ftrace/docs: Add documentation on how to use ftrace from within the kernel

=================================

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.