150 lines
5.7 KiB
Plaintext
150 lines
5.7 KiB
Plaintext
Kprobe-based Event Tracing
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==========================
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Documentation is written by Masami Hiramatsu
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Overview
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--------
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These events are similar to tracepoint based events. Instead of Tracepoint,
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this is based on kprobes (kprobe and kretprobe). So it can probe wherever
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kprobes can probe (this means, all functions body except for __kprobes
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functions). Unlike the Tracepoint based event, this can be added and removed
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dynamically, on the fly.
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To enable this feature, build your kernel with CONFIG_KPROBE_TRACING=y.
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Similar to the events tracer, this doesn't need to be activated via
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current_tracer. Instead of that, add probe points via
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/sys/kernel/debug/tracing/kprobe_events, and enable it via
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/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
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Synopsis of kprobe_events
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-------------------------
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p[:[GRP/]EVENT] SYMBOL[+offs]|MEMADDR [FETCHARGS] : Set a probe
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r[:[GRP/]EVENT] SYMBOL[+0] [FETCHARGS] : Set a return probe
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GRP : Group name. If omitted, use "kprobes" for it.
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EVENT : Event name. If omitted, the event name is generated
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based on SYMBOL+offs or MEMADDR.
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SYMBOL[+offs] : Symbol+offset where the probe is inserted.
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MEMADDR : Address where the probe is inserted.
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FETCHARGS : Arguments. Each probe can have up to 128 args.
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%REG : Fetch register REG
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@ADDR : Fetch memory at ADDR (ADDR should be in kernel)
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@SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
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$stackN : Fetch Nth entry of stack (N >= 0)
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$stack : Fetch stack address.
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$argN : Fetch function argument. (N >= 0)(*)
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$retval : Fetch return value.(**)
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+|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(***)
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NAME=FETCHARG: Set NAME as the argument name of FETCHARG.
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(*) aN may not correct on asmlinkaged functions and at the middle of
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function body.
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(**) only for return probe.
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(***) this is useful for fetching a field of data structures.
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Per-Probe Event Filtering
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-------------------------
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Per-probe event filtering feature allows you to set different filter on each
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probe and gives you what arguments will be shown in trace buffer. If an event
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name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
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under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
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'enabled', 'format' and 'filter'.
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enabled:
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You can enable/disable the probe by writing 1 or 0 on it.
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format:
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This shows the format of this probe event.
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filter:
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You can write filtering rules of this event.
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id:
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This shows the id of this probe event.
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Event Profiling
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---------------
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You can check the total number of probe hits and probe miss-hits via
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/sys/kernel/debug/tracing/kprobe_profile.
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The first column is event name, the second is the number of probe hits,
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the third is the number of probe miss-hits.
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Usage examples
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--------------
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To add a probe as a new event, write a new definition to kprobe_events
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as below.
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echo p:myprobe do_sys_open dfd=$arg0 filename=$arg1 flags=$arg2 mode=$arg3 > /sys/kernel/debug/tracing/kprobe_events
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This sets a kprobe on the top of do_sys_open() function with recording
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1st to 4th arguments as "myprobe" event. As this example shows, users can
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choose more familiar names for each arguments.
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echo r:myretprobe do_sys_open $retval >> /sys/kernel/debug/tracing/kprobe_events
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This sets a kretprobe on the return point of do_sys_open() function with
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recording return value as "myretprobe" event.
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You can see the format of these events via
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/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
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cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
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name: myprobe
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ID: 75
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format:
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field:unsigned short common_type; offset:0; size:2;
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field:unsigned char common_flags; offset:2; size:1;
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field:unsigned char common_preempt_count; offset:3; size:1;
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field:int common_pid; offset:4; size:4;
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field:int common_tgid; offset:8; size:4;
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field: unsigned long ip; offset:16;tsize:8;
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field: int nargs; offset:24;tsize:4;
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field: unsigned long dfd; offset:32;tsize:8;
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field: unsigned long filename; offset:40;tsize:8;
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field: unsigned long flags; offset:48;tsize:8;
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field: unsigned long mode; offset:56;tsize:8;
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print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->ip, REC->dfd, REC->filename, REC->flags, REC->mode
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You can see that the event has 4 arguments as in the expressions you specified.
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echo > /sys/kernel/debug/tracing/kprobe_events
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This clears all probe points.
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Right after definition, each event is disabled by default. For tracing these
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events, you need to enable it.
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echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
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echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
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And you can see the traced information via /sys/kernel/debug/tracing/trace.
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cat /sys/kernel/debug/tracing/trace
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# tracer: nop
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#
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# TASK-PID CPU# TIMESTAMP FUNCTION
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# | | | | |
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<...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
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<...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
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<...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
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<...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
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<...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
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<...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
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Each line shows when the kernel hits an event, and <- SYMBOL means kernel
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returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
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returns from do_sys_open to sys_open+0x1b).
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