Re: [RFC][PATCH 00/14 v2] function_graph: Rewrite to allow multiple users

[Masami Hiramatsu]

On Mon, 20 May 2019 10:20:01 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> 
> The background for this is explained in the V1 version found here:
> 
>  http://lkml.kernel.org/r/20181122012708.491151844@xxxxxxxxxxx
> 
> The TL;DR; is this:
> 
>  The function graph tracer required a rewrite, mainly because it
>  can only allow one callback registered at a time. The main motivation
>  for this change is to allow kretprobes to use the code of function
>  graph tracer, which should allow all archs that have function graph
>  tracing to also have kretprobes with no extra work.
> 
> Masami told me that one requirement was to allow the function entry
> callback to store data on the shadow stack that can be retrieved by
> the the function return callback. I added this, as well as a per-task
> variable (used by one of the function graph users).
> 
> The two functions to allow the storing of data on the stack and
> retrieval of it are:
> 
>  void *fgraph_reserve_data(int size_in_bytes)
> 
>     Allows the entry function to reserve up to 4 words of data on
>     the shadow stack. On success, a pointer to the contents is returned.
>     This may be only called once per entry function.
> 
>  void *fgraph_retrieve_data(void)
> 
>     Allows the return function to retrieve the reserved data that was
>     allocated by the entry function.

Nice! this seems good for kretprobe too. I'll review and try to port
kretprobe on this framework.

Thank you!

> 
> Note, this code has passed my full test suite.
> 
> Changes since v1:
> 
>   - Well, the first part of that series was already merged.
>     But that was just the preparation for this part.
> 
>   - Allocate a page for the shadow stack split it up that way.
>     When the stack is full, we stop allowing more to be added (stop tracing).
> 
>   - Added the reserve and retrieve of private data on the shadow stack
>     for individual entry/return callbacks to pass data to each other.
> 
>   - Added a "per task" data that can be used by a fgraph_ops for all
>     function callbacks for a specific task.
> 
> Steven Rostedt (VMware) (14):
>       function_graph: Convert ret_stack to a series of longs
>       function_graph: Add an array structure that will allow multiple callbacks
>       function_graph: Allow multiple users to attach to function graph
>       function_graph: Remove logic around ftrace_graph_entry and return
>       ftrace/function_graph: Pass fgraph_ops to function graph callbacks
>       ftrace: Allow function_graph tracer to be enabled in instances
>       ftrace: Allow ftrace startup flags exist without dynamic ftrace
>       function_graph: Have the instances use their own ftrace_ops for filtering
>       function_graph: Add "task variables" per task for fgraph_ops
>       function_graph: Move set_graph_function tests to shadow stack global var
>       function_graph: Move graph depth stored data to shadow stack global var
>       function_graph: Move graph notrace bit to shadow stack global var
>       function_graph: Implement fgraph_reserve_data() and fgraph_retrieve_data()
>       function_graph: Add selftest for passing local variables
> 
> ----
>  include/linux/ftrace.h               |  37 +-
>  include/linux/sched.h                |   2 +-
>  kernel/trace/fgraph.c                | 862 ++++++++++++++++++++++++++++-------
>  kernel/trace/ftrace.c                |  13 +-
>  kernel/trace/ftrace_internal.h       |   2 -
>  kernel/trace/trace.h                 | 132 +++---
>  kernel/trace/trace_functions.c       |   7 +
>  kernel/trace/trace_functions_graph.c |  96 ++--
>  kernel/trace/trace_irqsoff.c         |  10 +-
>  kernel/trace/trace_sched_wakeup.c    |  10 +-
>  kernel/trace/trace_selftest.c        | 168 ++++++-
>  11 files changed, 1048 insertions(+), 291 deletions(-)


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>