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diff --git a/Documentation/trace/uprobetracer.rst b/Documentation/trace/uprobetracer.rst new file mode 100644 index 000000000..d08228115 --- /dev/null +++ b/Documentation/trace/uprobetracer.rst @@ -0,0 +1,173 @@ +========================================= +Uprobe-tracer: Uprobe-based Event Tracing +========================================= + +:Author: Srikar Dronamraju + + +Overview +-------- +Uprobe based trace events are similar to kprobe based trace events. +To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y. + +Similar to the kprobe-event tracer, this doesn't need to be activated via +current_tracer. Instead of that, add probe points via +/sys/kernel/debug/tracing/uprobe_events, and enable it via +/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enable. + +However unlike kprobe-event tracer, the uprobe event interface expects the +user to calculate the offset of the probepoint in the object. + +Synopsis of uprobe_tracer +------------------------- +:: + + p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe + r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe) + -:[GRP/]EVENT : Clear uprobe or uretprobe event + + GRP : Group name. If omitted, "uprobes" is the default value. + EVENT : Event name. If omitted, the event name is generated based + on PATH+OFFSET. + PATH : Path to an executable or a library. + OFFSET : Offset 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 userspace) + @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) + $stackN : Fetch Nth entry of stack (N >= 0) + $stack : Fetch stack address. + $retval : Fetch return value.(*) + $comm : Fetch current task comm. + +|-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), hexadecimal types + (x8/x16/x32/x64), "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. Uprobe tracer will access memory +by given type. Prefix 's' and 'u' means those types are signed and unsigned +respectively. 'x' prefix implies it is unsigned. Traced arguments are shown +in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' +or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and +x86-64 uses x64). +String type is a special type, which fetches a "null-terminated" string from +user space. +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> + +For $comm, the default type is "string"; any other type is invalid. + + +Event Profiling +--------------- +You can check the total number of probe hits and probe miss-hits via +/sys/kernel/debug/tracing/uprobe_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 +-------------- + * Add a probe as a new uprobe event, write a new definition to uprobe_events + as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: + + echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events + + * Add a probe as a new uretprobe event:: + + echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events + + * Unset registered event:: + + echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events + + * Print out the events that are registered:: + + cat /sys/kernel/debug/tracing/uprobe_events + + * Clear all events:: + + echo > /sys/kernel/debug/tracing/uprobe_events + +Following example shows how to dump the instruction pointer and %ax register +at the probed text address. Probe zfree function in /bin/zsh:: + + # cd /sys/kernel/debug/tracing/ + # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp + 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh + # objdump -T /bin/zsh | grep -w zfree + 0000000000446420 g DF .text 0000000000000012 Base zfree + +0x46420 is the offset of zfree in object /bin/zsh that is loaded at +0x00400000. Hence the command to uprobe would be:: + + # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events + +And the same for the uretprobe would be:: + + # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events + +.. note:: User has to explicitly calculate the offset of the probe-point + in the object. + +We can see the events that are registered by looking at the uprobe_events file. +:: + + # cat uprobe_events + p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax + r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax + +Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. +:: + + # cat events/uprobes/zfree_entry/format + name: zfree_entry + ID: 922 + 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:int common_padding; offset:8; size:4; signed:1; + + field:unsigned long __probe_ip; offset:12; size:4; signed:0; + field:u32 arg1; offset:16; size:4; signed:0; + field:u32 arg2; offset:20; size:4; signed:0; + + print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 + +Right after definition, each event is disabled by default. For tracing these +events, you need to enable it by:: + + # echo 1 > events/uprobes/enable + +Lets disable the event after sleeping for some time. +:: + + # sleep 20 + # echo 0 > events/uprobes/enable + +And you can see the traced information via /sys/kernel/debug/tracing/trace. +:: + + # cat trace + # tracer: nop + # + # TASK-PID CPU# TIMESTAMP FUNCTION + # | | | | | + zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 + zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 + zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 + zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 + +Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 +and contents of ax register being 79. And uretprobe was triggered with ip at +0x446540 with counterpart function entry at 0x446420. |