. + +Unlike the Tracepoint based events tracer, this tracer can add and remove +probe points on the fly. + +Similar to the events tracer, this tracer doesn't need to be activated via +current_tracer, instead of that, just set probe points via +/sys/kernel/debug/tracing/kprobe_events. And you can set filters on each +probe events via /sys/kernel/debug/tracing/events/kprobes/<EVENT>/filter. + + +Synopsis of kprobe_events +------------------------- + p[:EVENT] SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS] : Set a probe + r[:EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe + + EVENT : Event name. + SYMBOL[+offs|-offs] : Symbol+offset where the probe is inserted. + MEMADDR : Address where the probe is inserted. + + FETCHARGS : Arguments. + %REG : Fetch register REG + sN : Fetch Nth entry of stack (N >= 0) + sa : Fetch stack address. + @ADDR : Fetch memory at ADDR (ADDR should be in kernel) + @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) + aN : Fetch function argument. (N >= 0)(*) + rv : Fetch return value.(**) + ra : Fetch return address.(**) + +|-offs(FETCHARG) : fetch memory at FETCHARG +|- offs address.(***) + + (*) aN may not correct on asmlinkaged functions and at the middle of + function body. + (**) only for return probe. + (***) this is useful for fetching a field of data structures. + + +Per-Probe Event Filtering +------------------------- + Per-probe event filtering feature allows you to set different filter on each +probe and gives you what arguments will be shown in trace buffer. If an event +name is specified right after 'p:' or 'r:' in kprobe_events, the tracer adds +an event under tracing/events/kprobes/<EVENT>, at the directory you can see +'id', 'enabled', 'format' and 'filter'. + +enabled: + You can enable/disable the probe by writing 1 or 0 on it. + +format: + It shows the format of this probe event. It also shows aliases of arguments + which you specified to kprobe_events. + +filter: + You can write filtering rules of this event. And you can use both of aliase + names and field names for describing filters. + + +Usage examples +-------------- +To add a probe as a new event, write a new definition to kprobe_events +as below. + + echo p:myprobe do_sys_open a0 a1 a2 a3 > /sys/kernel/debug/tracing/kprobe_events + + This sets a kprobe on the top of do_sys_open() function with recording +1st to 4th arguments as "myprobe" event. + + echo r:myretprobe do_sys_open rv ra >> /sys/kernel/debug/tracing/kprobe_events + + This sets a kretprobe on the return point of do_sys_open() function with +recording return value and return address as "myretprobe" event. + You can see the format of these events via +/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format. + + cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format +name: myprobe +ID: 23 +format: + field:unsigned short common_type; offset:0; size:2; + field:unsigned char common_flags; offset:2; size:1; + field:unsigned char common_preempt_count; offset:3; size:1; + field:int common_pid; offset:4; size:4; + field:int common_tgid; offset:8; size:4; + + field: unsigned long ip; offset:16;tsize:8; + field: int nargs; offset:24;tsize:4; + field: unsigned long arg0; offset:32;tsize:8; + field: unsigned long arg1; offset:40;tsize:8; + field: unsigned long arg2; offset:48;tsize:8; + field: unsigned long arg3; offset:56;tsize:8; + + alias: a0; original: arg0; + alias: a1; original: arg1; + alias: a2; original: arg2; + alias: a3; original: arg3; + +print fmt: "%lx: 0x%lx 0x%lx 0x%lx 0x%lx", ip, arg0, arg1, arg2, arg3 + + + You can see that the event has 4 arguments and alias expressions +corresponding to it. + + echo > /sys/kernel/debug/tracing/kprobe_events + + This clears all probe points. and you can see the traced information via +/sys/kernel/debug/tracing/trace. + + cat /sys/kernel/debug/tracing/trace +# tracer: nop +# +# TASK-PID CPU# TIMESTAMP FUNCTION +# | | | | | + <...>-1447 [001] 1038282.286875: do_sys_open+0x0/0xd6: 0x3 0x7fffd1ec4440 0x8000 0x0 + <...>-1447 [001] 1038282.286878: sys_openat+0xc/0xe <- do_sys_open: 0xfffffffffffffffe 0xffffffff81367a3a + <...>-1447 [001] 1038282.286885: do_sys_open+0x0/0xd6: 0xffffff9c 0x40413c 0x8000 0x1b6 + <...>-1447 [001] 1038282.286915: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a + <...>-1447 [001] 1038282.286969: do_sys_open+0x0/0xd6: 0xffffff9c 0x4041c6 0x98800 0x10 + <...>-1447 [001] 1038282.286976: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a + + + Each line shows when the kernel hits a probe, and <- SYMBOL means kernel +returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel +returns from do_sys_open to sys_open+0x1b). + + |
