From aba09b44a985f963f3ce22132694161f79e18984 Mon Sep 17 00:00:00 2001 From: Masami Hiramatsu Date: Tue, 15 Mar 2022 23:02:22 +0900 Subject: [PATCH] docs: fprobe: Add fprobe description to ftrace-use.rst Add a documentation of fprobe for the user who needs this interface. Signed-off-by: Masami Hiramatsu Signed-off-by: Steven Rostedt (Google) Tested-by: Steven Rostedt (Google) Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/164735294272.1084943.12372175959382037397.stgit@devnote2 --- Documentation/trace/fprobe.rst | 174 +++++++++++++++++++++++++++++++++ Documentation/trace/index.rst | 1 + 2 files changed, 175 insertions(+) create mode 100644 Documentation/trace/fprobe.rst diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst new file mode 100644 index 000000000000..b64bec1ce144 --- /dev/null +++ b/Documentation/trace/fprobe.rst @@ -0,0 +1,174 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================== +Fprobe - Function entry/exit probe +================================== + +.. Author: Masami Hiramatsu + +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. + +.. code-block:: c + + 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. + +.. code-block:: c + + 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. + +.. code-block:: c + + 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 + +Same as ftrace, the registered callbacks will start being called some time +after the register_fprobe() is called and before it returns. See +:file:`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 is as follows: + +.. code-block:: c + + void callback_func(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs); + +Note that both entry and exit callbacks have same ptototype. The @entry_ip is +saved at function entry and passed to exit handler. + +@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. + +Share the callbacks with kprobes +================================ + +Since the recursion safeness of the fprobe (and ftrace) is a bit different +from the kprobes, this may cause an issue if user wants to run the same +code from the fprobe and the kprobes. + +Kprobes has per-cpu 'current_kprobe' variable which protects the kprobe +handler from recursion in all cases. On the other hand, fprobe uses +only ftrace_test_recursion_trylock(). This allows interrupt context to +call another (or same) fprobe while the fprobe user handler is running. + +This is not a matter if the common callback code has its own recursion +detection, or it can handle the recursion in the different contexts +(normal/interrupt/NMI.) +But if it relies on the 'current_kprobe' recursion lock, it has to check +kprobe_running() and use kprobe_busy_*() APIs. + +Fprobe has FPROBE_FL_KPROBE_SHARED flag to do this. If your common callback +code will be shared with kprobes, please set FPROBE_FL_KPROBE_SHARED +*before* registering the fprobe, like: + +.. code-block:: c + + fprobe.flags = FPROBE_FL_KPROBE_SHARED; + + register_fprobe(&fprobe, "func*", NULL); + +This will protect your common callback from the nested call. + +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 +======================== + +.. kernel-doc:: include/linux/fprobe.h +.. kernel-doc:: kernel/trace/fprobe.c + diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst index 3769b9b7aed8..b9f3757f8269 100644 --- a/Documentation/trace/index.rst +++ b/Documentation/trace/index.rst @@ -9,6 +9,7 @@ Linux Tracing Technologies tracepoint-analysis ftrace ftrace-uses + fprobe kprobes kprobetrace uprobetracer