I played a little bit with nice library called libmonkey which gives nice interface for dynamic ELF patching on Linux. However, I'm trying to run it on FreeBSD; compilation passed, but I'm getting core dump in runtime (gdb is pointing header offset calculation code).
I'm not familiar with ELF, but aren't headers should be the same on linux and freebsd?
Anyone have idea what is happening?
Thanks
Related
I have seen a document here http://www.codeproject.com/Articles/3472/Finding-crash-information-using-the-MAP-file. This example is all about crash seen in Windows file? I am looking for the same mathematics that has been done here for the crash generated on Linux systems. If I get the crash on Linux, then how will I debug the issue in the similar lines like Microsoft document http://www.codeproject.com/Articles/3472/Finding-crash-information-using-the-MAP-file. Please help?
Is load address and the code segment address are same in Linux? what is the following in Linux as mentioned in the link "The first part of the binary is the Portable Executable (PE), which is 0x1000 bytes long."?
PE is windows format. Linux uses ELF. Of course you can parse ELF manually, but you shouldn't - gdb could do that for you. Even more, you can use addr2line utility to map address to file/line in source code (of course both of these will require debug build).
Map files are rarely used in linux - it usually just a part of debug executable. It could be dumped from debug build, however, but it don't have too much practical value.
Also, take a look at How to use addr2line command in linux
I'm writing a kernel for educational purposes and I want to integrate a disassembler into my kernel.
Since I'm going to integrate it into the kernel I want it to be very small and simple,
i.e I only need it to receive a memory address and return the assembly instruction at that address.
I've looked for an open source that can do that but all I found is big libraries which are way too big and complicated.
I'm talking about a few (2-4) source code files that does not require a complicated installation/integration and that is very limited in it's abilities and only provide the very basic functionalities.
Does anybody know such an open source (for C of course)?
You might have heard of the radare2 project (github). It includes rasm which is a quite simple disassembler.
You can find their code in their github repository.
If you're using GNU gcc & binutils, you can just use objdump. The way I usually use it is:
objdump -dSr my_file.o > my_file.s
Recently I tried to write a simple compiler on the linux platform by myself.
When it comes to the backend of the compiler, I decided to generate ELF-formatted binaries without using a third-party library, such as libelf.
Instead I want to try to write machine code directly into the file coresponding to the ELF ABI just by using the write() function and controlling all details of the ELF file.
The advantage of this approach is that I can control everything for my compiler.
But I am hesitating. Is that way feasible, considering how detailed the ELF ABI is?
I hope for any suggestions and pointers to good available resources available.
How easy/feasible this is depends on what features you want to support. If you want to use dynamic linking, you have to deal with the symbol table, relocations, etc. And of course if you want to be able to link with existing libraries, even static ones, you'll have to support whatever they need. But if your goal is just to make standalone static ELF binaries, it's really very easy. All you need is a main ELF header (100% boilerplate) and 2 PT_LOAD program headers: one to load your program's code segment, the other to load its data segment. In theory they could be combined, but security-hardened kernels do not allow a given page to be both writable and executable, so it would be smart to separate them.
Some suggested reading:
http://www.linuxjournal.com/article/1059
I recently received a closed-source SDK consisting of a C header file (.h), a library file (.lib), and a dynamic library (.dll). They were compiled using Microsoft's Visual C++. However, I am attempting to write my code using MinGW (GCC ported to Windows, for anyone unfamiliar with the project). It appears that ld is unable to link to the .lib file. I was wondering if it was possible to write a compatibility wrapper between the VS-compiled code and the GCC code I'm writing.
Is there an ABI mismatch or does it just not want to to link against the object format? If it's just a linking problem, you can extract the functions you care about, disassemble them, and then reassemble them into an object your linker can handle. Even easier, maybe objcopy(1) can speak both formats and can help you out?
If you do have an ABI problem to deal with, you can do the same but also add a shim layer to thunk the ABI so that the function calls will work. How complicated that layer is and how difficult it will be to write will depend on the interfaces of the functions you're trying to use.
Don't get too discouraged by the comments - it's software, so pretty much anything is possible.
This is a hard question to ask because I'm positive I'm about to be bombarded with haters commenting on "if I can't write an operating system already, I won't ever to be able to write an operating system". Well I've read Modern OS by Tanembaum, Linux Kernel Development, Understanding the Linux kernel and others I still don't know if or not I can write an operating system and only by pushing forward to write one will I realise what I don't know. On top of that none of the books I read even bother to describe the boot sequence / compilation sequence.
Anyway I hate to be negative but I would just like to build the example code from the bkerndev tutorial below and have an absolutely minimum operating system:
http://www.osdever.net/bkerndev/index.php?the_id=90
You can download the associated source code in a zip format from here:
http://www.osdever.net/bkerndev/bkerndev.zip
When you try and compile this kernel you run into all sorts of errors caused by the fact that some of the code is broken. Another user was seeking help for this here on stack overflow here:
compiling my own kernel (not from linux-kernel source)
Although didn't get much help. I have addressed those errors by adding the gcc flag fleading-underscores and by changing some of the data types. You can see my code here:
http://github.com/PhillipTaylor/farmix
The code will compile sucessfully and leave me with a kernel.bin executable but when I boot into it from grub I get:
Error 13: Unrecognised or unsupported format (or something to that nature)
When I take the kernel.bin directly from the authors zip file and run it on my eeepc it boots absolutely fine so I think I have a problem with compiling the code correctly. The author is building it from a Windows machine, I believe, but I am trying to compile it using Fedora 10 i386 with GNU GCC 4.3 and I think this is what is causing the issue so I ask you, how do I build a valid executable kernel? Am I missing the correct target or the wrong binary format?
I would really appreciate someone helping me over this embarrassing "first step"
My comment above wasn't very clear. What I meant is "What does the 'file' command report on your kernel.bin vs. theirs?". The output of the linker is a raw binary file. It should start with a few magic words that grub recognizes. They are defined in start.asm near "mboot". I suspect yours is different than theirs.
I don't have nasm handy so I can't build, but you might want to start by comparing the first few words of the .bin file.
It turns out that the line used to compile the app was explicitly set to compile to "aout" format which was what the guide said and what I assumed to be true. Only reading stuff in the "barebones" guide convinced me that I may have been confused. As soon as I changed that one line to "nasm -f elf" it all worked.
This is a tag in my repository that points to a basic WORKING version of bkerndev tutorial code (How to write your own Operating system) for future reference and people who were in my position..
It comes with a makefile for building it from a 32 bit Linux system.
http://github.com/PhillipTaylor/farmix/tree/bkerndev_tutorial_working