[PATCH v16 17/18] Documentation: probes: Update fprobe on function-graph tracer

Masami Hiramatsu (Google)
2024-10-15T01:31:53Z

Update fprobe documentation for the new fprobe on function-graph
tracer. This includes some bahvior changes and pt_regs to
ftrace_regs interface change.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v2:
- Update @fregs parameter explanation.
---
Documentation/trace/fprobe.rst | 42 ++++++++++++++++++++++++++--------------
1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst
index 196f52386aaa..f58bdc64504f 100644
--- a/Documentation/trace/fprobe.rst
+++ b/Documentation/trace/fprobe.rst
@@ -9,9 +9,10 @@ Fprobe - Function entry/exit probe
Introduction
============

-Fprobe is a function entry/exit probe mechanism based on ftrace.
-Instead of using ftrace full feature, if you only want to attach callbacks
-on function entry and exit, similar to the kprobes and kretprobes, you can
+Fprobe is a function entry/exit probe mechanism based on the function-graph
+tracer.
+Instead of tracing all functions, if you want to attach callbacks on specific
+function entry and exit, similar to the kprobes and kretprobes, you can
use fprobe. Compared with kprobes and kretprobes, fprobe gives faster
instrumentation for multiple functions with single handler. This document
describes how to use fprobe.
@@ -91,12 +92,14 @@ The prototype of the entry/exit callback function are as follows:

.. code-block:: c

- int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
+ int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data);

- void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
+ void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data);

-Note that the @entry_ip is saved at function entry and passed to exit handler.
-If the entry callback function returns !0, the corresponding exit callback will be cancelled.
+Note that the @entry_ip is saved at function entry and passed to exit
+handler.
+If the entry callback function returns !0, the corresponding exit callback
+will be cancelled.

@fp
This is the address of `fprobe` data structure related to this handler.
@@ -112,12 +115,10 @@ If the entry callback function returns !0, the corresponding exit callback will
This is the return address that the traced function will return to,
somewhere in the caller. This can be used at both entry and exit.

-@regs
- This is the `pt_regs` data structure at the entry and exit. Note that
- the instruction pointer of @regs may be different from the @entry_ip
- in the entry_handler. If you need traced instruction pointer, you need
- to use @entry_ip. On the other hand, in the exit_handler, the instruction
- pointer of @regs is set to the current return address.
+@fregs
+ This is the `ftrace_regs` data structure at the entry and exit. This
+ includes the function parameters, or the return values. So user can
+ access thos values via appropriate `ftrace_regs_*` APIs.

@entry_data
This is a local storage to share the data between entry and exit handlers.
@@ -125,6 +126,17 @@ If the entry callback function returns !0, the corresponding exit callback will
and `entry_data_size` field when registering the fprobe, the storage is
allocated and passed to both `entry_handler` and `exit_handler`.

+Entry data size and exit handlers on the same function
+======================================================
+
+Since the entry data is passed via per-task stack and it is has limited size,
+the entry data size per probe is limited to `15 * sizeof(long)`. You also need
+to take care that the different fprobes are probing on the same function, this
+limit becomes smaller. The entry data size is aligned to `sizeof(long)` and
+each fprobe which has exit handler uses a `sizeof(long)` space on the stack,
+you should keep the number of fprobes on the same function as small as
+possible.
+
Share the callbacks with kprobes
================================

@@ -165,8 +177,8 @@ This counter counts up when;
- fprobe fails to take ftrace_recursion lock. This usually means that a function
which is traced by other ftrace users is called from the entry_handler.

- - fprobe fails to setup the function exit because of the shortage of rethook
- (the shadow stack for hooking the function return.)
+ - fprobe fails to setup the function exit because of failing to allocate the
+ data buffer from the per-task shadow stack.

The `fprobe::nmissed` field counts up in both cases. Therefore, the former
skips both of entry and exit callback and the latter skips the exit