185 lines
7.0 KiB
ReStructuredText
185 lines
7.0 KiB
ReStructuredText
=========================================
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Uprobe-tracer: Uprobe-based Event Tracing
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=========================================
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:Author: Srikar Dronamraju
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Overview
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--------
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Uprobe based trace events are similar to kprobe based trace events.
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To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y.
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Similar to the kprobe-event 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/uprobe_events, and enable it via
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/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enable.
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However unlike kprobe-event tracer, the uprobe event interface expects the
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user to calculate the offset of the probepoint in the object.
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You can also use /sys/kernel/debug/tracing/dynamic_events instead of
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uprobe_events. That interface will provide unified access to other
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dynamic events too.
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Synopsis of uprobe_tracer
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-------------------------
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::
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p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
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r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
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-:[GRP/]EVENT : Clear uprobe or uretprobe event
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GRP : Group name. If omitted, "uprobes" is the default value.
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EVENT : Event name. If omitted, the event name is generated based
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on PATH+OFFSET.
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PATH : Path to an executable or a library.
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OFFSET : Offset 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 userspace)
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@+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH)
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$stackN : Fetch Nth entry of stack (N >= 0)
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$stack : Fetch stack address.
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$retval : Fetch return value.(\*1)
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$comm : Fetch current task comm.
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+|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)
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\IMM : Store an immediate value to the argument.
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NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
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FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
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(u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
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(x8/x16/x32/x64), "string" and bitfield are supported.
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(\*1) only for return probe.
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(\*2) this is useful for fetching a field of data structures.
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(\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe
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events can access only user-space memory.
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Types
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-----
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Several types are supported for fetch-args. Uprobe tracer will access memory
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by given type. Prefix 's' and 'u' means those types are signed and unsigned
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respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
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in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
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or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
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x86-64 uses x64).
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String type is a special type, which fetches a "null-terminated" string from
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user space.
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Bitfield is another special type, which takes 3 parameters, bit-width, bit-
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offset, and container-size (usually 32). The syntax is::
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b<bit-width>@<bit-offset>/<container-size>
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For $comm, the default type is "string"; any other type is invalid.
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Event Profiling
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---------------
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You can check the total number of probe hits per event via
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/sys/kernel/debug/tracing/uprobe_profile. The first column is the filename,
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the second is the event name, the third is the number of probe hits.
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Usage examples
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--------------
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* Add a probe as a new uprobe event, write a new definition to uprobe_events
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as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)::
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echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
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* Add a probe as a new uretprobe event::
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echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
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* Unset registered event::
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echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
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* Print out the events that are registered::
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cat /sys/kernel/debug/tracing/uprobe_events
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* Clear all events::
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echo > /sys/kernel/debug/tracing/uprobe_events
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Following example shows how to dump the instruction pointer and %ax register
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at the probed text address. Probe zfree function in /bin/zsh::
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# cd /sys/kernel/debug/tracing/
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# cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
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00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
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# objdump -T /bin/zsh | grep -w zfree
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0000000000446420 g DF .text 0000000000000012 Base zfree
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0x46420 is the offset of zfree in object /bin/zsh that is loaded at
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0x00400000. Hence the command to uprobe would be::
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# echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
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And the same for the uretprobe would be::
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# echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
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.. note:: User has to explicitly calculate the offset of the probe-point
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in the object.
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We can see the events that are registered by looking at the uprobe_events file.
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::
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# cat uprobe_events
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p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
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r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
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Format of events can be seen by viewing the file events/uprobes/zfree_entry/format.
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::
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# cat events/uprobes/zfree_entry/format
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name: zfree_entry
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ID: 922
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format:
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field:unsigned short common_type; offset:0; size:2; signed:0;
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field:unsigned char common_flags; offset:2; size:1; signed:0;
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field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
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field:int common_pid; offset:4; size:4; signed:1;
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field:int common_padding; offset:8; size:4; signed:1;
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field:unsigned long __probe_ip; offset:12; size:4; signed:0;
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field:u32 arg1; offset:16; size:4; signed:0;
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field:u32 arg2; offset:20; size:4; signed:0;
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print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
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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 by::
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# echo 1 > events/uprobes/enable
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Lets start tracing, sleep for some time and stop tracing.
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::
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# echo 1 > tracing_on
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# sleep 20
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# echo 0 > tracing_on
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Also, you can disable the event by::
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# echo 0 > events/uprobes/enable
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And you can see the traced information via /sys/kernel/debug/tracing/trace.
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::
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# cat 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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zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
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zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
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zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
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zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
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Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
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and contents of ax register being 79. And uretprobe was triggered with ip at
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0x446540 with counterpart function entry at 0x446420.
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