1 files changed, 348 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace-uses.rst b/Documentation/trace/ftrace-uses.rst
new file mode 100644
index 000000000..f7d98ae5b
--- /dev/null
+++ b/Documentation/trace/ftrace-uses.rst
@@ -0,0 +1,348 @@
+=================================
+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 originally 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.
+
+There are helper functions to help against recursion, and making sure
+RCU is watching. These are explained below.
+
+
+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::
+
+    register_ftrace_function(&ops);
+
+To disable tracing call::
+
+    unregister_ftrace_function(&ops);
+
+The above is defined by including the header::
+
+    #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.
+
+Protect your callback
+=====================
+
+As functions can be called from anywhere, and it is possible that a function
+called by a callback may also be traced, and call that same callback,
+recursion protection must be used. There are two helper functions that
+can help in this regard. If you start your code with:
+
+.. code-block:: c
+
+	int bit;
+
+	bit = ftrace_test_recursion_trylock(ip, parent_ip);
+	if (bit < 0)
+		return;
+
+and end it with:
+
+.. code-block:: c
+
+	ftrace_test_recursion_unlock(bit);
+
+The code in between will be safe to use, even if it ends up calling a
+function that the callback is tracing. Note, on success,
+ftrace_test_recursion_trylock() will disable preemption, and the
+ftrace_test_recursion_unlock() will enable it again (if it was previously
+enabled). The instruction pointer (ip) and its parent (parent_ip) is passed to
+ftrace_test_recursion_trylock() to record where the recursion happened
+(if CONFIG_FTRACE_RECORD_RECURSION is set).
+
+Alternatively, if the FTRACE_OPS_FL_RECURSION flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for recursion for the callback and no recursion test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+
+If your callback accesses any data or critical section that requires RCU
+protection, it is best to make sure that RCU is "watching", otherwise
+that data or critical section will not be protected as expected. In this
+case add:
+
+.. code-block:: c
+
+	if (!rcu_is_watching())
+		return;
+
+Alternatively, if the FTRACE_OPS_FL_RCU flag is set on the ftrace_ops
+(as explained below), then a helper trampoline will be used to test
+for rcu_is_watching for the callback and no other test needs to be done.
+But this is at the expense of a slightly more overhead from an extra
+function call.
+
+
+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
+	By default, it is expected that the callback can handle recursion.
+	But if the callback is not that worried about overehead, then
+	setting this bit will add the recursion protection around the
+	callback by calling a helper function that will do the recursion
+	protection and only call the callback if it did not recurse.
+
+	Note, if this flag is not set, and recursion does occur, it could
+	cause the system to crash, and possibly reboot via a triple fault.
+
+	Not, if this flag is set, then the callback will always be called
+	with preemption disabled. If it is not set, then it is possible
+	(but not guaranteed) that the callback will be called in
+	preemptable context.
+
+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.
+
+FTRACE_OPS_FL_PERMANENT
+        If this is set on any ftrace ops, then the tracing cannot disabled by
+        writing 0 to the proc sysctl ftrace_enabled. Equally, a callback with
+        the flag set cannot be registered if ftrace_enabled is 0.
+
+        Livepatch uses it not to lose the function redirection, so the system
+        stays protected.
+
+
+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.rst`.
+
+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.