964 lines
46 KiB
HTML
964 lines
46 KiB
HTML
|
<!DOCTYPE html>
|
||
|
<html lang="en">
|
||
|
<head>
|
||
|
<meta charset="UTF-8">
|
||
|
<!--[if IE]><meta http-equiv="X-UA-Compatible" content="IE=edge"><![endif]-->
|
||
|
<meta name="viewport" content="width=device-width, initial-scale=1.0">
|
||
|
<meta name="generator" content="Asciidoctor 1.5.8">
|
||
|
<title>How to recover an object from scratch</title>
|
||
|
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
|
||
|
<style>
|
||
|
/* Asciidoctor default stylesheet | MIT License | http://asciidoctor.org */
|
||
|
/* Uncomment @import statement below to use as custom stylesheet */
|
||
|
/*@import "https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700";*/
|
||
|
article,aside,details,figcaption,figure,footer,header,hgroup,main,nav,section,summary{display:block}
|
||
|
audio,canvas,video{display:inline-block}
|
||
|
audio:not([controls]){display:none;height:0}
|
||
|
script{display:none!important}
|
||
|
html{font-family:sans-serif;-ms-text-size-adjust:100%;-webkit-text-size-adjust:100%}
|
||
|
a{background:transparent}
|
||
|
a:focus{outline:thin dotted}
|
||
|
a:active,a:hover{outline:0}
|
||
|
h1{font-size:2em;margin:.67em 0}
|
||
|
abbr[title]{border-bottom:1px dotted}
|
||
|
b,strong{font-weight:bold}
|
||
|
dfn{font-style:italic}
|
||
|
hr{-moz-box-sizing:content-box;box-sizing:content-box;height:0}
|
||
|
mark{background:#ff0;color:#000}
|
||
|
code,kbd,pre,samp{font-family:monospace;font-size:1em}
|
||
|
pre{white-space:pre-wrap}
|
||
|
q{quotes:"\201C" "\201D" "\2018" "\2019"}
|
||
|
small{font-size:80%}
|
||
|
sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}
|
||
|
sup{top:-.5em}
|
||
|
sub{bottom:-.25em}
|
||
|
img{border:0}
|
||
|
svg:not(:root){overflow:hidden}
|
||
|
figure{margin:0}
|
||
|
fieldset{border:1px solid silver;margin:0 2px;padding:.35em .625em .75em}
|
||
|
legend{border:0;padding:0}
|
||
|
button,input,select,textarea{font-family:inherit;font-size:100%;margin:0}
|
||
|
button,input{line-height:normal}
|
||
|
button,select{text-transform:none}
|
||
|
button,html input[type="button"],input[type="reset"],input[type="submit"]{-webkit-appearance:button;cursor:pointer}
|
||
|
button[disabled],html input[disabled]{cursor:default}
|
||
|
input[type="checkbox"],input[type="radio"]{box-sizing:border-box;padding:0}
|
||
|
button::-moz-focus-inner,input::-moz-focus-inner{border:0;padding:0}
|
||
|
textarea{overflow:auto;vertical-align:top}
|
||
|
table{border-collapse:collapse;border-spacing:0}
|
||
|
*,*::before,*::after{-moz-box-sizing:border-box;-webkit-box-sizing:border-box;box-sizing:border-box}
|
||
|
html,body{font-size:100%}
|
||
|
body{background:#fff;color:rgba(0,0,0,.8);padding:0;margin:0;font-family:"Noto Serif","DejaVu Serif",serif;font-weight:400;font-style:normal;line-height:1;position:relative;cursor:auto;tab-size:4;-moz-osx-font-smoothing:grayscale;-webkit-font-smoothing:antialiased}
|
||
|
a:hover{cursor:pointer}
|
||
|
img,object,embed{max-width:100%;height:auto}
|
||
|
object,embed{height:100%}
|
||
|
img{-ms-interpolation-mode:bicubic}
|
||
|
.left{float:left!important}
|
||
|
.right{float:right!important}
|
||
|
.text-left{text-align:left!important}
|
||
|
.text-right{text-align:right!important}
|
||
|
.text-center{text-align:center!important}
|
||
|
.text-justify{text-align:justify!important}
|
||
|
.hide{display:none}
|
||
|
img,object,svg{display:inline-block;vertical-align:middle}
|
||
|
textarea{height:auto;min-height:50px}
|
||
|
select{width:100%}
|
||
|
.center{margin-left:auto;margin-right:auto}
|
||
|
.stretch{width:100%}
|
||
|
.subheader,.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{line-height:1.45;color:#7a2518;font-weight:400;margin-top:0;margin-bottom:.25em}
|
||
|
div,dl,dt,dd,ul,ol,li,h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6,pre,form,p,blockquote,th,td{margin:0;padding:0;direction:ltr}
|
||
|
a{color:#2156a5;text-decoration:underline;line-height:inherit}
|
||
|
a:hover,a:focus{color:#1d4b8f}
|
||
|
a img{border:none}
|
||
|
p{font-family:inherit;font-weight:400;font-size:1em;line-height:1.6;margin-bottom:1.25em;text-rendering:optimizeLegibility}
|
||
|
p aside{font-size:.875em;line-height:1.35;font-style:italic}
|
||
|
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{font-family:"Open Sans","DejaVu Sans",sans-serif;font-weight:300;font-style:normal;color:#ba3925;text-rendering:optimizeLegibility;margin-top:1em;margin-bottom:.5em;line-height:1.0125em}
|
||
|
h1 small,h2 small,h3 small,#toctitle small,.sidebarblock>.content>.title small,h4 small,h5 small,h6 small{font-size:60%;color:#e99b8f;line-height:0}
|
||
|
h1{font-size:2.125em}
|
||
|
h2{font-size:1.6875em}
|
||
|
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.375em}
|
||
|
h4,h5{font-size:1.125em}
|
||
|
h6{font-size:1em}
|
||
|
hr{border:solid #dddddf;border-width:1px 0 0;clear:both;margin:1.25em 0 1.1875em;height:0}
|
||
|
em,i{font-style:italic;line-height:inherit}
|
||
|
strong,b{font-weight:bold;line-height:inherit}
|
||
|
small{font-size:60%;line-height:inherit}
|
||
|
code{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;color:rgba(0,0,0,.9)}
|
||
|
ul,ol,dl{font-size:1em;line-height:1.6;margin-bottom:1.25em;list-style-position:outside;font-family:inherit}
|
||
|
ul,ol{margin-left:1.5em}
|
||
|
ul li ul,ul li ol{margin-left:1.25em;margin-bottom:0;font-size:1em}
|
||
|
ul.square li ul,ul.circle li ul,ul.disc li ul{list-style:inherit}
|
||
|
ul.square{list-style-type:square}
|
||
|
ul.circle{list-style-type:circle}
|
||
|
ul.disc{list-style-type:disc}
|
||
|
ol li ul,ol li ol{margin-left:1.25em;margin-bottom:0}
|
||
|
dl dt{margin-bottom:.3125em;font-weight:bold}
|
||
|
dl dd{margin-bottom:1.25em}
|
||
|
abbr,acronym{text-transform:uppercase;font-size:90%;color:rgba(0,0,0,.8);border-bottom:1px dotted #ddd;cursor:help}
|
||
|
abbr{text-transform:none}
|
||
|
blockquote{margin:0 0 1.25em;padding:.5625em 1.25em 0 1.1875em;border-left:1px solid #ddd}
|
||
|
blockquote cite{display:block;font-size:.9375em;color:rgba(0,0,0,.6)}
|
||
|
