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 use fprobe. Compared with kprobes and kretprobes, fprobe gives faster instrumentation for multiple functions with single handler. This document describes how to use fprobe.
The usage of fprobe¶
The fprobe is a wrapper of ftrace (+ kretprobe-like return callback) to attach callbacks to multiple function entry and exit. User needs to set up the struct fprobe and pass it to register_fprobe().
Typically, fprobe data structure is initialized with the entry_handler and/or exit_handler as below.
struct fprobe fp = {
.entry_handler = my_entry_callback,
.exit_handler = my_exit_callback,
};
To enable the fprobe, call one of register_fprobe(), register_fprobe_ips(), and register_fprobe_syms(). These functions register the fprobe with different types of parameters.
The register_fprobe() enables a fprobe by function-name filters. E.g. this enables @fp on "func*()" function except "func2()".:
register_fprobe(&fp, "func*", "func2");
The register_fprobe_ips() enables a fprobe by ftrace-location addresses. E.g.
unsigned long ips[] = { 0x.... };
register_fprobe_ips(&fp, ips, ARRAY_SIZE(ips));
And the register_fprobe_syms() enables a fprobe by symbol names. E.g.
char syms[] = {"func1", "func2", "func3"};
register_fprobe_syms(&fp, syms, ARRAY_SIZE(syms));
To disable (remove from functions) this fprobe, call:
unregister_fprobe(&fp);
You can temporally (soft) disable the fprobe by:
disable_fprobe(&fp);
and resume by:
enable_fprobe(&fp);
The above is defined by including the header:
#include <linux/fprobe.h>
Same as ftrace, the registered callbacks will start being called some time
after the register_fprobe() is called and before it returns. See
Documentation/trace/ftrace.rst
.
Also, the unregister_fprobe() will guarantee that the both enter and exit handlers are no longer being called by functions after unregister_fprobe() returns as same as unregister_ftrace_function().
The fprobe entry/exit handler¶
The prototype of the entry/exit callback function are as follows:
int entry_callback(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs, void *entry_data);
void exit_callback(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs, 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.
- @fp
This is the address of fprobe data structure related to this handler. You can embed the fprobe to your data structure and get it by container_of() macro from @fp. The @fp must not be NULL.
- @entry_ip
This is the ftrace address of the traced function (both entry and exit). Note that this may not be the actual entry address of the function but the address where the ftrace is instrumented.
- @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 currect return address.
- @entry_data
This is a local storage to share the data between entry and exit handlers. This storage is NULL by default. If the user specify exit_handler field and entry_data_size field when registering the fprobe, the storage is allocated and passed to both entry_handler and exit_handler.
The missed counter¶
The fprobe data structure has fprobe::nmissed counter field as same as kprobes. 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.)
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 callback, but in both case the counter will increase by 1.
Note that if you set the FTRACE_OPS_FL_RECURSION and/or FTRACE_OPS_FL_RCU to fprobe::ops::flags (ftrace_ops::flags) when registering the fprobe, this counter may not work correctly, because ftrace skips the fprobe function which increase the counter.
Functions and structures¶
Error
kernel-doc missing
Error
kernel-doc missing