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+=================================
+Using ftrace to hook to functions
+=================================
+
+.. Copyright 2017 VMware Inc.
+..   Author:   Steven Rostedt <srostedt@goodmis.org>
+..  License:   The GNU Free Documentation License, Version 1.2
+..               (dual licensed under the GPL v2)
+
+Written for: 4.14
+
+Introduction
+============
+
+The ftrace infrastructure was originially created to attach callbacks to the
+beginning of functions in order to record and trace the flow of the kernel.
+But callbacks to the start of a function can have other use cases. Either
+for live kernel patching, or for security monitoring. This document describes
+how to use ftrace to implement your own function callbacks.
+
+
+The ftrace context
+==================
+
+WARNING: The ability to add a callback to almost any function within the
+kernel comes with risks. A callback can be called from any context
+(normal, softirq, irq, and NMI). Callbacks can also be called just before
+going to idle, during CPU bring up and takedown, or going to user space.
+This requires extra care to what can be done inside a callback. A callback
+can be called outside the protective scope of RCU.
+
+The ftrace infrastructure has some protections agains recursions and RCU
+but one must still be very careful how they use the callbacks.
+
+
+The ftrace_ops structure
+========================
+
+To register a function callback, a ftrace_ops is required. This structure
+is used to tell ftrace what function should be called as the callback
+as well as what protections the callback will perform and not require
+ftrace to handle.
+
+There is only one field that is needed to be set when registering
+an ftrace_ops with ftrace::
+
+.. code-block: c
+
+ struct ftrace_ops ops = {
+       .func                    = my_callback_func,
+       .flags                   = MY_FTRACE_FLAGS
+       .private                 = any_private_data_structure,
+ };
+
+Both .flags and .private are optional. Only .func is required.
+
+To enable tracing call::
+
+.. c:function::  register_ftrace_function(&ops);
+
+To disable tracing call::
+
+.. c:function::  unregister_ftrace_function(&ops);
+
+The above is defined by including the header::
+
+.. c:function:: #include <linux/ftrace.h>
+
+The registered callback will start being called some time after the
+register_ftrace_function() is called and before it returns. The exact time
+that callbacks start being called is dependent upon architecture and scheduling
+of services. The callback itself will have to handle any synchronization if it
+must begin at an exact moment.
+
+The unregister_ftrace_function() will guarantee that the callback is
+no longer being called by functions after the unregister_ftrace_function()
+returns. Note that to perform this guarantee, the unregister_ftrace_function()
+may take some time to finish.
+
+
+The callback function
+=====================
+
+The prototype of the callback function is as follows (as of v4.14)::
+
+.. code-block: c
+
+ void callback_func(unsigned long ip, unsigned long parent_ip,
+                    struct ftrace_ops *op, struct pt_regs *regs);
+
+@ip
+         This is the instruction pointer of the function that is being traced.
+         (where the fentry or mcount is within the function)
+
+@parent_ip
+        This is the instruction pointer of the function that called the
+        the function being traced (where the call of the function occurred).
+
+@op
+        This is a pointer to ftrace_ops that was used to register the callback.
+        This can be used to pass data to the callback via the private pointer.
+
+@regs
+        If the FTRACE_OPS_FL_SAVE_REGS or FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
+        flags are set in the ftrace_ops structure, then this will be pointing
+        to the pt_regs structure like it would be if an breakpoint was placed
+        at the start of the function where ftrace was tracing. Otherwise it
+        either contains garbage, or NULL.
+
+
+The ftrace FLAGS
+================
+
+The ftrace_ops flags are all defined and documented in include/linux/ftrace.h.
+Some of the flags are used for internal infrastructure of ftrace, but the
+ones that users should be aware of are the following:
+
+FTRACE_OPS_FL_SAVE_REGS
+        If the callback requires reading or modifying the pt_regs
+        passed to the callback, then it must set this flag. Registering
+        a ftrace_ops with this flag set on an architecture that does not
+        support passing of pt_regs to the callback will fail.
+
+FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
+        Similar to SAVE_REGS but the registering of a
+        ftrace_ops on an architecture that does not support passing of regs
+        will not fail with this flag set. But the callback must check if
+        regs is NULL or not to determine if the architecture supports it.
+
+FTRACE_OPS_FL_RECURSION_SAFE
+        By default, a wrapper is added around the callback to
+        make sure that recursion of the function does not occur. That is,
+        if a function that is called as a result of the callback's execution
+        is also traced, ftrace will prevent the callback from being called
+        again. But this wrapper adds some overhead, and if the callback is
+        safe from recursion, it can set this flag to disable the ftrace
+        protection.
+
+        Note, if this flag is set, and recursion does occur, it could cause
+        the system to crash, and possibly reboot via a triple fault.
+
+        It is OK if another callback traces a function that is called by a
+        callback that is marked recursion safe. Recursion safe callbacks
+        must never trace any function that are called by the callback
+        itself or any nested functions that those functions call.
