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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
| commit | abdaadbcae30fe0c9a66c7516798279fdfd97750 (patch) | |
| tree | 00a54a6e25601e43876d03c1a4a12a749d4a914c /share/doc/gdb/Unwinding-Frames-in-Python.html | |
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diff --git a/share/doc/gdb/Unwinding-Frames-in-Python.html b/share/doc/gdb/Unwinding-Frames-in-Python.html new file mode 100644 index 0000000..e759f7c --- /dev/null +++ b/share/doc/gdb/Unwinding-Frames-in-Python.html @@ -0,0 +1,282 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Free Software" and "Free Software Needs +Free Documentation", with the Front-Cover Texts being "A GNU Manual," +and with the Back-Cover Texts as in (a) below. + +(a) The FSF's Back-Cover Text is: "You are free to copy and modify +this GNU Manual. Buying copies from GNU Press supports the FSF in +developing GNU and promoting software freedom." --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: Unwinding Frames in Python</title> + +<meta name="description" content="Debugging with GDB: Unwinding Frames in Python"> +<meta name="keywords" content="Debugging with GDB: Unwinding Frames in Python"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Python-API.html#Python-API" rel="up" title="Python API"> +<link href="Xmethods-In-Python.html#Xmethods-In-Python" rel="next" title="Xmethods In Python"> +<link href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" rel="previous" title="Writing a Frame Filter"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Unwinding-Frames-in-Python"></a> +<div class="header"> +<p> +Next: <a href="Xmethods-In-Python.html#Xmethods-In-Python" accesskey="n" rel="next">Xmethods In Python</a>, Previous: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="p" rel="previous">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Unwinding-Frames-in-Python-1"></a> +<h4 class="subsubsection">23.3.2.12 Unwinding Frames in Python</h4> +<a name="index-unwinding-frames-in-Python"></a> + +<p>In <small>GDB</small> terminology “unwinding” is the process of finding +the previous frame (that is, caller’s) from the current one. An +unwinder has three methods. The first one checks if it can handle +given frame (“sniff” it). For the frames it can sniff an unwinder +provides two additional methods: it can return frame’s ID, and it can +fetch registers from the previous frame. A running <small>GDB</small> +mantains a list of the unwinders and calls each unwinder’s sniffer in +turn until it finds the one that recognizes the current frame. There +is an API to register an unwinder. +</p> +<p>The unwinders that come with <small>GDB</small> handle standard frames. +However, mixed language applications (for example, an application +running Java Virtual Machine) sometimes use frame layouts that cannot +be handled by the <small>GDB</small> unwinders. You can write Python code +that can handle such custom frames. +</p> +<p>You implement a frame unwinder in Python as a class with which has two +attributes, <code>name</code> and <code>enabled</code>, with obvious meanings, and +a single method <code>__call__</code>, which examines a given frame and +returns an object (an instance of <code>gdb.UnwindInfo class)</code> +describing it. If an unwinder does not recognize a frame, it should +return <code>None</code>. The code in <small>GDB</small> that enables writing +unwinders in Python uses this object to return frame’s ID and previous +frame registers when <small>GDB</small> core asks for them. +</p> +<p>An unwinder should do as little work as possible. Some otherwise +innocuous operations can cause problems (even crashes, as this code is +not not well-hardened yet). For example, making an inferior call from +an unwinder is unadvisable, as an inferior call will reset +<small>GDB</small>’s stack unwinding process, potentially causing re-entrant +unwinding. +</p> +<a name="Unwinder-Input"></a> +<h4 class="subheading">Unwinder Input</h4> + +<p>An object passed to an unwinder (a <code>gdb.PendingFrame</code> instance) +provides a method to read frame’s registers: +</p> +<dl> +<dt><a name="index-PendingFrame_002eread_005fregister"></a>Function: <strong>PendingFrame.read_register</strong> <em>(reg)</em></dt> +<dd><p>This method returns the contents of the register <var>reg</var> in the +frame as a <code>gdb.Value</code> object. For a description of the +acceptable values of <var>reg</var> see +<a href="Frames-In-Python.html#gdbpy_005fframe_005fread_005fregister">Frame.read_register</a>. If <var>reg</var> +does not name a register for the current architecture, this method +will throw an exception. +</p> +<p>Note that this method will always return a <code>gdb.Value</code> for a +valid register name. This does not mean that the value will be valid. +For example, you may request a register that an earlier unwinder could +not unwind—the value will be unavailable. Instead, the +<code>gdb.Value</code> returned from this method will be lazy; that is, its +underlying bits will not be fetched until it is first used. So, +attempting to use such a value will cause an exception at the point of +use. +</p> +<p>The type of the returned <code>gdb.Value</code> depends on the register and +the architecture. It is common for registers to have a scalar type, +like <code>long long</code>; but many other types are possible, such as +pointer, pointer-to-function, floating point or vector types. +</p></dd></dl> + +<p>It also provides a factory method to create a <code>gdb.UnwindInfo</code> +instance to be returned to <small>GDB</small>: +</p> +<dl> +<dt><a name="index-PendingFrame_002ecreate_005funwind_005finfo"></a>Function: <strong>PendingFrame.create_unwind_info</strong> <em>(frame_id)</em></dt> +<dd><p>Returns a new <code>gdb.UnwindInfo</code> instance