Hi,
Here is the 10th version of the series to re-implement the fprobe on
function-graph tracer. The previous version is;
https://lore.kernel.org/all/171318533841.254850.15841395205784342850.stgit@devnote2/
This version is ported on the latest kernel (v6.9-rc6 + probes/for-next)
and fixed some bugs + performance optimizations.
- [7/36] Fix terminology in comments and code. Use "offset" instead
of "index" for shadow stack. This also update macros.
- [18/36] Fix supported data size bug.
- [29/36] Define bpf_kprobe_multi_pt_regs only if it is used.
- [36/36] Add likely() to skip timestamp.
Overview
This series does major 2 changes, enable multiple function-graphs on
the ftrace (e.g. allow function-graph on sub instances) and rewrite the
fprobe on this function-graph.
The former changes had been sent from Steven Rostedt 4 years ago (*),
which allows users to set different setting function-graph tracer (and
other tracers based on function-graph) in each trace-instances at the
same time.
(*) https://lore.kernel.org/all/20190525031633.811342...@goodmis.org/
The purpose of latter change are;
1) Remove dependency of the rethook from fprobe so that we can reduce
the return hook code and shadow stack.
2) Make 'ftrace_regs' the common trace interface for the function
boundary.
1) Currently we have 2(or 3) different function return hook codes,
the function-graph tracer and rethook (and legacy kretprobe).
But since this is redundant and needs double maintenance cost,
I would like to unify those. From the user's viewpoint, function-
graph tracer is very useful to grasp the execution path. For this
purpose, it is hard to use the rethook in the function-graph
tracer, but the opposite is possible. (Strictly speaking, kretprobe
can not use it because it requires 'pt_regs' for historical reasons.)
2) Now the fprobe provides the 'pt_regs' for its handler, but that is
wrong for the function entry and exit. Moreover, depending on the
architecture, there is no way to accurately reproduce 'pt_regs'
outside of interrupt or exception handlers. This means fprobe should
not use 'pt_regs' because it does not use such exceptions.
(Conversely, kprobe should use 'pt_regs' because it is an abstract
interface of the software breakpoint exception.)
This series changes fprobe to use function-graph tracer for tracing
function entry and exit, instead of mixture of ftrace and rethook.
Unlike the rethook which is a per-task list of system-wide allocated
nodes, the function graph's ret_stack is a per-task shadow stack.
Thus it does not need to set 'nr_maxactive' (which is the number of
pre-allocated nodes).
Also the handlers will get the 'ftrace_regs' instead of 'pt_regs'.
Since eBPF mulit_kprobe/multi_kretprobe events still use 'pt_regs' as
their register interface, this changes it to convert 'ftrace_regs' to
'pt_regs'. Of course this conversion makes an incomplete 'pt_regs',
so users must access only registers for function parameters or
return value.
Design
--
Instead of using ftrace's function entry hook directly, the new fprobe
is built on top of the function-graph's entry and return callbacks
with 'ftrace_regs'.
Since the fprobe requires access to 'ftrace_regs', the architecture
must support CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS and
CONFIG_HAVE_FTRACE_GRAPH_FUNC, which enables to call function-graph
entry callback with 'ftrace_regs', and also
CONFIG_HAVE_FUNCTION_GRAPH_FREGS, which passes the ftrace_regs to
return_to_handler.
All fprobes share a single function-graph ops (means shares a common
ftrace filter) similar to the kprobe-on-ftrace. This needs another
layer to find corresponding fprobe in the common function-graph
callbacks, but has much better scalability, since the number of
registered function-graph ops is limited.
In the entry callback, the fprobe runs its entry_handler and saves the
address of 'fprobe' on the function-graph's shadow stack as data. The
return callback decodes the data to get the 'fprobe' address, and runs
the exit_handler.
The fprobe introduces two hash-tables, one is for entry callback which
searches fprobes related to the given function address passed by entry
callback. The other is for a return callback which checks if the given
'fprobe' data structure pointer is still valid. Note that it is
possible to unregister fprobe before the return callback runs. Thus
the address validation must be done before using it in the return
callback.
This series can be applied against the probes/for-next branch, which
is based on v6.9-rc6.
This series can also be found below branch.
https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/log/?h=topic/fprobe-on-fgraph
Thank you,
---
Masami Hiramatsu (Google) (21):
tracing: Add a comment about ftrace_regs definition
tracing: Rename ftrace_regs_return_value to ftrace_regs_get_return_value
x86: tracing: Add ftrace_regs definition in the header
function_graph: Use a simple LRU f