blockquote cite::before{content:"\2014 \0020"}
|
||
|
blockquote cite a,blockquote cite a:visited{color:rgba(0,0,0,.6)}
|
||
|
blockquote,blockquote p{line-height:1.6;color:rgba(0,0,0,.85)}
|
||
|
@media screen and (min-width:768px){h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2}
|
||
|
h1{font-size:2.75em}
|
||
|
h2{font-size:2.3125em}
|
||
|
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.6875em}
|
||
|
h4{font-size:1.4375em}}
|
||
|
table{background:#fff;margin-bottom:1.25em;border:solid 1px #dedede}
|
||
|
table thead,table tfoot{background:#f7f8f7}
|
||
|
table thead tr th,table thead tr td,table tfoot tr th,table tfoot tr td{padding:.5em .625em .625em;font-size:inherit;color:rgba(0,0,0,.8);text-align:left}
|
||
|
table tr th,table tr td{padding:.5625em .625em;font-size:inherit;color:rgba(0,0,0,.8)}
|
||
|
table tr.even,table tr.alt,table tr:nth-of-type(even){background:#f8f8f7}
|
||
|
table thead tr th,table tfoot tr th,table tbody tr td,table tr td,table tfoot tr td{display:table-cell;line-height:1.6}
|
||
|
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2;word-spacing:-.05em}
|
||
|
h1 strong,h2 strong,h3 strong,#toctitle strong,.sidebarblock>.content>.title strong,h4 strong,h5 strong,h6 strong{font-weight:400}
|
||
|
.clearfix::before,.clearfix::after,.float-group::before,.float-group::after{content:" ";display:table}
|
||
|
.clearfix::after,.float-group::after{clear:both}
|
||
|
*:not(pre)>code{font-size:.9375em;font-style:normal!important;letter-spacing:0;padding:.1em .5ex;word-spacing:-.15em;background-color:#f7f7f8;-webkit-border-radius:4px;border-radius:4px;line-height:1.45;text-rendering:optimizeSpeed;word-wrap:break-word}
|
||
|
*:not(pre)>code.nobreak{word-wrap:normal}
|
||
|
*:not(pre)>code.nowrap{white-space:nowrap}
|
||
|
pre,pre>code{line-height:1.45;color:rgba(0,0,0,.9);font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;text-rendering:optimizeSpeed}
|
||
|
em em{font-style:normal}
|
||
|
strong strong{font-weight:400}
|
||
|
.keyseq{color:rgba(51,51,51,.8)}
|
||
|
kbd{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;display:inline-block;color:rgba(0,0,0,.8);font-size:.65em;line-height:1.45;background-color:#f7f7f7;border:1px solid #ccc;-webkit-border-radius:3px;border-radius:3px;-webkit-box-shadow:0 1px 0 rgba(0,0,0,.2),0 0 0 .1em white inset;box-shadow:0 1px 0 rgba(0,0,0,.2),0 0 0 .1em #fff inset;margin:0 .15em;padding:.2em .5em;vertical-align:middle;position:relative;top:-.1em;white-space:nowrap}
|
||
|
.keyseq kbd:first-child{margin-left:0}
|
||
|
.keyseq kbd:last-child{margin-right:0}
|
||
|
.menuseq,.menuref{color:#000}
|
||
|
.menuseq b:not(.caret),.menuref{font-weight:inherit}
|
||
|
.menuseq{word-spacing:-.02em}
|
||
|
.menuseq b.caret{font-size:1.25em;line-height:.8}
|
||
|
.menuseq i.caret{font-weight:bold;text-align:center;width:.45em}
|
||
|
b.button::before,b.button::after{position:relative;top:-1px;font-weight:400}
|
||
|
b.button::before{content:"[";padding:0 3px 0 2px}
|
||
|
b.button::after{content:"]";padding:0 2px 0 3px}
|
||
|
p a>code:hover{color:rgba(0,0,0,.9)}
|
||
|
#header,#content,#footnotes,#footer{width:100%;margin-left:auto;margin-right:auto;margin-top:0;margin-bottom:0;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
|
||
|
#header::before,#header::after,#content::before,#content::after,#footnotes::before,#footnotes::after,#footer::before,#footer::after{content:" ";display:table}
|
||
|
#header::after,#content::after,#footnotes::after,#footer::after{clear:both}
|
||
|
#content{margin-top:1.25em}
|
||
|
#content::before{content:none}
|
||
|
#header>h1:first-child{color:rgba(0,0,0,.85);margin-top:2.25rem;margin-bottom:0}
|
||
|
#header>h1:first-child+#toc{margin-top:8px;border-top:1px solid #dddddf}
|
||
|
#header>h1:only-child,body.toc2 #header>h1:nth-last-child(2){border-bottom:1px solid #dddddf;padding-bottom:8px}
|
||
|
#header .details{border-bottom:1px solid #dddddf;line-height:1.45;padding-top:.25em;padding-bottom:.25em;padding-left:.25em;color:rgba(0,0,0,.6);display:-ms-flexbox;display:-webkit-flex;display:flex;-ms-flex-flow:row wrap;-webkit-flex-flow:row wrap;flex-flow:row wrap}
|
||
|
#header .details span:first-child{margin-left:-.125em}
|
||
|
#header .details span.email a{color:rgba(0,0,0,.85)}
|
||
|
#header .details br{display:none}
|
||
|
#header .details br+span::before{content:"\00a0\2013\00a0"}
|
||
|
#header .details br+span.author::before{content:"\00a0\22c5\00a0";color:rgba(0,0,0,.85)}
|
||
|
#header .details br+span#revremark::before{content:"\00a0|\00a0"}
|
||
|
#header #revnumber{text-transform:capitalize}
|
||
|
#header #revnumber::after{content:"\00a0"}
|
||
|
#content>h1:first-child:not([class]){color:rgba(0,0,0,.85);border-bottom:1px solid #dddddf;padding-bottom:8px;margin-top:0;padding-top:1rem;margin-bottom:1.25rem}
|
||
|
#toc{border-bottom:1px solid #e7e7e9;padding-bottom:.5em}
|
||
|
#toc>ul{margin-left:.125em}
|
||
|
#toc ul.sectlevel0>li>a{font-style:italic}
|
||
|
#toc ul.sectlevel0 ul.sectlevel1{margin:.5em 0}
|
||
|
#toc ul{font-family:"Open Sans","DejaVu Sans",sans-serif;list-style-type:none}
|
||
|
#toc li{line-height:1.3334;margin-top:.3334em}
|
||
|
#toc a{text-decoration:none}
|
||
|
#toc a:active{text-decoration:underline}
|
||
|
#toctitle{color:#7a2518;font-size:1.2em}
|
||
|
@media screen and (min-width:768px){#toctitle{font-size:1.375em}
|
||
|
body.toc2{padding-left:15em;padding-right:0}
|
||
|
#toc.toc2{margin-top:0!important;background-color:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #e7e7e9;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto}
|
||
|
#toc.toc2 #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
|
||
|
#toc.toc2>ul{font-size:.9em;margin-bottom:0}
|
||
|
#toc.toc2 ul ul{margin-left:0;padding-left:1em}
|
||
|
#toc.toc2 ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
|
||
|
body.toc2.toc-right{padding-left:0;padding-right:15em}
|
||
|
body.toc2.toc-right #toc.toc2{border-right-width:0;border-left:1px solid #e7e7e9;left:auto;right:0}}
|
||
|
@media screen and (min-width:1280px){body.toc2{padding-left:20em;padding-right:0}
|
