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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 <mhiramat@kernel.org> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org> Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/164735294272.1084943.12372175959382037397.stgit@devnote2
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Documentation/trace/fprobe.rst
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Documentation/trace/fprobe.rst
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.. SPDX-License-Identifier: GPL-2.0
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==================================
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Fprobe - Function entry/exit probe
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==================================
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.. Author: Masami Hiramatsu <mhiramat@kernel.org>
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Introduction
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============
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Fprobe is a function entry/exit probe mechanism based on ftrace.
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Instead of using ftrace full feature, if you only want to attach callbacks
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on function entry and exit, similar to the kprobes and kretprobes, you can
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use fprobe. Compared with kprobes and kretprobes, fprobe gives faster
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instrumentation for multiple functions with single handler. This document
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describes how to use fprobe.
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The usage of fprobe
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===================
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The fprobe is a wrapper of ftrace (+ kretprobe-like return callback) to
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attach callbacks to multiple function entry and exit. User needs to set up
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the `struct fprobe` and pass it to `register_fprobe()`.
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Typically, `fprobe` data structure is initialized with the `entry_handler`
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and/or `exit_handler` as below.
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.. code-block:: c
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struct fprobe fp = {
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.entry_handler = my_entry_callback,
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.exit_handler = my_exit_callback,
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};
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To enable the fprobe, call one of register_fprobe(), register_fprobe_ips(), and
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register_fprobe_syms(). These functions register the fprobe with different types
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of parameters.
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The register_fprobe() enables a fprobe by function-name filters.
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E.g. this enables @fp on "func*()" function except "func2()".::
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register_fprobe(&fp, "func*", "func2");
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The register_fprobe_ips() enables a fprobe by ftrace-location addresses.
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E.g.
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.. code-block:: c
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unsigned long ips[] = { 0x.... };
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register_fprobe_ips(&fp, ips, ARRAY_SIZE(ips));
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And the register_fprobe_syms() enables a fprobe by symbol names.
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E.g.
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.. code-block:: c
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char syms[] = {"func1", "func2", "func3"};
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register_fprobe_syms(&fp, syms, ARRAY_SIZE(syms));
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To disable (remove from functions) this fprobe, call::
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unregister_fprobe(&fp);
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You can temporally (soft) disable the fprobe by::
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disable_fprobe(&fp);
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and resume by::
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enable_fprobe(&fp);
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The above is defined by including the header::
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#include <linux/fprobe.h>
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Same as ftrace, the registered callbacks will start being called some time
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after the register_fprobe() is called and before it returns. See
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:file:`Documentation/trace/ftrace.rst`.
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Also, the unregister_fprobe() will guarantee that the both enter and exit
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handlers are no longer being called by functions after unregister_fprobe()
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returns as same as unregister_ftrace_function().
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The fprobe entry/exit handler
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=============================
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The prototype of the entry/exit callback function is as follows:
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.. code-block:: c
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void callback_func(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs);
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Note that both entry and exit callbacks have same ptototype. The @entry_ip is
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saved at function entry and passed to exit handler.
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@fp
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This is the address of `fprobe` data structure related to this handler.
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You can embed the `fprobe` to your data structure and get it by
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container_of() macro from @fp. The @fp must not be NULL.
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@entry_ip
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This is the ftrace address of the traced function (both entry and exit).
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Note that this may not be the actual entry address of the function but
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the address where the ftrace is instrumented.
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@regs
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This is the `pt_regs` data structure at the entry and exit. Note that
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the instruction pointer of @regs may be different from the @entry_ip
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in the entry_handler. If you need traced instruction pointer, you need
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to use @entry_ip. On the other hand, in the exit_handler, the instruction
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pointer of @regs is set to the currect return address.
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Share the callbacks with kprobes
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================================
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Since the recursion safeness of the fprobe (and ftrace) is a bit different
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from the kprobes, this may cause an issue if user wants to run the same
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code from the fprobe and the kprobes.
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Kprobes has per-cpu 'current_kprobe' variable which protects the kprobe
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handler from recursion in all cases. On the other hand, fprobe uses
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only ftrace_test_recursion_trylock(). This allows interrupt context to
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call another (or same) fprobe while the fprobe user handler is running.
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This is not a matter if the common callback code has its own recursion
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detection, or it can handle the recursion in the different contexts
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(normal/interrupt/NMI.)
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But if it relies on the 'current_kprobe' recursion lock, it has to check
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kprobe_running() and use kprobe_busy_*() APIs.
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Fprobe has FPROBE_FL_KPROBE_SHARED flag to do this. If your common callback
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code will be shared with kprobes, please set FPROBE_FL_KPROBE_SHARED
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*before* registering the fprobe, like:
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.. code-block:: c
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fprobe.flags = FPROBE_FL_KPROBE_SHARED;
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register_fprobe(&fprobe, "func*", NULL);
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This will protect your common callback from the nested call.
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The missed counter
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==================
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The `fprobe` data structure has `fprobe::nmissed` counter field as same as
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kprobes.
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This counter counts up when;
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- fprobe fails to take ftrace_recursion lock. This usually means that a function
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which is traced by other ftrace users is called from the entry_handler.
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- fprobe fails to setup the function exit because of the shortage of rethook
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(the shadow stack for hooking the function return.)
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The `fprobe::nmissed` field counts up in both cases. Therefore, the former
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skips both of entry and exit callback and the latter skips the exit
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callback, but in both case the counter will increase by 1.
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Note that if you set the FTRACE_OPS_FL_RECURSION and/or FTRACE_OPS_FL_RCU to
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`fprobe::ops::flags` (ftrace_ops::flags) when registering the fprobe, this
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counter may not work correctly, because ftrace skips the fprobe function which
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increase the counter.
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Functions and structures
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========================
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.. kernel-doc:: include/linux/fprobe.h
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.. kernel-doc:: kernel/trace/fprobe.c
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@ -9,6 +9,7 @@ Linux Tracing Technologies
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tracepoint-analysis
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ftrace
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ftrace-uses
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fprobe
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kprobes
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kprobetrace
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uprobetracer
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