GDB符号从何而来? [英] Where are GDB symbols coming from?

问题描述

当我将Fedora 28的/usr/bin/ls文件加载到GDB中时，即使它不以字符串形式出现也不存在于二进制文件的符号表中，我也可以访问符号abformat_init.

When I load Fedora 28's /usr/bin/ls file into GDB, I can access to the symbol abformat_init, even if it is not present as a string nor in the symbols table of the binary file.

$ file /usr/bin/ls
/usr/bin/ls: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=d6d0ea6be508665f5586e90a30819d090710842f, stripped, too many notes (256)
$ readelf -S /usr/bin/ls | grep abformat
$ nm /usr/bin/ls
nm: /usr/bin/ls: no symbols
$ strings /usr/bin/ls | grep abformat
$ gdb /usr/bin/ls
[...]
Reading symbols from /usr/bin/ls...Reading symbols from /usr/bin/ls...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Missing separate debuginfos, use: dnf debuginfo-install coreutils-8.29-7.fc28.x86_64
(gdb) info symbol abformat_init 
abformat_init in section .text of /usr/bin/ls

此符号来自何处?是否有一个程序可以将它们提取到GDB之外?

Where does this symbol comes from? Is there a program that allows to extract them outside of GDB?

推荐答案

TL; DR:

1. GDB读取的Fedora二进制文件中有一个特殊的.gnu_debugdata压缩节，其中包含符号.
2. 可以使用eu-readelf -Ws --elf-section /usr/bin/ls
方便地打印该部分的内容

1. There is a special .gnu_debugdata compressed section in Fedora binaries that GDB reads, and which contains mini-symbols.
2. Contents of that section can be conveniently printed with eu-readelf -Ws --elf-section /usr/bin/ls

---

readelf -S /usr/bin/ls | grep abformat

该命令正在转储节.您需要符号:

That command is dumping sections. You want symbols instead:

readelf -s /usr/bin/ls | grep abformat
readelf --all /usr/bin/ls | grep abformat

strings /usr/bin/ls | grep abformat

字符串尝试猜测您想要的是什么，并且不会输出二进制文件中找到的所有字符串.参见此博客文章并尝试:

Strings tries to guess what you want, and doesn't output all strings found in the binary. See this blog post and try:

strings -a /usr/bin/ls | grep abformat

更新:我确认了您观察到的结果:abformat没有出现在任何地方，但是GDB知道它.

Update: I confirmed the results you've observed: abformat does not appear anywhere, yet GDB knows about it.

原来，有一个.gnu_debugdata compressed 部分(描述为迷你符号.

Turns out, there is a .gnu_debugdata compressed section (described here), which has mini-symbols.

要提取此数据，通常可以这样做:

To extract this data, normally you would do:

objcopy -O binary -j .gnu_debugdata /usr/bin/ls ls.mini.xz

但是，我的系统上的损坏了(产生空输出) ，所以我改用dd:

However, that is broken on my system (produces empty output), so instead I used dd:

# You may need to adjust the numbers below from "readelf -WS /usr/bin/ls"
dd if=/usr/bin/ls of=ls.mini.xz bs=1 skip=151896 count=3764
xz -d ls.mini.xz
nm ls.mini | grep abformat

产生的结果:

00000000000005db0 t abformat_init

QED.

其他信息:

1. 令人困惑的GDB no debugging symbols在此错误中得到了解决.. li>
2. objcopy拒绝复制.gnu_debugdata是此错误的主题.

3. 有 个工具，可以方便地转储此信息:

1. Confusing GDB no debugging symbols is addressed in this bug.
2. objcopy refusing to copy .gnu_debugdata is the subject of this bug.

3. There is a tool that can conveniently dump this info:

eu-readelf -Ws --elf-section /usr/bin/ls | grep abformat 37: 0000000000005db0 593 FUNC LOCAL DEFAULT 14 abformat_init

eu-readelf -Ws --elf-section /usr/bin/ls | grep abformat 37: 0000000000005db0 593 FUNC LOCAL DEFAULT 14 abformat_init

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