||
|
#toc.toc2{width:20em}
|
||
|
#toc.toc2 #toctitle{font-size:1.375em}
|
||
|
#toc.toc2>ul{font-size:.95em}
|
||
|
#toc.toc2 ul ul{padding-left:1.25em}
|
||
|
body.toc2.toc-right{padding-left:0;padding-right:20em}}
|
||
|
#content #toc{border-style:solid;border-width:1px;border-color:#e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;-webkit-border-radius:4px;border-radius:4px}
|
||
|
#content #toc>:first-child{margin-top:0}
|
||
|
#content #toc>:last-child{margin-bottom:0}
|
||
|
#footer{max-width:100%;background-color:rgba(0,0,0,.8);padding:1.25em}
|
||
|
#footer-text{color:rgba(255,255,255,.8);line-height:1.44}
|
||
|
#content{margin-bottom:.625em}
|
||
|
.sect1{padding-bottom:.625em}
|
||
|
@media screen and (min-width:768px){#content{margin-bottom:1.25em}
|
||
|
.sect1{padding-bottom:1.25em}}
|
||
|
.sect1:last-child{padding-bottom:0}
|
||
|
.sect1+.sect1{border-top:1px solid #e7e7e9}
|
||
|
#content h1>a.anchor,h2>a.anchor,h3>a.anchor,#toctitle>a.anchor,.sidebarblock>.content>.title>a.anchor,h4>a.anchor,h5>a.anchor,h6>a.anchor{position:absolute;z-index:1001;width:1.5ex;margin-left:-1.5ex;display:block;text-decoration:none!important;visibility:hidden;text-align:center;font-weight:400}
|
||
|
#content h1>a.anchor::before,h2>a.anchor::before,h3>a.anchor::before,#toctitle>a.anchor::before,.sidebarblock>.content>.title>a.anchor::before,h4>a.anchor::before,h5>a.anchor::before,h6>a.anchor::before{content:"\00A7";font-size:.85em;display:block;padding-top:.1em}
|
||
|
#content h1:hover>a.anchor,#content h1>a.anchor:hover,h2:hover>a.anchor,h2>a.anchor:hover,h3:hover>a.anchor,#toctitle:hover>a.anchor,.sidebarblock>.content>.title:hover>a.anchor,h3>a.anchor:hover,#toctitle>a.anchor:hover,.sidebarblock>.content>.title>a.anchor:hover,h4:hover>a.anchor,h4>a.anchor:hover,h5:hover>a.anchor,h5>a.anchor:hover,h6:hover>a.anchor,h6>a.anchor:hover{visibility:visible}
|
||
|
#content h1>a.link,h2>a.link,h3>a.link,#toctitle>a.link,.sidebarblock>.content>.title>a.link,h4>a.link,h5>a.link,h6>a.link{color:#ba3925;text-decoration:none}
|
||
|
#content h1>a.link:hover,h2>a.link:hover,h3>a.link:hover,#toctitle>a.link:hover,.sidebarblock>.content>.title>a.link:hover,h4>a.link:hover,h5>a.link:hover,h6>a.link:hover{color:#a53221}
|
||
|
.audioblock,.imageblock,.literalblock,.listingblock,.stemblock,.videoblock{margin-bottom:1.25em}
|
||
|
.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
|
||
|
table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
|
||
|
.paragraph.lead>p,#preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
|
||
|
table.tableblock #preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:inherit}
|
||
|
.admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
|
||
|
.admonitionblock>table td.icon{text-align:center;width:80px}
|
||
|
.admonitionblock>table td.icon img{max-width:none}
|
||
|
.admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
|
||
|
.admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6)}
|
||
|
.admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
|
||
|
.exampleblock>.content{border-style:solid;border-width:1px;border-color:#e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;-webkit-border-radius:4px;border-radius:4px}
|
||
|
.exampleblock>.content>:first-child{margin-top:0}
|
||
|
.exampleblock>.content>:last-child{margin-bottom:0}
|
||
|
.sidebarblock{border-style:solid;border-width:1px;border-color:#e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;-webkit-border-radius:4px;border-radius:4px}
|
||
|
.sidebarblock>:first-child{margin-top:0}
|
||
|
.sidebarblock>:last-child{margin-bottom:0}
|
||
|
.sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
|
||
|
.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
|
||
|
.literalblock pre,.listingblock pre:not(.highlight),.listingblock pre[class="highlight"],.listingblock pre[class^="highlight "],.listingblock pre.CodeRay,.listingblock pre.prettyprint{background:#f7f7f8}
|
||
|
.sidebarblock .literalblock pre,.sidebarblock .listingblock pre:not(.highlight),.sidebarblock .listingblock pre[class="highlight"],.sidebarblock .listingblock pre[class^="highlight "],.sidebarblock .listingblock pre.CodeRay,.sidebarblock .listingblock pre.prettyprint{background:#f2f1f1}
|
||
|
.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{-webkit-border-radius:4px;border-radius:4px;word-wrap:break-word;overflow-x:auto;padding:1em;font-size:.8125em}
|
||
|
@media screen and (min-width:768px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:.90625em}}
|
||
|
@media screen and (min-width:1280px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:1em}}
|
||
|
.literalblock pre.nowrap,.literalblock pre.nowrap pre,.listingblock pre.nowrap,.listingblock pre.nowrap pre{white-space:pre;word-wrap:normal}
|
||
|
.literalblock.output pre{color:#f7f7f8;background-color:rgba(0,0,0,.9)}
|
||
|
.listingblock pre.highlightjs{padding:0}
|
||
|
.listingblock pre.highlightjs>code{padding:1em;-webkit-border-radius:4px;border-radius:4px}
|
||
|
.listingblock pre.prettyprint{border-width:0}
|
||
|
.listingblock>.content{position:relative}
|
||
|
.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:#999}
|
||
|
.listingblock:hover code[data-lang]::before{display:block}
|
||
|
.listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:#999}
|
||
|
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
|
||
|
table.pyhltable{border-collapse:separate;border:0;margin-bottom:0;background:none}
|
||
|
table.pyhltable td{vertical-align:top;padding-top:0;padding-bottom:0;line-height:1.45}
|
||
|
table.pyhltable td.code{padding-left:.75em;padding-right:0}
|
||
|
pre.pygments .lineno,table.pyhltable td:not(.code){color:#999;padding-left:0;padding-right:.5em;border-right:1px solid #dddddf}
|
||
|
pre.pygments .lineno{display:inline-block;margin-right:.25em}
|
||
|
table.pyhltable .linenodiv{background:none!important;padding-right:0!important}
|
||
|
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
|
||
|
.quoteblock>.title{margin-left:-1.5em;margin-bottom:.75em}
|
||
|
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
