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编译并vscode可视化调试Linux 4.18内核

1. 安装必要软件

# 安装编译依赖组件。
sudo apt install -y libncurses5-dev libssl-dev bison flex libelf-dev gcc make openssl libc6-dev
sudo apt install -y flex bison make gcc libssl-dev bc libelf-dev
wget https://mirrors.tuna.tsinghua.edu.cn/kernel/v4.x/linux-4.18.tar.gz
tar -zxf linux-4.18.tar.gz
cd linux-4.18.tar.gz

# 设置调试的编译菜单
export ARCH=x86
make x86_64_defconfig
make menuconfig

# 下面选项如果没有选上的，选上（点击空格键），然后 save 保存设置，退出 exit。
##################################################################
General setup  --->
    [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support

Device Drivers  --->
    [*] Block devices  --->
        <*> RAM block device support
            (65536) Default RAM disk size (kbytes)

Processor type and features  --->
    [*] Randomize the address of the kernel image (KASLR)

Kernel hacking  --->
    Compile-time checks and compiler options  ---> 
        [*] Compile the kernel with debug info
            [*] Provide GDB scripts for kernel debugging

Device Drivers --> 
    Network device support --> 
        <*> Universal TUN/TAP device driver support

[*] Networking support --> 
        Networking options --> 
            <*> 802.1d Ethernet Bridging

# 编译内核。
make -j8

2. 编译内核

编译内核会遇见很多问题

2.1 网络库的问题

In file included from scripts/selinux/genheaders/genheaders.c:19:
# scripts/selinux/mdp/mdp.c 也会出现这种现象
./security/selinux/include/classmap.h:249:2: error: #error New address family defined, please update secclass_map.
  249 | #error New address family defined, please update secclass_map.
      |  ^~~~~
make[3]: *** [scripts/Makefile.host:90: scripts/selinux/genheaders/genheaders] Error 1
make[2]: *** [scripts/Makefile.build:558: scripts/selinux/genheaders] Error 2
make[1]: *** [scripts/Makefile.build:558: scripts/selinux] Error 2
make: *** [Makefile:1045: scripts] Error 2

1.找到错误中提示的.c文件，genheader.c文件和mdp.c文件编辑，去掉两个文件的头部引用中的

#include <sys/socket.h>

2.找到错误提示中的classmap.h文件
a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
编辑classmap.h，在头文件中添加

#include <linux/socket.h>

重新编译即可

2.2 连接的问题

arch/x86/entry/entry_64.S: Assembler messages:
arch/x86/entry/entry_64.S:1667: Warning: no instruction mnemonic suffix given and no register operands; using default for `sysret'
  AS      arch/x86/entry/thunk_64.o
arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table

使用这个patch

Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/elf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index be89c741ba9a..2b0f4f52f7b5 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -380,8 +380,11 @@ static int read_symbols(struct elf *elf)

symtab = find_section_by_name(elf, ".symtab");
 	if (!symtab) {
-		WARN("missing symbol table");
-		return -1;
+		/*
+		 * A missing symbol table is actually possible if it's an empty
+		 * .o file.  This can happen for thunk_64.o.
+		 */
+		return 0;
 	}

symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
-- 
2.29.2

3. 安装修改版的gdb

# 删除 gdb
gdb -v | grep gdb
apt remove gdb -y

# 安装其它组件。
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt install software-properties-common
sudo apt-get update

# 安装高版本 gcc。
gcc --version
sudo apt install gcc-9 g++-9 -y
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 90 --slave /usr/bin/g++ g++ /usr/bin/g++-9
gcc --version

