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diff --git a/development/ld.html b/development/ld.html new file mode 100644 index 0000000..b1a3fbb --- /dev/null +++ b/development/ld.html @@ -0,0 +1,416 @@ +<!DOCTYPE HTML> +<html lang="en" class="light sidebar-visible" dir="ltr"> + <head> + <!-- Book generated using mdBook --> + <meta charset="UTF-8"> + <title>ld - Notes</title> + + + <!-- Custom HTML head --> + + <meta name="description" content=""> + <meta name="viewport" content="width=device-width, initial-scale=1"> + <meta name="theme-color" content="#ffffff"> + + <link rel="icon" href="../favicon.svg"> + <link rel="shortcut icon" href="../favicon.png"> + <link rel="stylesheet" href="../css/variables.css"> + <link rel="stylesheet" href="../css/general.css"> + <link rel="stylesheet" href="../css/chrome.css"> + <link rel="stylesheet" href="../css/print.css" media="print"> + + <!-- Fonts --> + <link rel="stylesheet" href="../FontAwesome/css/font-awesome.css"> + <link rel="stylesheet" href="../fonts/fonts.css"> + + <!-- Highlight.js Stylesheets --> + <link rel="stylesheet" id="highlight-css" href="../highlight.css"> + <link rel="stylesheet" id="tomorrow-night-css" href="../tomorrow-night.css"> + <link rel="stylesheet" id="ayu-highlight-css" href="../ayu-highlight.css"> + + <!-- Custom theme stylesheets --> + + + <!-- Provide site root and default themes to javascript --> + <script> + const path_to_root = "../"; + const default_light_theme = "light"; + const default_dark_theme = "navy"; + </script> + <!-- Start loading toc.js asap --> + <script src="../toc.js"></script> + </head> + <body> + <div id="body-container"> + <!-- Work around some values being stored in localStorage wrapped in quotes --> + <script> + try { + let theme = localStorage.getItem('mdbook-theme'); + let sidebar = localStorage.getItem('mdbook-sidebar'); + + if (theme.startsWith('"') && theme.endsWith('"')) { + localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1)); + } + + if (sidebar.startsWith('"') && sidebar.endsWith('"')) { + localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1)); + } + } catch (e) { } + </script> + + <!-- Set the theme before any content is loaded, prevents flash --> + <script> + const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme; + let theme; + try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { } + if (theme === null || theme === undefined) { theme = default_theme; } + const html = document.documentElement; + html.classList.remove('light') + html.classList.add(theme); + html.classList.add("js"); + </script> + + <input type="checkbox" id="sidebar-toggle-anchor" class="hidden"> + + <!-- Hide / unhide sidebar before it is displayed --> + <script> + let sidebar = null; + const sidebar_toggle = document.getElementById("sidebar-toggle-anchor"); + if (document.body.clientWidth >= 1080) { + try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } + sidebar = sidebar || 'visible'; + } else { + sidebar = 'hidden'; + } + sidebar_toggle.checked = sidebar === 'visible'; + html.classList.remove('sidebar-visible'); + html.classList.add("sidebar-" + sidebar); + </script> + + <nav id="sidebar" class="sidebar" aria-label="Table of contents"> + <!-- populated by js --> + <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox> + <noscript> + <iframe class="sidebar-iframe-outer" src="../toc.html"></iframe> + </noscript> + <div id="sidebar-resize-handle" class="sidebar-resize-handle"> + <div class="sidebar-resize-indicator"></div> + </div> + </nav> + + <div id="page-wrapper" class="page-wrapper"> + + <div class="page"> + <div id="menu-bar-hover-placeholder"></div> + <div id="menu-bar" class="menu-bar sticky"> + <div class="left-buttons"> + <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> + <i class="fa fa-bars"></i> + </label> + <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list"> + <i class="fa fa-paint-brush"></i> + </button> + <ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu"> + <li role="none"><button role="menuitem" class="theme" id="default_theme">Auto</button></li> + <li role="none"><button role="menuitem" class="theme" id="light">Light</button></li> + <li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li> + <li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li> + <li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li> + <li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li> + </ul> + <button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> + <i class="fa fa-search"></i> + </button> + </div> + + <h1 