[WIP] 实验：用gdb观察gdb

Can I use gdb to observe gdb?

I always forget how gdb was made. for me, I went though the website on stackoverflow of Zhihu serveral times only for this question, and I always forget everything about ptrace. And I think it's annoying me, since I am just hanging around on the same field. So one day a crazy and funny idea comes to me:

Can I use gdb to observe gdb?

That was a great idea! I am going to do that!

how gdb was made?

If you know ptrace well, you can skip to next section!

ptrace is a great syscall, it can provide many infomation that we want for a process in Linux, and gdb was built upon it. Here I make a simple program

#include <stdio.h>
#include <stdlib.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/user.h>
#include <sys/reg.h>

int main() {

    pid_t child_pid = fork();

    if (child_pid == 0) {
        printf("Child process starting 'aaa'...\n");
        ptrace(PTRACE_TRACEME, 0, NULL, NULL);  
        execl("./pTrace", "pTrace", NULL);
        perror("execl failed");
        exit(EXIT_FAILURE);
    } else {
        int status;
        waitpid(child_pid, &status, 0);
        while (1) {
            if (WIFEXITED(status)) {
                printf("Child process terminated normally.\n");
                break;
            }
            long rax = ptrace(PTRACE_PEEKUSER, child_pid, 8 * RAX, NULL);
            if (rax == -1) {
                perror("ptrace failed");
                break;
            }
            printf("Child process RAX register value: %lx\n", rax);

            ptrace(PTRACE_CONT, child_pid, NULL, NULL);
            waitpid(child_pid, &status, 0);  
        }
        ptrace(PTRACE_DETACH, child_pid, NULL, NULL);
        waitpid(child_pid, &status, 0);
    }
    return 0;
}

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下载：
lib

wget https://gcc.gnu.org/pub/gcc/infrastructure/mpfr-4.1.0.tar.bz2
wget https://gcc.gnu.org/pub/gcc/infrastructure/gmp-6.2.1.tar.bz2
wget https://gcc.gnu.org/pub/gcc/infrastructure/mpc-1.2.1.tar.gz

gdb
Index of /gnu/gdb

wget https://ftp.gnu.org/gnu/gdb/gdb-15.2.tar.xz

编译：

gmp

位置：

/home/haibin/experiment/GMP/install

1.   
    ./configure --prefix=$(pwd)
    /home/haibin/experiment/GMP/

2. make

3. make install

mpfr

./configure --prefix=$(pwd) --with-gmp-include=/home/haibin/experiment/gmp-6.2.1/include --with-gmp-lib=/home/haibin/experiment/gmp-6.2.1/lib

mpc

tar -xvzf mpc-1.2.1.tar.gz
cd mpc-1.2.1/

./configure --prefix=$(pwd) \
    --with-gmp-include=/home/haibin/experiment/gmp-6.2.1/include \
    --with-gmp-lib=/home/haibin/experiment/gmp-6.2.1/lib \
    --with-mpfr-include=/home/haibin/experiment/mpfr-4.1.0/include \
    --with-mpfr-lib=/home/haibin/experiment/mpfr-4.1.0/lib \

make
make install

export LD_LIBRARY_PATH=$(pwd)/gmp-6.2.1/lib:$(pwd)/mpfr-4.1.0/lib:$(pwd)/mpc-1.2.1/lib:$LD_LIBRARY_PATH

编译好这些lib后，我们可以编译自己的gdb

./configure --with-gmp=/home/haibin/experiment/gmp-6.2.1/ --with-mpfr=/home/haibin/experiment/mpfr-4.1.0

随后，

./
gdb ./gdb

2316    in rtld.c
(gdb) n 500
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Single stepping until exit from function _dl_start_user,
which has no line number information.
Cannot find bounds of current function
(gdb) n 500
Cannot find bounds of current function
(gdb) n 500
Cannot find bounds of current function
(gdb) n 500
Cannot find bounds of current function
(gdb) s
Cannot find bounds of current function
(gdb) continue
Continuing.
[Detaching after vfork from child process 1011434]
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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(gdb) 

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