|
||
|
.quoteblock blockquote{margin:0;padding:0;border:0}
|
||
|
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
|
||
|
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
|
||
|
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
|
||
|
.verseblock{margin:0 1em 1.25em}
|
||
|
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
|
||
|
.verseblock pre strong{font-weight:400}
|
||
|
.verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
|
||
|
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
|
||
|
.quoteblock .attribution br,.verseblock .attribution br{display:none}
|
||
|
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
|
||
|
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
|
||
|
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
|
||
|
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
|
||
|
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
|
||
|
.quoteblock.excerpt,.quoteblock .quoteblock{margin:0 0 1.25em;padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
|
||
|
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
|
||
|
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;text-align:left;margin-right:0}
|
||
|
table.tableblock{max-width:100%;border-collapse:separate}
|
||
|
p.tableblock:last-child{margin-bottom:0}
|
||
|
td.tableblock>.content{margin-bottom:-1.25em}
|
||
|
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
|
||
|
table.grid-all>thead>tr>.tableblock,table.grid-all>tbody>tr>.tableblock{border-width:0 1px 1px 0}
|
||
|
table.grid-all>tfoot>tr>.tableblock{border-width:1px 1px 0 0}
|
||
|
table.grid-cols>*>tr>.tableblock{border-width:0 1px 0 0}
|
||
|
table.grid-rows>thead>tr>.tableblock,table.grid-rows>tbody>tr>.tableblock{border-width:0 0 1px}
|
||
|
table.grid-rows>tfoot>tr>.tableblock{border-width:1px 0 0}
|
||
|
table.grid-all>*>tr>.tableblock:last-child,table.grid-cols>*>tr>.tableblock:last-child{border-right-width:0}
|
||
|
table.grid-all>tbody>tr:last-child>.tableblock,table.grid-all>thead:last-child>tr>.tableblock,table.grid-rows>tbody>tr:last-child>.tableblock,table.grid-rows>thead:last-child>tr>.tableblock{border-bottom-width:0}
|
||
|
table.frame-all{border-width:1px}
|
||
|
table.frame-sides{border-width:0 1px}
|
||
|
table.frame-topbot,table.frame-ends{border-width:1px 0}
|
||
|
table.stripes-all tr,table.stripes-odd tr:nth-of-type(odd){background:#f8f8f7}
|
||
|
table.stripes-none tr,table.stripes-odd tr:nth-of-type(even){background:none}
|
||
|
th.halign-left,td.halign-left{text-align:left}
|
||
|
th.halign-right,td.halign-right{text-align:right}
|
||
|
th.halign-center,td.halign-center{text-align:center}
|
||
|
th.valign-top,td.valign-top{vertical-align:top}
|
||
|
th.valign-bottom,td.valign-bottom{vertical-align:bottom}
|
||
|
th.valign-middle,td.valign-middle{vertical-align:middle}
|
||
|
table thead th,table tfoot th{font-weight:bold}
|
||
|
tbody tr th{display:table-cell;line-height:1.6;background:#f7f8f7}
|
||
|
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
|
||
|
p.tableblock>code:only-child{background:none;padding:0}
|
||
|
p.tableblock{font-size:1em}
|
||
|
td>div.verse{white-space:pre}
|
||
|
ol{margin-left:1.75em}
|
||
|
ul li ol{margin-left:1.5em}
|
||
|
dl dd{margin-left:1.125em}
|
||
|
dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
|
||
|
ol>li p,ul>li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
|
||
|
ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
|
||
|
ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
|
||
|
ul.unstyled,ol.unstyled{margin-left:0}
|
||
|
ul.checklist{margin-left:.625em}
|
||
|
ul.checklist li>p:first-child>.fa-square-o:first-child,ul.checklist li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
|
||
|
ul.checklist li>p:first-child>input[type="checkbox"]:first-child{margin-right:.25em}
|
||
|
ul.inline{display:-ms-flexbox;display:-webkit-box;display:flex;-ms-flex-flow:row wrap;-webkit-flex-flow:row wrap;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
|
||
|
ul.inline>li{margin-left:1.25em}
|
||
|
.unstyled dl dt{font-weight:400;font-style:normal}
|
||
|
ol.arabic{list-style-type:decimal}
|
||
|
ol.decimal{list-style-type:decimal-leading-zero}
|
||
|
ol.loweralpha{list-style-type:lower-alpha}
|
||
|
ol.upperalpha{list-style-type:upper-alpha}
|
||
|
ol.lowerroman{list-style-type:lower-roman}
|
||
|
ol.upperroman{list-style-type:upper-roman}
|
||
|
ol.lowergreek{list-style-type:lower-greek}
|
||
|
.hdlist>table,.colist>table{border:0;background:none}
|
||
|
.hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
|
||
|
td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
|
||
|
td.hdlist1{font-weight:bold;padding-bottom:1.25em}
|
||
|
.literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
|
||
|
.colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
|
||
|
.colist td:not([class]):first-child img{max-width:none}
|
||
|
.colist td:not([class]):last-child{padding:.25em 0}
|
||
|
.thumb,.th{line-height:0;display:inline-block;border:solid 4px #fff;-webkit-box-shadow:0 0 0 1px #ddd;box-shadow:0 0 0 1px #ddd}
|
||
|
.imageblock.left{margin:.25em .625em 1.25em 0}
|
||
|
.imageblock.right{margin:.25em 0 1.25em .625em}
|
||
|
.imageblock>.title{margin-bottom:0}
|
||
|
.imageblock.thumb,.imageblock.th{border-width:6px}
|
||
|
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
|
||
|
.image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
|
||
|
.image.left{margin-right:.625em}
|
||
|
.image.right{margin-left:.625em}
|
||
|
a.image{text-decoration:none;display:inline-block}
|
||
|
a.image object{pointer-events:none}
|
||
|
sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
|
||
|
sup.footnote a,sup.footnoteref a{text-decoration:none}
|
||
|
sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
|
||
|
#footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
|
||
|
#footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
|
||
|
#footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
|
||
|
#footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
|
||
|
#footnotes .footnote:last-of-type{margin-bottom:0}