# 下载解压 gdb
cd /root
wget http://ftp.gnu.org/gnu/gdb/gdb-8.3.tar.gz
tar zxf gdb-8.3.tar.gz

# 修改 gdb/remote.c 代码。
cd gdb-8.3
vim gdb/remote.c

#在gdb/remote.c文件下作如下修改
    /* Further sanity checks, with knowledge of the architecture.  */
    // if (buf_len > 2 * rsa->sizeof_g_packet)
    //   error (_("Remote 'g' packet reply is too long (expected %ld bytes, got %d "
    //      "bytes): %s"),
    //    rsa->sizeof_g_packet, buf_len / 2,
    //    rs->buf.data ());
  if (buf_len > 2 * rsa->sizeof_g_packet) {
    rsa->sizeof_g_packet = buf_len;
    for (i = 0; i < gdbarch_num_regs (gdbarch); i++){
            if (rsa->regs[i].pnum == -1)
                continue;
            if (rsa->regs[i].offset >= rsa->sizeof_g_packet)
                rsa->regs[i].in_g_packet = 0;
            else
                rsa->regs[i].in_g_packet = 1;
        }
    }

#编译GDB
./configure
./make -j8
# 有可能会有这个报错
# error: "Please include config.h first."
# vim ./gdb/build-gnulib/import/unistd.h
# 加入 #include "../config.h"
# 总之缺啥就在哪儿加上 #include "../config.h"
# 打开GDB，看是否为新版本

sudo apt install texinfo
sudo ./make install
# makeinfo: command not found

gdb -v

4. 安装qemu并配置qemu

sudo apt install -y qemu qemu-system-x86

制作helloworld的rootfs用于测试

touch main.c

键入以下代码

#include <stdio>
int main()
{
    printf("hello world!");
    printf("hello world!");
    printf("hello world!");
    printf("hello world!");
    fflush(stdout);
    while(1);
    return 0;
}

gcc --static -o helloworld main.c
echo helloworld | cpio -o --format=newc > rootfs

Qemu直接运行测试（非必须）

qemu-system-x86_64 \
    -kernel ./arch/x86/boot/bzImage \
    -initrd ./rootfs \
    -append "root=/dev/ram rdinit=/helloworld console=ttyS0" \
    -nographic -serial mon:stdio

但是实践证明还是有问题，最好是来一个图形化界面

Qemu 开启GDB调试

qemu-system-x86_64  \
 -kernel ./arch/x86/boot/bzImage  \
 -initrd ./rootfs  \
 -append "root=/dev/ram rdinit=/helloworld" \
 -smp 2  \
 -s -S  \
 -append nokaslr

-s= -gdb tcp:1234, 这个可以改

-S 表示进入调试模式，在这个模式下已开始CPU是冻结的，在gdb中按下c来继续

这里的root=/dev/ram 因此需要在内核里面重新编译，开启对ext2的支持

进行以上会打开Qemu并进入等待调试状态，此时可以直接gdb调试，如下（非必须）

gdb ./vmLinux
#以下进行调试
target remote:1234
b start_kernel
c

可以发现内核被断点在start_kernel函数上

5. 使用vscode远程阅读linux内核源码

首先直接打开的话会发现有很多地方没法跳转，需要安装C/C++ GNU Global

在VScode远程窗口中安装C/C++ GNU Global 这个扩展

在远程机器中

sudo apt install global

在vscode设置中，写入

{
    "gnuGlobal.globalExecutable": "/usr/bin/global",
    "gnuGlobal.gtagsExecutable": "/usr/bin/gtags",
    "gnuGlobal.objDirPrefix": "/home/damaoooo/.global"
}

shift + command + P，然后 rebuild Gtags Data

之后就可以进行Linux源代码阅读了

如果需要debug，需要安装 GDB debug 插件，随后配置launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "kernel-debug",
            "type": "cppdbg",
            "request": "launch",
            "miDebuggerServerAddress": "192.168.1.114:1234",
            "program": "${workspaceFolder}/vmlinux",
            "args": [],
            "stopAtEntry": true,
            "cwd": "${workspaceFolder}",
            "environment": [],
            "externalConsole": false,
            "logging": {
                "engineLogging": false
            },
            "MIMode": "gdb",
        }
    ]
    }

启动qemu，就可以对内核进行调试了

但是有问题，如图所示，很多的变量会被optimized out，是因为默认编译的时候开了O2，而开O0选项，直接编译不过，需要做一些修改，我懒得改了直接单步调试看内存了。