我向 GDB 添加了一个符号文件。为什么地址不一样？

Question

我在 GDB 中添加了一个符号文件来调试内核模块。 但是GDB输出的和我注册的地址不一样。

为什么cardev_write()函数的地址是0x13d？ 我想把cardev_write()函数的地址设为0xffffffffffc0904119。

模块内存

@ubuntu:~$ sudo cat /proc/kallsyms |grep chardev
ffffffffc0904000 t chardev_release  [chardev]
ffffffffc090403a t copy_overflow.constprop.0    [chardev]
ffffffffc0904056 t chardev_read [chardev]
ffffffffc0904119 t chardev_write    [chardev]
ffffffffc090420c t chardev_init [chardev]
ffffffffc09065a0 b chardev_cdev [chardev]
ffffffffc0906608 b chardev_major    [chardev]
ffffffffc0906580 b __key.28485  [chardev]
ffffffffc0906580 b chardev_class    [chardev]
ffffffffc0904390 t chardev_exit [chardev]
ffffffffc09051d5 r .LC2 [chardev]
ffffffffc0906140 d __this_module    [chardev]
ffffffffc0906000 d chardev_fops [chardev]
ffffffffc0904390 t cleanup_module   [chardev]
ffffffffc090420c t init_module  [chardev]
ffffffffc0906480 b G_Data   [chardev]
ffffffffc04b5120 d kvm_chardev_ops  [kvm]
@ubuntu:~$

系统信息

$ uname -a
Linux ubuntu 4.18.0-13-generic #14-Ubuntu SMP Wed Dec 5 09:04:24 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

设置gdb

$ gdb /usr/lib/debug/boot/vmlinux-4.18.0-13-generic
GNU gdb (Ubuntu 8.2-0ubuntu1) 8.2
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/lib/debug/boot/vmlinux-4.18.0-13-generic...done.
(gdb) add-symbol-file Kernel/Module/SS/chardev.ko 0xffffffffc0904000
add symbol table from file "Kernel/Module/SS/chardev.ko" at
    .text_addr = 0xffffffffc0904000
(y or n) y
Reading symbols from Kernel/Module/SS/chardev.ko...(no debugging symbols found)...done.
(gdb) b chardev_write
Breakpoint 1 at 0x13d
(gdb)

生成文件

obj-m := chardev.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) modules
clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(shell pwd) clean

+添加信息

章节标题

@ubuntu:~/Kernel/Module/SS$ readelf -WS chardev.ko | grep '\.text$'
@ubuntu:~/Kernel/Module/SS$ readelf -WS chardev.ko | grep '.text'
  [ 2] .text             PROGBITS        0000000000000000 000064 000000 00  AX  0   0  1
  [ 3] .text.unlikely    PROGBITS        0000000000000000 000064 0003bb 00  AX  0   0  1
  [ 4] .rela.text.unlikely RELA            0000000000000000 001278 0006a8 18   I 17   3  8
@ubuntu:~/Kernel/Module/SS$

符号信息

@ubuntu:~/Kernel/Module/SS$ nm chardev.ko | grep ' chardev_release$'
000000000000006d t chardev_release
@ubuntu:~/Kernel/Module/SS$

/proc/kallsyms

@ubuntu:~/Kernel/Module/SS$ sudo cat /proc/kallsyms |grep chardev
ffffffffc0587000 t device_lseek [chardev]
ffffffffc058706d t chardev_release  [chardev]
ffffffffc05870a7 t chardev_write    [chardev]
ffffffffc0587136 t chardev_read [chardev]
ffffffffc05871f0 t chardev_init [chardev]
ffffffffc0589460 b chardev_cdev [chardev]
ffffffffc05894c8 b chardev_major    [chardev]
ffffffffc0589440 b __key.28489  [chardev]
ffffffffc0589440 b chardev_class    [chardev]
ffffffffc0587348 t chardev_exit [chardev]
ffffffffc0589100 d __this_module    [chardev]
ffffffffc0589000 d chardev_fops [chardev]
ffffffffc0587348 t cleanup_module   [chardev]
ffffffffc05871f0 t init_module  [chardev]
@ubuntu:~/Kernel/Module/SS$

调试

(gdb) add-symbol-file Kernel/Module/SS/chardev.ko 0xffffffffc058706d
add symbol table from file "Kernel/Module/SS/chardev.ko" at
    .text_addr = 0xffffffffc058706d
(y or n) y
Reading symbols from Kernel/Module/SS/chardev.ko...(no debugging symbols found)...done.
(gdb) p chardev_write 
Cannot access memory at address 0x8d
(gdb)

Answer 1

(gdb) add-symbol-file Kernel/Module/SS/chardev.ko 0xffffffffc0904000

这很可能是在错误的地址添加模块。

要找出正确的地址，您需要两个数字：

• chardev.ko 中.text 部分的开头（使用readelf -WS chardev.ko | grep '\.text$' 查找）
• 内核在加载chardev.ko 时应用的重定位。找到此重定位的最简单方法是计算&chardev_release 与/proc/kallsyms 之间的增量——0xffffffffc0904000 和chardev.ko 中符号的地址——使用nm chardev.ko | grep ' chardev_release$' 得到它）。李>

把这两个数字加起来，就是你想add-symbol-file的地址。