|
||
|
#content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
|
||
|
.gist .file-data>table{border:0;background:#fff;width:100%;margin-bottom:0}
|
||
|
.gist .file-data>table td.line-data{width:99%}
|
||
|
div.unbreakable{page-break-inside:avoid}
|
||
|
.big{font-size:larger}
|
||
|
.small{font-size:smaller}
|
||
|
.underline{text-decoration:underline}
|
||
|
.overline{text-decoration:overline}
|
||
|
.line-through{text-decoration:line-through}
|
||
|
.aqua{color:#00bfbf}
|
||
|
.aqua-background{background-color:#00fafa}
|
||
|
.black{color:#000}
|
||
|
.black-background{background-color:#000}
|
||
|
.blue{color:#0000bf}
|
||
|
.blue-background{background-color:#0000fa}
|
||
|
.fuchsia{color:#bf00bf}
|
||
|
.fuchsia-background{background-color:#fa00fa}
|
||
|
.gray{color:#606060}
|
||
|
.gray-background{background-color:#7d7d7d}
|
||
|
.green{color:#006000}
|
||
|
.green-background{background-color:#007d00}
|
||
|
.lime{color:#00bf00}
|
||
|
.lime-background{background-color:#00fa00}
|
||
|
.maroon{color:#600000}
|
||
|
.maroon-background{background-color:#7d0000}
|
||
|
.navy{color:#000060}
|
||
|
.navy-background{background-color:#00007d}
|
||
|
.olive{color:#606000}
|
||
|
.olive-background{background-color:#7d7d00}
|
||
|
.purple{color:#600060}
|
||
|
.purple-background{background-color:#7d007d}
|
||
|
.red{color:#bf0000}
|
||
|
.red-background{background-color:#fa0000}
|
||
|
.silver{color:#909090}
|
||
|
.silver-background{background-color:#bcbcbc}
|
||
|
.teal{color:#006060}
|
||
|
.teal-background{background-color:#007d7d}
|
||
|
.white{color:#bfbfbf}
|
||
|
.white-background{background-color:#fafafa}
|
||
|
.yellow{color:#bfbf00}
|
||
|
.yellow-background{background-color:#fafa00}
|
||
|
span.icon>.fa{cursor:default}
|
||
|
a span.icon>.fa{cursor:inherit}
|
||
|
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
|
||
|
.admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
|
||
|
.admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
|
||
|
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
|
||
|
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
|
||
|
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
|
||
|
.conum[data-value]{display:inline-block;color:#fff!important;background-color:rgba(0,0,0,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
|
||
|
.conum[data-value] *{color:#fff!important}
|
||
|
.conum[data-value]+b{display:none}
|
||
|
.conum[data-value]::after{content:attr(data-value)}
|
||
|
pre .conum[data-value]{position:relative;top:-.125em}
|
||
|
b.conum *{color:inherit!important}
|
||
|
.conum:not([data-value]):empty{display:none}
|
||
|
dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
|
||
|
h1,h2,p,td.content,span.alt{letter-spacing:-.01em}
|
||
|
p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
|
||
|
p,blockquote,dt,td.content,span.alt{font-size:1.0625rem}
|
||
|
p{margin-bottom:1.25rem}
|
||
|
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
|
||
|
.exampleblock>.content{background-color:#fffef7;border-color:#e0e0dc;-webkit-box-shadow:0 1px 4px #e0e0dc;box-shadow:0 1px 4px #e0e0dc}
|
||
|
.print-only{display:none!important}
|
||
|
@page{margin:1.25cm .75cm}
|
||
|
@media print{*{-webkit-box-shadow:none!important;box-shadow:none!important;text-shadow:none!important}
|
||
|
html{font-size:80%}
|
||
|
a{color:inherit!important;text-decoration:underline!important}
|
||
|
a.bare,a[href^="#"],a[href^="mailto:"]{text-decoration:none!important}
|
||
|
a[href^="http:"]:not(.bare)::after,a[href^="https:"]:not(.bare)::after{content:"(" attr(href) ")";display:inline-block;font-size:.875em;padding-left:.25em}
|
||
|
abbr[title]::after{content:" (" attr(title) ")"}
|
||
|
pre,blockquote,tr,img,object,svg{page-break-inside:avoid}
|
||
|
thead{display:table-header-group}
|
||
|
svg{max-width:100%}
|
||
|
p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
|
||
|
h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
|
||
|
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
|
||
|
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
|
||
|
body.book #header{text-align:center}
|
||
|
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
|
||
|
body.book #header .details{border:0!important;display:block;padding:0!important}
|
||
|
body.book #header .details span:first-child{margin-left:0!important}
|
||
|
body.book #header .details br{display:block}
|
||
|
body.book #header .details br+span::before{content:none!important}
|
||
|
body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
|
||
|
body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
|
||
|
.listingblock code[data-lang]::before{display:block}
|
||
|
#footer{padding:0 .9375em}
|
||
|
.hide-on-print{display:none!important}
|
||
|
.print-only{display:block!important}
|
||
|
.hide-for-print{display:none!important}
|
||
|
.show-for-print{display:inherit!important}}
|
||
|
@media print,amzn-kf8{#header>h1:first-child{margin-top:1.25rem}
|
||
|
.sect1{padding:0!important}
|
||
|
.sect1+.sect1{border:0}
|
||
|
#footer{background:none}
|
||
|
#footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
|
||
|
@media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
|
||
|
</style>
|
||
|
</head>
|
||
|
<body class="article">
|
||
|
<div id="header">
|
||
|
<h1>How to recover an object from scratch</h1>
|
||
|
</div>
|
||
|
<div id="content">
|
||
|
<div id="preamble">
|
||
|
<div class="sectionbody">
|
||
|
<div class="paragraph">
|
||
|
<p>I was recently presented with a repository with a corrupted packfile,
|
||
|
and was asked if the data was recoverable. This post-mortem describes
|
||
|
the steps I took to investigate and fix the problem. I thought others
|
||
|
might find the process interesting, and it might help somebody in the
|
||
|
same situation.</p>
|
||
|
</div>
|
||
|
<div class="sidebarblock">
|
||
|
<div class="content">
|
||
|
<div class="paragraph">
|
||
|
<p>Note: In this case, no good copy of the repository was available. For
|
||
|
the much easier case where you can get the corrupted object from
|
||
|
