| Uprobe-tracer: Uprobe-based Event Tracing |
| ========================================= |
| |
| Documentation written by Srikar Dronamraju |
| |
| |
| Overview |
| -------- |
| Uprobe based trace events are similar to kprobe based trace events. |
| To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=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>/enabled. |
| |
| 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 '-: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 |
| |
| Please 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. |