identified by given +<var>frame_id</var>. The argument is used to build <small>GDB</small>’s frame ID +using one of functions provided by <small>GDB</small>. <var>frame_id</var>’s attributes +determine which function will be used, as follows: +</p> +<dl compact="compact"> +<dt><code>sp, pc</code></dt> +<dd><p>The frame is identified by the given stack address and PC. The stack +address must be chosen so that it is constant throughout the lifetime +of the frame, so a typical choice is the value of the stack pointer at +the start of the function—in the DWARF standard, this would be the +“Call Frame Address”. +</p> +<p>This is the most common case by far. The other cases are documented +for completeness but are only useful in specialized situations. +</p> +</dd> +<dt><code>sp, pc, special</code></dt> +<dd><p>The frame is identified by the stack address, the PC, and a +“special” address. The special address is used on architectures +that can have frames that do not change the stack, but which are still +distinct, for example the IA-64, which has a second stack for +registers. Both <var>sp</var> and <var>special</var> must be constant +throughout the lifetime of the frame. +</p> +</dd> +<dt><code>sp</code></dt> +<dd><p>The frame is identified by the stack address only. Any other stack +frame with a matching <var>sp</var> will be considered to match this frame. +Inside gdb, this is called a “wild frame”. You will never need +this. +</p></dd> +</dl> + +<p>Each attribute value should be an instance of <code>gdb.Value</code>. +</p> +</dd></dl> + +<dl> +<dt><a name="index-PendingFrame_002earchitecture"></a>Function: <strong>PendingFrame.architecture</strong> <em>()</em></dt> +<dd><p>Return the <code>gdb.Architecture</code> (see <a href="Architectures-In-Python.html#Architectures-In-Python">Architectures In Python</a>) +for this <code>gdb.PendingFrame</code>. This represents the architecture of +the particular frame being unwound. +</p></dd></dl> + +<dl> +<dt><a name="index-PendingFrame_002elevel"></a>Function: <strong>PendingFrame.level</strong> <em>()</em></dt> +<dd><p>Return an integer, the stack frame level for this frame. +See <a href="Frames.html#Frames">Stack Frames</a>. +</p></dd></dl> + +<a name="Unwinder-Output_003a-UnwindInfo"></a> +<h4 class="subheading">Unwinder Output: UnwindInfo</h4> + +<p>Use <code>PendingFrame.create_unwind_info</code> method described above to +create a <code>gdb.UnwindInfo</code> instance. Use the following method to +specify caller registers that have been saved in this frame: +</p> +<dl> +<dt><a name="index-gdb_002eUnwindInfo_002eadd_005fsaved_005fregister"></a>Function: <strong>gdb.UnwindInfo.add_saved_register</strong> <em>(reg, value)</em></dt> +<dd><p><var>reg</var> identifies the register, for a description of the acceptable +values see <a href="Frames-In-Python.html#gdbpy_005fframe_005fread_005fregister">Frame.read_register</a>. +<var>value</var> is a register value (a <code>gdb.Value</code> object). +</p></dd></dl> + +<a name="Unwinder-Skeleton-Code"></a> +<h4 class="subheading">Unwinder Skeleton Code</h4> + +<p><small>GDB</small> comes with the module containing the base <code>Unwinder</code> +class. Derive your unwinder class from it and structure the code as +follows: +</p> +<div class="smallexample"> +<pre class="smallexample">from gdb.unwinders import Unwinder + +class FrameId(object): + def __init__(self, sp, pc): + self.sp = sp + self.pc = pc + + +class MyUnwinder(Unwinder): + def __init__(....): + super(MyUnwinder, self).__init___(<expects unwinder name argument>) + + def __call__(pending_frame): + if not <we recognize frame>: + return None + # Create UnwindInfo. Usually the frame is identified by the stack + # pointer and the program counter. + sp = pending_frame.read_register(<SP number>) + pc = pending_frame.read_register(<PC number>) + unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc)) + + # Find the values of the registers in the caller's frame and + # save them in the result: + unwind_info.add_saved_register(<register>, <value>) + .... + + # Return the result: + return unwind_info + +</pre></div> + +<a name="Registering-an-Unwinder"></a> +<h4 class="subheading">Registering an Unwinder</h4> + +<p>Object files and program spaces can have unwinders registered with +them. In addition, you can also register unwinders globally. +</p> +<p>The <code>gdb.unwinders</code> module provides the function to register an +unwinder: +</p> +<dl> +<dt><a name="index-gdb_002eunwinder_002eregister_005funwinder"></a>Function: <strong>gdb.unwinder.register_unwinder</strong> <em>(locus, unwinder, replace=False)</em></dt> +<dd><p><var>locus</var> specifies to which unwinder list to prepend the +<var>unwinder</var>. It can be either an object file, a program space, or +<code>None</code>, in which case the unwinder is registered globally. The +newly added <var>unwinder</var> will be called before any other unwinder from the +same locus. Two unwinders in the same locus cannot have the same +name. An attempt to add an unwinder with an already existing name raises +an exception unless <var>replace</var> is <code>True</code>, in which case the +old unwinder is deleted. +</p></dd></dl> + +<a name="Unwinder-Precedence"></a> +<h4 class="subheading">Unwinder Precedence</h4> + +<p><small>GDB</small> first calls the unwinders from all the object files in no +particular order, then the unwinders from the current program space, +and finally the unwinders from <small>GDB</small>. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Xmethods-In-Python.html#Xmethods-In-Python" accesskey="n" rel="next">Xmethods In Python</a>, Previous: <a href="Writing-a-Frame-Filter.html#Writing-a-Frame-Filter" accesskey="p" rel="previous">Writing a Frame Filter</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |