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

From: Steven Rostedt
Date: Mon Oct 09 2017 - 15:32:38 EST



With the coming removal of jprobes, using ftrace callbacks is one of the
utilities that replace the jprobes functionality. Having a document that
explains how to use ftrace as such will help in the transition from jprobes
to ftrace. This document is for kernel developers that require attaching a
callback to a function within the kernel.

Link: http://lkml.kernel.org/r/150724519527.5014.10207042218696587159.stgit@devbox

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
Documentation/trace/ftrace-uses.rst | 297 ++++++++++++++++++++++++++++++++++++
1 file changed, 297 insertions(+)
create mode 100644 Documentation/trace/ftrace-uses.rst

diff --git a/Documentation/trace/ftrace-uses.rst b/Documentation/trace/ftrace-uses.rst
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+ Using ftrace to hook to functions
+ =================================
+
+Copyright 2017 VMware Inc.
+ Author: Steven Rostedt <srostedt@xxxxxxxxxxx>
+ 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::
+
+ 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)::
+
+ 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_PER_CPU
+ When set, the callback is only when the defined CPU has been
+ "enabled" to do so. After the callback is registered, all
+ CPUs are set to disable, and the callback is not called.
+ In order to enable or disalble a CPU to call a given callback,
+ use one of the following functions.
+
+::
+
+| void ftrace_function_local_enable(struct ftrace_ops *ops);
+| void ftrace_function_local_disable(struct ftrace_ops *ops);
+
+ These two functions must be called with preemption disabled.
+
+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.
+
+::
+
+ 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::
+
+ 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::
+
+
+ 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.
+
+::
+
+ 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.
+
+::
+
+ 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::
+
+ 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.
+
+::
+
+ 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:
+
+::
+
+ 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.
--
2.9.5