elsewhere, see <a href="recover-corrupted-blob-object.html">this howto</a>.</p>
|
||
|
</div>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I started with an fsck, which found a problem with exactly one object
|
||
|
(I’ve used $pack and $obj below to keep the output readable, and also
|
||
|
because I’ll refer to them later):</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> $ git fsck
|
||
|
error: $pack SHA1 checksum mismatch
|
||
|
error: index CRC mismatch for object $obj from $pack at offset 51653873
|
||
|
error: inflate: data stream error (incorrect data check)
|
||
|
error: cannot unpack $obj from $pack at offset 51653873</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>The pack checksum failing means a byte is munged somewhere, and it is
|
||
|
presumably in the object mentioned (since both the index checksum and
|
||
|
zlib were failing).</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>Reading the zlib source code, I found that "incorrect data check" means
|
||
|
that the adler-32 checksum at the end of the zlib data did not match the
|
||
|
inflated data. So stepping the data through zlib would not help, as it
|
||
|
did not fail until the very end, when we realize the CRC does not match.
|
||
|
The problematic bytes could be anywhere in the object data.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>The first thing I did was pull the broken data out of the packfile. I
|
||
|
needed to know how big the object was, which I found out with:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> $ git show-index <$idx | cut -d' ' -f1 | sort -n | grep -A1 51653873
|
||
|
51653873
|
||
|
51664736</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>Show-index gives us the list of objects and their offsets. We throw away
|
||
|
everything but the offsets, and then sort them so that our interesting
|
||
|
offset (which we got from the fsck output above) is followed immediately
|
||
|
by the offset of the next object. Now we know that the object data is
|
||
|
10863 bytes long, and we can grab it with:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> dd if=$pack of=object bs=1 skip=51653873 count=10863</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I inspected a hexdump of the data, looking for any obvious bogosity
|
||
|
(e.g., a 4K run of zeroes would be a good sign of filesystem
|
||
|
corruption). But everything looked pretty reasonable.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>Note that the "object" file isn’t fit for feeding straight to zlib; it
|
||
|
has the git packed object header, which is variable-length. We want to
|
||
|
strip that off so we can start playing with the zlib data directly. You
|
||
|
can either work your way through it manually (the format is described in
|
||
|
<a href="../technical/pack-format.html">Documentation/technical/pack-format.txt</a>),
|
||
|
or you can walk through it in a debugger. I did the latter, creating a
|
||
|
valid pack like:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> # pack magic and version
|
||
|
printf 'PACK\0\0\0\2' >tmp.pack
|
||
|
# pack has one object
|
||
|
printf '\0\0\0\1' >>tmp.pack
|
||
|
# now add our object data
|
||
|
cat object >>tmp.pack
|
||
|
# and then append the pack trailer
|
||
|
/path/to/git.git/t/helper/test-tool sha1 -b <tmp.pack >trailer
|
||
|
cat trailer >>tmp.pack</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>and then running "git index-pack tmp.pack" in the debugger (stop at
|
||
|
unpack_raw_entry). Doing this, I found that there were 3 bytes of header
|
||
|
(and the header itself had a sane type and size). So I stripped those
|
||
|
off with:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> dd if=object of=zlib bs=1 skip=3</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I ran the result through zlib’s inflate using a custom C program. And
|
||
|
while it did report the error, I did get the right number of output
|
||
|
bytes (i.e., it matched git’s size header that we decoded above). But
|
||
|
feeding the result back to "git hash-object" didn’t produce the same
|
||
|
sha1. So there were some wrong bytes, but I didn’t know which. The file
|
||
|
happened to be C source code, so I hoped I could notice something
|
||
|
obviously wrong with it, but I didn’t. I even got it to compile!</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I also tried comparing it to other versions of the same path in the
|
||
|
repository, hoping that there would be some part of the diff that didn’t
|
||
|
make sense. Unfortunately, this happened to be the only revision of this
|
||
|
particular file in the repository, so I had nothing to compare against.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>So I took a different approach. Working under the guess that the
|
||
|
corruption was limited to a single byte, I wrote a program to munge each
|
||
|
byte individually, and try inflating the result. Since the object was
|
||
|
only 10K compressed, that worked out to about 2.5M attempts, which took
|
||
|
a few minutes.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>The program I used is here:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre>#include <stdio.h>
|
||
|
#include <unistd.h>
|
||
|
#include <string.h>
|
||
|
#include <signal.h>
|
||
|
#include <zlib.h>
|
||
|
|
||
|
static int try_zlib(unsigned char *buf, int len)
|
||
|
{
|
||
|
/* make this absurdly large so we don't have to loop */
|
||
|
static unsigned char out[1024*1024];
|
||
|
z_stream z;
|
||
|
int ret;
|
||
|
|
||
|
memset(&z, 0, sizeof(z));
|
||
|
inflateInit(&z);
|
||
|
|
||
|
z.next_in = buf;
|
||
|
z.avail_in = len;
|
||
|
z.next_out = out;
|
||
|
z.avail_out = sizeof(out);
|
||
|
|
||
|
ret = inflate(&z, 0);
|
||
|
inflateEnd(&z);
|
||
|
return ret >= 0;
|
||
|
}
|
||
|
|
||
|
/* eye candy */
|
||
|
static int counter = 0;
|
||
|
static void progress(int sig)
|
||
|
{
|
||
|
fprintf(stderr, "\r%d", counter);
|
||
|
alarm(1);
|
||
|
}
|
||
|
|
||
|
int main(void)
|
||
|
{
|
||
|
/* oversized so we can read the whole buffer in */
|
||
|
