[PATCH v5 3.1.0-rc4-tip 21/26] tracing: uprobes Documentation

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

Documents how to trace user space applications using uprobe tracer.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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+		Uprobe-tracer: Uprobe-based Event Tracing
+		=========================================
+                 Documentation is written by Srikar Dronamraju
+
+Overview
+--------
+These events are similar to kprobe based 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>/enabled.
+
+
+Synopsis of uprobe_tracer
+-------------------------
+  p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS]	: Set a probe
+
+ GRP		: Group name. If omitted, use "uprobes" for it.
+ EVENT		: Event name. If omitted, the event name is generated
+		  based on SYMBOL+offs.
+ PATH		: path to an executable or a library.
+ SYMBOL[+offs]	: Symbol+offset where the probe is inserted.
+
+ FETCHARGS	: Arguments. Each probe can have up to 128 args.
+  %REG		: Fetch register REG
+
+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
+--------------
+To add a probe as a new event, write a new definition to uprobe_events
+as below.
+
+  echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
+
+ This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash
+
+
+  echo > /sys/kernel/debug/tracing/uprobe_events
+
+ This clears all probe points.
+
+The following example shows how to dump the instruction pointer and %ax
+a register at the probed text address.  Here we are trying to probe
+function zfree 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 probe would be :
+
+    # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
+
+We can see the events that are registered by looking at the uprobe_events
+file.
+
+    # cat uprobe_events
+    p:uprobes/p_zsh_0x46420 /bin/zsh:0x0000000000046420
+
+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: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
+                 zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
+                 zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
+                 zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
+
+Each line shows us probes were triggered for a pid 24842 with ip being
+0x446421 and contents of ax register being 79.
+

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