class="menu-title">Notes</h1> + + <div class="right-buttons"> + <a href="../print.html" title="Print this book" aria-label="Print this book"> + <i id="print-button" class="fa fa-print"></i> + </a> + <a href="https://github.com/johannst/notes" title="Git repository" aria-label="Git repository"> + <i id="git-repository-button" class="fa fa-github"></i> + </a> + + </div> + </div> + + <div id="search-wrapper" class="hidden"> + <form id="searchbar-outer" class="searchbar-outer"> + <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> + </form> + <div id="searchresults-outer" class="searchresults-outer hidden"> + <div id="searchresults-header" class="searchresults-header"></div> + <ul id="searchresults"> + </ul> + </div> + </div> + + <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> + <script> + document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); + document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); + Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { + link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); + }); + </script> + + <div id="content" class="content"> + <main> + <h1 id="ld1"><a class="header" href="#ld1">ld(1)</a></h1> +<pre><code>ld [opts] files... + -T <script> use <script> as linker script + --trace report each file the linker touches + --start-group archives --end-group + search archives repearepeatedly until no new + undefined references are created + (eg helpfull with list of static libraries) +</code></pre> +<h2 id="linker-script"><a class="header" href="#linker-script">Linker Script</a></h2> +<p><code>output</code> sections are defined as follows (full description at <a href="https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html">output +section</a> and <a href="https://sourceware.org/binutils/docs/ld/Input-Section.html">input section</a>).</p> +<pre><code>section_name [vaddr] : [AT(paddr)] { + file_pattern (section_pattern) +} +</code></pre> +<p>The following gives an example of an <code>output</code> section with two <code>input</code> section rules.</p> +<pre><code>.foo : { + abc.o (.foo) + *.o (.foo.*) +} +</code></pre> +<h3 id="example-virtual-vs-physical-load-address"><a class="header" href="#example-virtual-vs-physical-load-address">Example: virtual vs physical (load) address</a></h3> +<p>Sometimes code is initially located at a different location as when being run. +For example in embedded cases, where code may initially resides in a <em>rom</em> and +startup code will copy a section with writable <em>data</em> into <em>ram</em>. Code accessing +the writable data accesses the data in the <em>ram</em>.</p> +<p>In this case we need different addresses for the same data.</p> +<ul> +<li>The <code>virtual</code> or <em>runtime</em> address, this is the address used when the linker +resolves accesses to the data. Hence, this is the address the data will have +when the code is running.</li> +<li>The <code>physical</code> or <em>load</em> address, this is the address the data is stored at +initially. Startup code typically copies the initial values from the +<code>physical</code> to the <code>virtual</code> address.</li> +</ul> +<p>The following shows an example linker script which uses <em>virtual</em> and <em>physical</em> +addresses. The full source files can be found <a href="https://github.com/johannst/notes/tree/master/src/development/ld">here</a>.</p> +<pre><code>OUTPUT_FORMAT(elf64-x86-64) +ENTRY(_entry) + +SECTIONS { + /* Set the initial location counter (vaddr) */ + . = 0x00800000; + + /* Create .text output section at current vaddr */ + .text : { + *(.text*) + } + + ASSERT(. == 0x00800000 + SIZEOF(.text), "inc loc counter automatically") + + /* Create .data section at location counter aligned to the next 0x100 (vaddr) */ + /* Set the load address to 0x00100000 (paddr) */ + .data ALIGN(0x100) : AT(0x00100000) { + HIDDEN(_data_vaddr = .); + HIDDEN(_data_paddr = LOADADDR(.data)); + *(.data*) + } + + /* Create .rodata with explicit vaddr */ + /* Re-adjust the paddr location counter */ + .rodata 0x00804000 : AT(ADDR(.rodata)) { + *(.rodata*) + } + + ASSERT(. == 0x00804000 + SIZEOF(.rodata), "inc loc counter automatically") + + .stack ALIGN (0x1000) : { + . += 0x1000; + HIDDEN(_stack_top = .); + } + + /DISCARD/ : { + *(.*) + } +} + +/* Some example assertions */ +ASSERT(ADDR(.data) != LOADADDR(.data), "DATA vaddr and paddr must be different") +ASSERT(SIZEOF(.stack) == 0x1000, "STACK section must be 0x1000") +</code></pre> +<p>We can use the following assembly snippet to explore the linker script.