unsigned char buf[1024*1024];
|
||
|
int len;
|
||
|
unsigned i, j;
|
||
|
|
||
|
signal(SIGALRM, progress);
|
||
|
alarm(1);
|
||
|
|
||
|
len = read(0, buf, sizeof(buf));
|
||
|
for (i = 0; i < len; i++) {
|
||
|
unsigned char c = buf[i];
|
||
|
for (j = 0; j <= 0xff; j++) {
|
||
|
buf[i] = j;
|
||
|
|
||
|
counter++;
|
||
|
if (try_zlib(buf, len))
|
||
|
printf("i=%d, j=%x\n", i, j);
|
||
|
}
|
||
|
buf[i] = c;
|
||
|
}
|
||
|
|
||
|
alarm(0);
|
||
|
fprintf(stderr, "\n");
|
||
|
return 0;
|
||
|
}</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I compiled and ran with:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> gcc -Wall -Werror -O3 munge.c -o munge -lz
|
||
|
./munge <zlib</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>There were a few false positives early on (if you write "no data" in the
|
||
|
zlib header, zlib thinks it’s just fine :) ). But I got a hit about
|
||
|
halfway through:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> i=5642, j=c7</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I let it run to completion, and got a few more hits at the end (where it
|
||
|
was munging the CRC to match our broken data). So there was a good
|
||
|
chance this middle hit was the source of the problem.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I confirmed by tweaking the byte in a hex editor, zlib inflating the
|
||
|
result (no errors!), and then piping the output into "git hash-object",
|
||
|
which reported the sha1 of the broken object. Success!</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>I fixed the packfile itself with:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> chmod +w $pack
|
||
|
printf '\xc7' | dd of=$pack bs=1 seek=51659518 conv=notrunc
|
||
|
chmod -w $pack</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>The <code>\xc7</code> comes from the replacement byte our "munge" program found.
|
||
|
The offset 51659518 is derived by taking the original object offset
|
||
|
(51653873), adding the replacement offset found by "munge" (5642), and
|
||
|
then adding back in the 3 bytes of git header we stripped.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>After that, "git fsck" ran clean.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>As for the corruption itself, I was lucky that it was indeed a single
|
||
|
byte. In fact, it turned out to be a single bit. The byte 0xc7 was
|
||
|
corrupted to 0xc5. So presumably it was caused by faulty hardware, or a
|
||
|
cosmic ray.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>And the aborted attempt to look at the inflated output to see what was
|
||
|
wrong? I could have looked forever and never found it. Here’s the diff
|
||
|
between what the corrupted data inflates to, versus the real data:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre> - cp = strtok (arg, "+");
|
||
|
+ cp = strtok (arg, ".");</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>It tweaked one byte and still ended up as valid, readable C that just
|
||
|
happened to do something totally different! One takeaway is that on a
|
||
|
less unlucky day, looking at the zlib output might have actually been
|
||
|
helpful, as most random changes would actually break the C code.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>But more importantly, git’s hashing and checksumming noticed a problem
|
||
|
that easily could have gone undetected in another system. The result
|
||
|
still compiled, but would have caused an interesting bug (that would
|
||
|
have been blamed on some random commit).</p>
|
||
|
</div>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="sect1">
|
||
|
<h2 id="_the_adventure_continues">The adventure continues…​</h2>
|
||
|
<div class="sectionbody">
|
||
|
<div class="paragraph">
|
||
|
<p>I ended up doing this again! Same entity, new hardware. The assumption
|
||
|
at this point is that the old disk corrupted the packfile, and then the
|
||
|
corruption was migrated to the new hardware (because it was done by
|
||
|
rsync or similar, and no fsck was done at the time of migration).</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>This time, the affected blob was over 20 megabytes, which was far too
|
||
|
large to do a brute-force on. I followed the instructions above to
|
||
|
create the <code>zlib</code> file. I then used the <code>inflate</code> program below to pull
|
||
|
the corrupted data from that. Examining that output gave me a hint about
|
||
|
where in the file the corruption was. But now I was working with the
|
||
|
file itself, not the zlib contents. So knowing the sha1 of the object
|
||
|
and the approximate area of the corruption, I used the <code>sha1-munge</code>
|
||
|
program below to brute-force the correct byte.</p>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>Here’s the inflate program (it’s essentially <code>gunzip</code> but without the
|
||
|
<code>.gz</code> header processing):</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre>#include <stdio.h>
|
||
|
#include <string.h>
|
||
|
#include <zlib.h>
|
||
|
#include <stdlib.h>
|
||
|
|
||
|
int main(int argc, char **argv)
|
||
|
{
|
||
|
/*
|
||
|
* oversized so we can read the whole buffer in;
|
||
|
* this could actually be switched to streaming
|
||
|
* to avoid any memory limitations
|
||
|
*/
|
||
|
static unsigned char buf[25 * 1024 * 1024];
|
||
|
static unsigned char out[25 * 1024 * 1024];
|
||
|
int len;
|
||
|
z_stream z;
|
||
|
int ret;
|
||
|
|
||
|
len = read(0, buf, sizeof(buf));
|
||
|
memset(&z, 0, sizeof(z));
|
||
|
inflateInit(&z);
|
||
|
|
||
|
z.next_in = buf;
|
||
|
z.avail_in = len;
|
||
|
z.next_out = out;
|
||
|
z.avail_out = sizeof(out);
|
||
|
|
||
|
ret = inflate(&z, 0);
|
||
|
if (ret != Z_OK && ret != Z_STREAM_END)
|
||
|
fprintf(stderr, "initial inflate failed (%d)\n", ret);
|
||
|
|
||
|
fprintf(stderr, "outputting %lu bytes", z.total_out);
|
||
|
fwrite(out, 1, z.total_out, stdout);
|
||
|
return 0;
|
||
|
}</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div class="paragraph">
|
||
|
<p>And here is the <code>sha1-munge</code> program:</p>
