*:first-child {
margin-top: 0 !important;
}

body>*:last-child {
margin-bottom: 0 !important;
}

/* BLOCKS
=============================================================================*/

p, blockquote, ul, ol, dl, table, pre {
margin: 15px 0;
}

/* HEADERS
=============================================================================*/

h1, h2, h3, h4, h5, h6 {
margin: 20px 0 10px;
padding: 0;
font-weight: bold;
-webkit-font-smoothing: antialiased;
}

h1 tt, h1 code, h2 tt, h2 code, h3 tt, h3 code, h4 tt, h4 code, h5 tt, h5 code, h6 tt, h6 code {
font-size: inherit;
}

h1 {
font-size: 28px;
color: #000;
}

h2 {
font-size: 24px;
border-bottom: 1px solid #ccc;
color: #000;
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h3 {
font-size: 18px;
}

h4 {
font-size: 16px;
}

h5 {
font-size: 14px;
}

h6 {
color: #777;
font-size: 14px;
}

body>h2:first-child, body>h1:first-child, body>h1:first-child+h2, body>h3:first-child, body>h4:first-child, body>h5:first-child, body>h6:first-child {
margin-top: 0;
padding-top: 0;
}

a:first-child h1, a:first-child h2, a:first-child h3, a:first-child h4, a:first-child h5, a:first-child h6 {
margin-top: 0;
padding-top: 0;
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h1+p, h2+p, h3+p, h4+p, h5+p, h6+p {
margin-top: 10px;
}

/* LINKS
=============================================================================*/

a {
color: #4183C4;
text-decoration: none;
}

a:hover {
text-decoration: underline;
}

/* LISTS
=============================================================================*/

ul, ol {
padding-left: 30px;
}

ul li > :first-child,
ol li > :first-child,
ul li ul:first-of-type,
ol li ol:first-of-type,
ul li ol:first-of-type,
ol li ul:first-of-type {
margin-top: 0px;
}

ul ul, ul ol, ol ol, ol ul {
margin-bottom: 0;
}

dl {
padding: 0;
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dl dt {
font-size: 14px;
font-weight: bold;
font-style: italic;
padding: 0;
margin: 15px 0 5px;
}

dl dt:first-child {
padding: 0;
}

dl dt>:first-child {
margin-top: 0px;
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dl dt>:last-child {
margin-bottom: 0px;
}

dl dd {
margin: 0 0 15px;
padding: 0 15px;
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dl dd>:first-child {
margin-top: 0px;
}

dl dd>:last-child {
margin-bottom: 0px;
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pre, code, tt {
font-size: 12px;
font-family: Consolas, "Liberation Mono", Courier, monospace;
}

code, tt {
margin: 0 0px;
padding: 0px 0px;
white-space: nowrap;
border: 1px solid #eaeaea;
background-color: #f8f8f8;
border-radius: 3px;
}

pre>code {
margin: 0;
padding: 0;
white-space: pre;
border: none;
background: transparent;
}

pre {
background-color: #f8f8f8;
border: 1px solid #ccc;
font-size: 13px;
line-height: 19px;
overflow: auto;
padding: 6px 10px;
border-radius: 3px;
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pre code, pre tt {
background-color: transparent;
border: none;
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kbd {
-moz-border-bottom-colors: none;
-moz-border-left-colors: none;
-moz-border-right-colors: none;
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background-color: #DDDDDD;
background-image: linear-gradient(#F1F1F1, #DDDDDD);
background-repeat: repeat-x;
border-color: #DDDDDD #CCCCCC #CCCCCC #DDDDDD;
border-image: none;
border-radius: 2px 2px 2px 2px;
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border-width: 1px;
font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;
line-height: 10px;
padding: 1px 4px;
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/* QUOTES
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blockquote {
border-left: 4px solid #DDD;
padding: 0 15px;
color: #777;
}

blockquote>:first-child {
margin-top: 0px;
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blockquote>:last-child {
margin-bottom: 0px;
}

/* HORIZONTAL RULES
=============================================================================*/

hr {
clear: both;
margin: 15px 0;
height: 0px;
overflow: hidden;
border: none;
background: transparent;
border-bottom: 4px solid #ddd;
padding: 0;
}

/* TABLES
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table th {
font-weight: bold;
}

table th, table td {
border: 1px solid #ccc;
padding: 6px 13px;
}

table tr {
border-top: 1px solid #ccc;
background-color: #fff;
}

table tr:nth-child(2n) {
background-color: #f8f8f8;
}

/* IMAGES
=============================================================================*/

img {
max-width: 100%
}
-->

pwnable.kr-fd-Writeup

• 根据题目描述Mommy! what is a file descriptor in Linux? 知该题与Linux系统下的文件描述有关；
• ssh远程登录如下：
  
• 根据题目提示，ls -l查看文件及权限如下,由下图，用户fd只具有读文件fd.c的权限（尝试sudo chmod增加权限，但失败）：
  
• cat fd.c读取fd.c中的内容，可得到如下代码：

 fd@ubuntu:~$ cat fd.c

 #include <stdio.h>

 #include <stdlib.h>

 #include <string.h>

 char buf[];

 int main(int argc, char* argv[], char* envp[]){

         if(argc<){

                 printf("pass argv[1] a number\n");

                 return ;

         }

         int fd = atoi( argv[] ) - 0x1234;

         int len = ;

         len = read(fd, buf, );

         if(!strcmp("LETMEWIN\n", buf)){

                 printf("good job :)\n");

                 system("/bin/cat flag");

                 exit();

         }

         printf("learn about Linux file IO\n");

         return ;

 }

 fd@ubuntu:~$

• 分析上述代码，执行system("/bin/cat flag");语句，即可获得flag;
• 执行system("/bin/cat flag");需要使buf == "LETMEWIN";
• 继续分析，需要buf通过read函数读入"LETMEWIN\n"，有read函数的定义，需要使fd==0；
  
• 则有atoi( argv[1] ) == 0x1234(即十进制下的4660），由atoi函数的定义，需要argv[1] == "4660";
  
• 进而由LinuxC下argv[]的相应定义可以构造输入，LinuxC下argc，argv[]的意义请参考http://www.cnblogs.com/WangAoBo/p/6366600.html

• echo "LETMEWIN" | ./fd 4660

如下，flag为：mommy! I think I know what a file descriptor is!!

2017-2-4 22:24;39