349 lines
12 KiB
ReStructuredText
349 lines
12 KiB
ReStructuredText
=================================
|
|
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.
|