</p> +<pre><code class="language-x86asm">.section .text, "ax", @progbits +.global _entry +_entry: + mov $_stack_top, %rsp + mov $asm_array, %rax + mov (asm_len), %eax + + hlt + jmp _entry + +.section .data.asm, "aw", @progbits +asm_array: + .4byte 0xa + .4byte 0xb + .4byte 0xc + .4byte 0xd +.rept 4 + .4byte 0xff +.endr + +.section .rodata.asm, "a", @progbits +asm_len: + .4byte 8 +</code></pre> +<blockquote> +<p><code>gcc -c data.S && ld -o link-nomem -T link-nomem.ld data.o</code></p> +</blockquote> +<p>The elf load segments show the difference in <em>physical</em> and <em>virtual</em> address +for the segment containing the <code>.data</code> section.</p> +<pre><code class="language-sh">> readelf -W -l link-nomem +# There are 4 program headers, starting at offset 64 +# +# Program Headers: +# Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align +# LOAD 0x001100 0x0000000000800100 0x0000000000100000 0x000020 0x000020 RW 0x1000 +# LOAD 0x002000 0x0000000000800000 0x0000000000800000 0x000018 0x000018 R E 0x1000 +# LOAD 0x003000 0x0000000000804000 0x0000000000804000 0x000004 0x000004 R 0x1000 +# LOAD 0x000000 0x0000000000805000 0x0000000000805000 0x000000 0x001000 RW 0x1000 +# +# Section to Segment mapping: +# Segment Sections... +# 00 .data +# 01 .text +# 02 .rodata +# 03 .stack +</code></pre> +<p>Startup code could copy data from <code>_data_paddr</code> to <code>_data_vaddr</code>.</p> +<pre><code class="language-sh">> nm link-nomem +# 0000000000800100 d asm_array +# 0000000000804000 r asm_len +# 0000000000100000 a _data_paddr +# 0000000000800100 d _data_vaddr +# 0000000000800000 T _entry +# 0000000000806000 b _stack_top +</code></pre> +<p>The linker resolves symbols to their <em>virtual</em> address, this can be seen by the +access to the <code>asm_array</code> variable.</p> +<pre><code class="language-sh">> objdump -d link-nomem +# Disassembly of section .text: +# +# 0000000000800000 <_entry>: +# 800000: 48 c7 c4 00 60 80 00 mov $0x806000,%rsp +# 800007: 48 c7 c0 00 01 80 00 mov $0x800100,%rax ;; mov $asm_array, %rax +# 80000e: 8b 04 25 00 40 80 00 mov 0x804000,%eax +# 800015: f4 hlt +# 800016: eb e8 jmp 800000 <_entry> +</code></pre> +<p>The following linker script shows an example with the <code>MEMORY</code> command.</p> +<pre><code>OUTPUT_FORMAT(elf64-x86-64) +ENTRY(_entry) + +MEMORY { + ROM : ORIGIN = 0x00100000, LENGTH = 0x4000 + RAM : ORIGIN = 0x00800000, LENGTH = 0x4000 +} + +SECTIONS { + /* Create .text output section at ROM (vaddr) */ + .text : { + *(.text*) + } > ROM + + ASSERT(. == ORIGIN(ROM) + SIZEOF(.text), "inc loc counter automatically") + + /* Create .data output section at RAM (vaddr) */ + /* Set load addr to ROM, right after .text (paddr) */ + .data : { + HIDDEN(_data_vaddr = .); + HIDDEN(_data_paddr = LOADADDR(.data)); + *(.data*) + } > RAM AT > ROM + + /* Append .rodata output section at ROM (vaddr) */ + .rodata : { + *(.rodata*) + } > ROM + + /* Append .stack output section at RAM (vaddr) aligned up to next 0x1000 */ + .stack : ALIGN (0x1000) { + . += 0x1000; + HIDDEN(_stack_top = .); + } > RAM + + /DISCARD/ : { + *(.*) + } +} + +/* Some example assertions */ +ASSERT(ADDR(.data) != LOADADDR(.data), "DATA vaddr and paddr must be different") +ASSERT(ADDR(.rodata) == LOADADDR(.rodata), "RODATA vaddr and paddr must be euqal") +ASSERT(ADDR(.stack) == ORIGIN(RAM) + 0x1000, "STACK section must aligned to 0x1000") +ASSERT(SIZEOF(.stack) == 0x1000, "STACK section must be 0x1000") +</code></pre> +<h2 id="references"><a class="header" href="#references">References</a></h2> +<ul> +<li><a href="https://sourceware.org/binutils/docs/ld/">ld manual</a></li> +<li><a href="https://sourceware.org/binutils/docs/ld/Input-Section.html">ld script: input sections</a></li> +<li><a href="https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html">ld script: output sections</a></li> +<li><a href="https://github.com/johannst/notes/tree/master/src/development/ld">notes/ld example files</a></li> +</ul> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + <a rel="prev" href="../development/gas.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + <a rel="next prefetch" href="../development/git.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + <a rel="prev" href="../development/gas.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + <a rel="next prefetch" href="../development/git.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + </nav> + + </div> + + + + + <script> + window.playground_copyable = true; 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