|
||
|
</div>
|
||
|
<div class="listingblock">
|
||
|
<div class="content">
|
||
|
<pre>#include <stdio.h>
|
||
|
#include <unistd.h>
|
||
|
#include <string.h>
|
||
|
#include <signal.h>
|
||
|
#include <openssl/sha.h>
|
||
|
#include <stdlib.h>
|
||
|
|
||
|
/* eye candy */
|
||
|
static int counter = 0;
|
||
|
static void progress(int sig)
|
||
|
{
|
||
|
fprintf(stderr, "\r%d", counter);
|
||
|
alarm(1);
|
||
|
}
|
||
|
|
||
|
static const signed char hexval_table[256] = {
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 00-07 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 08-0f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 10-17 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 18-1f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 20-27 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 28-2f */
|
||
|
0, 1, 2, 3, 4, 5, 6, 7, /* 30-37 */
|
||
|
8, 9, -1, -1, -1, -1, -1, -1, /* 38-3f */
|
||
|
-1, 10, 11, 12, 13, 14, 15, -1, /* 40-47 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 48-4f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 50-57 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 58-5f */
|
||
|
-1, 10, 11, 12, 13, 14, 15, -1, /* 60-67 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 68-67 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 70-77 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 78-7f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 80-87 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 88-8f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 90-97 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* 98-9f */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* a0-a7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* a8-af */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* b0-b7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* b8-bf */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* c0-c7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* c8-cf */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* d0-d7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* d8-df */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* e0-e7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* e8-ef */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* f0-f7 */
|
||
|
-1, -1, -1, -1, -1, -1, -1, -1, /* f8-ff */
|
||
|
};
|
||
|
|
||
|
static inline unsigned int hexval(unsigned char c)
|
||
|
{
|
||
|
return hexval_table[c];
|
||
|
}
|
||
|
|
||
|
static int get_sha1_hex(const char *hex, unsigned char *sha1)
|
||
|
{
|
||
|
int i;
|
||
|
for (i = 0; i < 20; i++) {
|
||
|
unsigned int val;
|
||
|
/*
|
||
|
* hex[1]=='\0' is caught when val is checked below,
|
||
|
* but if hex[0] is NUL we have to avoid reading
|
||
|
* past the end of the string:
|
||
|
*/
|
||
|
if (!hex[0])
|
||
|
return -1;
|
||
|
val = (hexval(hex[0]) << 4) | hexval(hex[1]);
|
||
|
if (val & ~0xff)
|
||
|
return -1;
|
||
|
*sha1++ = val;
|
||
|
hex += 2;
|
||
|
}
|
||
|
return 0;
|
||
|
}
|
||
|
|
||
|
int main(int argc, char **argv)
|
||
|
{
|
||
|
/* oversized so we can read the whole buffer in */
|
||
|
static unsigned char buf[25 * 1024 * 1024];
|
||
|
char header[32];
|
||
|
int header_len;
|
||
|
unsigned char have[20], want[20];
|
||
|
int start, len;
|
||
|
SHA_CTX orig;
|
||
|
unsigned i, j;
|
||
|
|
||
|
if (!argv[1] || get_sha1_hex(argv[1], want)) {
|
||
|
fprintf(stderr, "usage: sha1-munge <sha1> [start] <file.in\n");
|
||
|
return 1;
|
||
|
}
|
||
|
|
||
|
if (argv[2])
|
||
|
start = atoi(argv[2]);
|
||
|
else
|
||
|
start = 0;
|
||
|
|
||
|
len = read(0, buf, sizeof(buf));
|
||
|
header_len = sprintf(header, "blob %d", len) + 1;
|
||
|
fprintf(stderr, "using header: %s\n", header);
|
||
|
|
||
|
/*
|
||
|
* We keep a running sha1 so that if you are munging
|
||
|
* near the end of the file, we do not have to re-sha1
|
||
|
* the unchanged earlier bytes
|
||
|
*/
|
||
|
SHA1_Init(&orig);
|
||
|
SHA1_Update(&orig, header, header_len);
|
||
|
if (start)
|
||
|
SHA1_Update(&orig, buf, start);
|
||
|
|
||
|
signal(SIGALRM, progress);
|
||
|
alarm(1);
|
||
|
|
||
|
for (i = start; i < len; i++) {
|
||
|
unsigned char c;
|
||
|
SHA_CTX x;
|
||
|
|
||
|
#if 0
|
||
|
/*
|
||
|
* deletion -- this would not actually work in practice,
|
||
|
* I think, because we've already committed to a
|
||
|
* particular size in the header. Ditto for addition
|
||
|
* below. In those cases, you'd have to do the whole
|
||
|
* sha1 from scratch, or possibly keep three running
|
||
|
* "orig" sha1 computations going.
|
||
|
*/
|
||
|
memcpy(&x, &orig, sizeof(x));
|
||
|
SHA1_Update(&x, buf + i + 1, len - i - 1);
|
||
|
SHA1_Final(have, &x);
|
||
|
if (!memcmp(have, want, 20))
|
||
|
printf("i=%d, deletion\n", i);
|
||
|
#endif
|
||
|
|
||
|
/*
|
||
|
* replacement -- note that this tries each of the 256
|
||
|
* possible bytes. If you suspect a single-bit flip,
|
||
|
* it would be much shorter to just try the 8
|
||
|
* bit-flipped variants.
|
||
|
*/
|
||
|
c = buf[i];
|
||
|
for (j = 0; j <= 0xff; j++) {
|
||
|
buf[i] = j;
|
||
|
|
||
|
memcpy(&x, &orig, sizeof(x));
|
||
|
SHA1_Update(&x, buf + i, len - i);
|
||
|
SHA1_Final(have, &x);
|
||
|
if (!memcmp(have, want, 20))
|
||
|
printf("i=%d, j=%02x\n", i, j);
|
||
|
}
|
||
|
buf[i] = c;
|
||
|
|
||
|
#if 0
|
||
|
/* addition */
|
||
|
for (j = 0; j <= 0xff; j++) {
|
||
|
unsigned char extra = j;
|
||
|
memcpy(&x, &orig, sizeof(x));
|
||
|
SHA1_Update(&x, &extra, 1);
|
||
|
SHA1_Update(&x, buf + i, len - i);
|
||
|
SHA1_Final(have, &x);
|
||
|
if (!memcmp(have, want, 20))
|
||
|
printf("i=%d, addition=%02x", i, j);
|
||
|
}
|
||
|
#endif
|
||
|
|
||
|
SHA1_Update(&orig, buf + i, 1);
|
||
|
counter++;
|
||
|
}
|
||
|
|
||
|
alarm(0);
|
||
|
fprintf(stderr, "\r%d\n", counter);
|
||
|
return 0;
|
||
|
}</pre>
|
||
|
</div>
|
||
|
</div>
|
||
|
</div>
|
||
|
</div>
|
||
|
</div>
|
||
|
<div id="footer">
|
||
|
<div id="footer-text">
|
||
|
Last updated 2019-02-26 19:31:11 UTC
|
||
|
</div>
|
||
|
</div>
|
||
|
</body>
|
||
|
</html>
|