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Diffstat (limited to 'kernel/Documentation/trace/kprobetrace.txt')
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diff --git a/kernel/Documentation/trace/kprobetrace.txt b/kernel/Documentation/trace/kprobetrace.txt new file mode 100644 index 000000000..d68ea5fc8 --- /dev/null +++ b/kernel/Documentation/trace/kprobetrace.txt @@ -0,0 +1,172 @@ + Kprobe-based Event Tracing + ========================== + + Documentation is written by Masami Hiramatsu + + +Overview +-------- +These events are similar to tracepoint based events. Instead of Tracepoint, +this is based on kprobes (kprobe and kretprobe). So it can probe wherever +kprobes can probe (this means, all functions body except for __kprobes +functions). Unlike the Tracepoint based event, this can be added and removed +dynamically, on the fly. + +To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y. + +Similar to the events tracer, this doesn't need to be activated via +current_tracer. Instead of that, add probe points via +/sys/kernel/debug/tracing/kprobe_events, and enable it via +/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled. + + +Synopsis of kprobe_events +------------------------- + p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe + r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe + -:[GRP/]EVENT : Clear a probe + + GRP : Group name. If omitted, use "kprobes" for it. + EVENT : Event name. If omitted, the event name is generated + based on SYM+offs or MEMADDR. + MOD : Module name which has given SYM. + SYM[+offs] : Symbol+offset where the probe is inserted. + MEMADDR : Address where the probe is inserted. + + FETCHARGS : Arguments. Each probe can have up to 128 args. + %REG : Fetch register REG + @ADDR : Fetch memory at ADDR (ADDR should be in kernel) + @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol) + $stackN : Fetch Nth entry of stack (N >= 0) + $stack : Fetch stack address. + $retval : Fetch return value.(*) + +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**) + NAME=FETCHARG : Set NAME as the argument name of FETCHARG. + FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types + (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield + are supported. + + (*) only for return probe. + (**) this is useful for fetching a field of data structures. + +Types +----- +Several types are supported for fetch-args. Kprobe tracer will access memory +by given type. Prefix 's' and 'u' means those types are signed and unsigned +respectively. Traced arguments are shown in decimal (signed) or hex (unsigned). +String type is a special type, which fetches a "null-terminated" string from +kernel space. This means it will fail and store NULL if the string container +has been paged out. +Bitfield is another special type, which takes 3 parameters, bit-width, bit- +offset, and container-size (usually 32). The syntax is; + + b<bit-width>@<bit-offset>/<container-size> + + +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, it 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: + This shows the format of this probe event. + +filter: + You can write filtering rules of this event. + +id: + This shows the id of this probe event. + + +Event Profiling +--------------- + You can check the total number of probe hits and probe miss-hits via +/sys/kernel/debug/tracing/kprobe_profile. + The first column is event name, the second is the number of probe hits, +the third is the number of probe miss-hits. + + +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 dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /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. Note, which register/stack entry is +assigned to each function argument depends on arch-specific ABI. If you unsure +the ABI, please try to use probe subcommand of perf-tools (you can find it +under tools/perf/). +As this example shows, users can choose more familiar names for each arguments. + + echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events + + This sets a kretprobe on the return point of do_sys_open() function with +recording return value 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: 780 +format: + field:unsigned short common_type; offset:0; size:2; signed:0; + field:unsigned char common_flags; offset:2; size:1; signed:0; + field:unsigned char common_preempt_count; offset:3; size:1;signed:0; + field:int common_pid; offset:4; size:4; signed:1; + + field:unsigned long __probe_ip; offset:12; size:4; signed:0; + field:int __probe_nargs; offset:16; size:4; signed:1; + field:unsigned long dfd; offset:20; size:4; signed:0; + field:unsigned long filename; offset:24; size:4; signed:0; + field:unsigned long flags; offset:28; size:4; signed:0; + field:unsigned long mode; offset:32; size:4; signed:0; + + +print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip, +REC->dfd, REC->filename, REC->flags, REC->mode + + You can see that the event has 4 arguments as in the expressions you specified. + + echo > /sys/kernel/debug/tracing/kprobe_events + + This clears all probe points. + + Or, + + echo -:myprobe >> kprobe_events + + This clears probe points selectively. + + Right after definition, each event is disabled by default. For tracing these +events, you need to enable it. + + echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable + echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable + + 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: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0 + <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe + <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6 + <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 + <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10 + <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3 + + + Each line shows when the kernel hits an event, 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). + |