+
+        If this flag is set, it is possible that the callback will also
+        be called with preemption enabled (when CONFIG_PREEMPT is set),
+        but this is not guaranteed.
+
+FTRACE_OPS_FL_IPMODIFY
+        Requires FTRACE_OPS_FL_SAVE_REGS set. If the callback is to "hijack"
+        the traced function (have another function called instead of the
+        traced function), it requires setting this flag. This is what live
+        kernel patches uses. Without this flag the pt_regs->ip can not be
+        modified.
+
+        Note, only one ftrace_ops with FTRACE_OPS_FL_IPMODIFY set may be
+        registered to any given function at a time.
+
+FTRACE_OPS_FL_RCU
+        If this is set, then the callback will only be called by functions
+        where RCU is "watching". This is required if the callback function
+        performs any rcu_read_lock() operation.
+
+        RCU stops watching when the system goes idle, the time when a CPU
+        is taken down and comes back online, and when entering from kernel
+        to user space and back to kernel space. During these transitions,
+        a callback may be executed and RCU synchronization will not protect
+        it.
+
+
+Filtering which functions to trace
+==================================
+
+If a callback is only to be called from specific functions, a filter must be
+set up. The filters are added by name, or ip if it is known.
+
+.. code-block: c
+
+ int ftrace_set_filter(struct ftrace_ops *ops, unsigned char *buf,
+                       int len, int reset);
+
+@ops
+        The ops to set the filter with
+
+@buf
+        The string that holds the function filter text.
+@len
+        The length of the string.
+
+@reset
+        Non-zero to reset all filters before applying this filter.
+
+Filters denote which functions should be enabled when tracing is enabled.
+If @buf is NULL and reset is set, all functions will be enabled for tracing.
+
+The @buf can also be a glob expression to enable all functions that
+match a specific pattern.
+
+See Filter Commands in :file:`Documentation/trace/ftrace.txt`.
+
+To just trace the schedule function::
+
+.. code-block: c
+
+ ret = ftrace_set_filter(&ops, "schedule", strlen("schedule"), 0);
+
+To add more functions, call the ftrace_set_filter() more than once with the
+@reset parameter set to zero. To remove the current filter set and replace it
+with new functions defined by @buf, have @reset be non-zero.
+
+To remove all the filtered functions and trace all functions::
+
+.. code-block: c
+
+  ret = ftrace_set_filter(&ops, NULL, 0, 1);
+
+
+Sometimes more than one function has the same name. To trace just a specific
+function in this case, ftrace_set_filter_ip() can be used.
+
+.. code-block: c
+
+ ret = ftrace_set_filter_ip(&ops, ip, 0, 0);
+
+Although the ip must be the address where the call to fentry or mcount is
+located in the function. This function is used by perf and kprobes that
+gets the ip address from the user (usually using debug info from the kernel).
+
+If a glob is used to set the filter, functions can be added to a "notrace"
+list that will prevent those functions from calling the callback.
+The "notrace" list takes precedence over the "filter" list. If the
+two lists are non-empty and contain the same functions, the callback will not
+be called by any function.
+
+An empty "notrace" list means to allow all functions defined by the filter
+to be traced.
+
+.. code-block: c
+
+  int ftrace_set_notrace(struct ftrace_ops *ops, unsigned char *buf,
+                         int len, int reset);
+
+This takes the same parameters as ftrace_set_filter() but will add the
+functions it finds to not be traced. This is a separate list from the
+filter list, and this function does not modify the filter list.
+
+A non-zero @reset will clear the "notrace" list before adding functions
+that match @buf to it.
+
+Clearing the "notrace" list is the same as clearing the filter list
+
+.. code-block: c
+
+  ret = ftrace_set_notrace(&ops, NULL, 0, 1);
+
+The filter and notrace lists may be changed at any time. If only a set of
+functions should call the callback, it is best to set the filters before
+registering the callback. But the changes may also happen after the callback
+has been registered.
+
+If a filter is in place, and the @reset is non-zero, and @buf contains a
+matching glob to functions, the switch will happen during the time of
+the ftrace_set_filter() call. At no time will all functions call the callback.
+
+.. code-block: c
+
+        ftrace_set_filter(&ops, "schedule", strlen("schedule"), 1);
+
+        register_ftrace_function(&ops);
+
+        msleep(10);
+
+        ftrace_set_filter(&ops, "try_to_wake_up", strlen("try_to_wake_up"), 1);
+
+is not the same as:
+
+.. code-block: c
+
+        ftrace_set_filter(&ops, "schedule", strlen("schedule"), 1);
+
+        register_ftrace_function(&ops);
+
+        msleep(10);
+
+        ftrace_set_filter(&ops, NULL, 0, 1);
+
+        ftrace_set_filter(&ops, "try_to_wake_up", strlen("try_to_wake_up"), 0);
+
+As the latter will have a short time where all functions will call
+the callback, between the time of the reset, and the time of the
+new setting of the filter.