tartanllama / minidbg

A mini x86 linux debugger for teaching purposes

This project has been expanded into a book! It covers many more topics in much greater detail. You can now pre-order Building a Debugger.

minidbg

A mini x86 linux debugger for teaching purposes

See my Writing a linux debugger blog post series for a tutorial on how to write something like this.

minidbg's Issues

The link in the readme is 404

Cannot find function

i can use neither "next" nor "step".

A tiny enhancement

Awesome project!

But I think if add \n for this line of code in every branch:

std::cerr << "Program name not specified\n";

It will be better!

Thanks!

Development environment issues

I like your course very much. I hope you can provide your development environment configuration.
1、gcc version?
2、g++ version?
3、Is the operating system environment ubuntu or centos?

Error during libelfin submodule initialization

alexander@alexander-pc:~/code/test/debugger/minidbg$ git submodule init
Submodule 'ext/libelfin' (https://github.com/aclements/libelfin.git) registered for path 'ext/libelfin'
Submodule 'ext/linenoise' (https://github.com/antirez/linenoise.git) registered for path 'ext/linenoise'

alexander@alexander-pc:~/code/test/debugger/minidbg$ git submodule update
Cloning into 'ext/libelfin'...
remote: Counting objects: 818, done.
remote: Total 818 (delta 0), reused 0 (delta 0), pack-reused 818
Receiving objects: 100% (818/818), 190.57 KiB | 235.00 KiB/s, done.
Resolving deltas: 100% (487/487), done.
Checking connectivity... done.
fatal: reference is not a tree: cc25b47c11f6798f79f09ec2345b18d663f9d548
Cloning into 'ext/linenoise'...
remote: Counting objects: 396, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 396 (delta 0), reused 1 (delta 0), pack-reused 393
Receiving objects: 100% (396/396), 165.21 KiB | 248.00 KiB/s, done.
Resolving deltas: 100% (225/225), done.
Checking connectivity... done.
Submodule path 'ext/linenoise': checked out 'c894b9e59f02203dbe4e2be657572cf88c4230c3'
Unable to checkout 'cc25b47c11f6798f79f09ec2345b18d663f9d548' in submodule path 'ext/libelfin'

I was able to fix it by changing the submodule url to your fork.

alexander@alexander-pc:~/code/test/debugger/minidbg$ git diff
diff --git a/.gitmodules b/.gitmodules
index 13aada1..4017271 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,6 +1,6 @@
 [submodule "ext/libelfin"]
        path = ext/libelfin
-       url = https://github.com/aclements/libelfin.git
+       url = https://github.com/TartanLlama/libelfin.git
 [submodule "ext/linenoise"]
        path = ext/linenoise
        url = https://github.com/antirez/linenoise.git

Some mistakes about the data type.

std::intptr_t is a better choice than uint64_t when applies to an address. Once I use uint64_t data type to describe an address, step_in, step_out, stepi, step_over... funcs will not work normally.

So, this problem has to be fixed.

also, std::sto-functions may be used properly, I chose stoull when i need to trans string 0xADDR to std::intptr_t instead stoi, or something wrong may occur. like:
m_load_address = std::stoull(addr, 0, 16);
that could be better.

I also noticed that this project seems dead, so I will not pr, just reminding other people who are doing this small project.

dwarf::format_error

When I test miniDebugger, I encountered the following problem:
Start debugger process 132818 terminate called after throwing an instance of 'dwarf::format_error'
what(): unknown compilation unit version 5 Aborted (core dumped)
then I use gdb to debug miniDebugger, I got the following info, how can I fix this bug
(gdb) run hello
Starting program: /home/ccy/Desktop/miniDebugger/build/miniDebugger hello
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Detaching after fork from child process 132695]
Start debugger process 132695
terminate called after throwing an instance of 'dwarf::format_error'
what(): unknown compilation unit version 5

Program received signal SIGABRT, Aborted.
__pthread_kill_implementation (no_tid=0, signo=6, threadid=140737347836800) at ./nptl/pthread_kill.c:44
44 ./nptl/pthread_kill.c: No such file or directory.

some problem of run ./minidbg hello

Did i wrong in some step?

Build failure

I git cloned this project and did a 'cmake CMakeLists.txt'. It failed because I did not have linenoise.c under ext/linenoise. I then placed linenoise.c under ext/linenoise and repeated the above command and it succeeded. It generated a Makefile. But when I hit 'make', I get the following error. Can you please tell me what I should do? I git cloned the libelfin project separately, built it and then placed the .so files under ext/libelfin. Please advise.

[ 22%] Built target hello
make[3]: *** No targets specified and no makefile found. Stop.
CMakeFiles/libelfin.dir/build.make:57: recipe for target 'CMakeFiles/libelfin' failed
make[2]: *** [CMakeFiles/libelfin] Error 2
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/libelfin.dir/all' failed
make[1]: *** [CMakeFiles/libelfin.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

linenoise.c file missing

CMake Error at CMakeLists.txt:7 (add_executable):
Cannot find source file:

ext/linenoise/linenoise.c

In main branch, an error in minidbg.cpp

Obviously, when we want to get entry from pc, we must pass an off_set pc, so this needs to be fixed like:

 else if(is_prefix(command, "stepi")) {
        single_step_instruction_with_breakpoint_check();
        auto line_entry = get_line_entry_from_pc(get_pc());
        print_source(line_entry->file->path, line_entry->line);
    }
    else {
        std::cerr << "Unknown command\n";
    }

 else if(is_prefix(command, "stepi")) {
        single_step_instruction_with_breakpoint_check();
        auto line_entry = get_line_entry_from_pc(get_offset_pc());
        print_source(line_entry->file->path, line_entry->line);
    }
    else {
        std::cerr << "Unknown command\n";
    }

After compiling, stepi works normall.

address read from dwarf is not absolute

Thank you for your good tutorial!

But I can't make a breakpoint on a simple function following the code.
It seems dwarf++ read a relative PC offset, which begins from 0x0000000, thus I cannot write INT 3 on it.

How can I solve it?

an error in main branch

void Debugger::print_backtrace() {
  auto output_frame = [frame_number = 0](auto &&func) mutable {
    std::cout << "frame #" << frame_number++ << ": 0x" << dwarf::at_low_pc(func)
              << ' ' << dwarf::at_name(func) << std::endl;
  };

  auto current_func = get_function_from_pc(get_offset_pc());
  output_frame(current_func);

  std::intptr_t frame_pointer = get_register_value(m_pid, registers::reg::rbp);
  std::intptr_t return_address = read_memory(frame_pointer + 8);

  while (dwarf::at_name(current_func) != "main") {
    current_func = get_function_from_pc(offset_load_address(return_address));
    output_frame(current_func);
    frame_pointer = read_memory(frame_pointer);
    return_address = read_memory(frame_pointer + 8);
  }
}

function should be fixed like this, when we want to get function by pc, the pc value must a an off-set one.Also, data type to describe an addr must be std::intptr_t in case of occurring bizarre problems.

build failed with LLVM 10

As mentioned in aclements/libelfin#44, a small typo in libelfin will cause build failure with LLVM 10

diff --git a/elf/data.hh b/elf/data.hh
index ed5c7a1..4a60944 100644
--- a/elf/data.hh
+++ b/elf/data.hh
@@ -553,7 +553,7 @@ struct Sym<Elf64, Order>
                 return (stb)(info >> 4);
         }

-        void set_binding(stb v) const
+        void set_binding(stb v)
         {
                 info = (info & 0xF) | ((unsigned char)v << 4);
         }