Opening file modes in x86 assembly Linux - linux

I'm learning x86 assembly Linux from the book "Programming from the Ground up", and currently I'm learning how to open a file and read from or write to it.
I'm having trouble with the options for opening files. I know 0 is for read-only, 03101 is for write and truncate, but where can I get the full documentation to all the open options?

It's not about WSL, it's about what Linux distro you chose to install in your WSL. ​Different distros will put things in different places in the filesystem.
​ locate '*fcntl*.h' is a good way to find the appropriate headers.
You can always compile a C program that includes the documented headers (which will pull in the "real" headers), and look at its gcc -E -dM macro defines. Or even
gcc -E -dM /usr/include/fcntl.h | grep ' O_'
to filter just the O_ macro constants. (That fcntl.h is I think likely to be in the plain /usr/include, not buried somewhere, but maybe that's just my Arch GNU/Linux distro keeping it simple. It keeps Linux-specific libc headers like <asm/unistd.h> in /usr/include/asm/, where you can find unistd_32.h and unistd_64.h for 32 and 64-bit call numbers, respectively.) Or let the usual include-path search happen:
echo '#include <fcntl.h>' | gcc -E -dM - | grep ' O_'
Or write code that does printf("%x, %x\n", O_CREAT, O_TRUNC) or whatever to print out some constants you're interested in, whatever header they came from. (Or print out their bitwise OR, like O_CREAT|O_TRUNC).
The permission mode bit constants like S_IRUSR are defined in terms of other constants like __S_IREAD so it's a bit of a rats nest to follow; probably just printing it out is a good idea. Or simply write permission bits in octal, like mov edx, 0o666 (NASM) or mov $0666, %edx (GAS). (Letting umask clear the write-for-other bit on file creation).
The names of the constants to look for can be found in the man page, open(2).

Using: Windows Subsystem
I have found the correct file for x86 asm linux in /usr/include/asm-generic/fcntl.h

Related

How to debug assembly?

I have the following assembly file test which I want to debug,
How can I do that?
Note I am working with x86-64 and att syntax, plus I don't have access to c code.
I want to stop after each line and being able to see the registers in a table (I remember there is such an option).
I tried:
gdb test
r
but I get:
Starting program:
No executable file specified.
Use the "file" or "exec-file" command.
After running GDB on the executable1:
Use start or starti to set a breakpoint in main or _start respectively and run the program.
Or set breakpoints yourself with b 12 to set a breakpoint on source line 12 (if you built with enough debug info for this to work), or b *0x00401007 to set a breakpoint on an address you copy/pasted from disas output.
layout asm / layout reg puts GDB into text-UI mode with "windows" in your terminal for disassembly and registers. (This can be a bit flaky, you sometimes need control-L to redraw the screen, and sometimes GDB crashes when your process exits although I'm not sure if that's specifically from TUI.)
Otherwise without TUI mode, info reg and disas can be useful.
See the bottom of https://stackoverflow.com/tags/x86/info for more asm debugging tips.
Especially strace ./test is highly useful to see the system calls your program makes, decoded into C style. In toy programs you're playing with for your own experimentation, this basically works as an alternative to checking for error return values.
Footnote 1: You're not doing that part correctly:
No executable file specified.
That means no file called test existed in the directory where you ran gdb test.
You have to assemble + link test.S into an executable called test before you can run GDB on that file. If ls -l test shows it, then gdb test can debug it. (And ./test can run it.)
Often gcc -no-pie foo.S is a good choice to make debugging easier: addresses will be fixed at link time, so objdump -drwC -Mintel test output will match the addresses you see at run-time. And the addresses will be numerically smaller, so it's easier to visually spot a code (.text) address vs. .rodata (modern ld puts it in a separate page so it can avoid exec permission) vs. .data / .bss.
Either way, stack addresses are still easy to distinguish from code either way, 0x555... or 0x0000...XXXXXX is in the executable, 0x7fffff... is in the stack, other addresses from mmap are randomized. (But libc also gets mapped to a high address near the stack, with or without PIE.)
(Or if you're writing _start instead of main, gcc -nostdlib -static foo.S implies -no-pie)

How do you assemble, link and run a .s file in linux?

I'm getting a weird error message when trying to assemble and run a .s file using AT&T Intel Syntax. Not sure if I'm even using the correct architecture to begin with, or if I'm having syntax errors, if I'm not using the correct commands to assemble and link, etc. Completely lost and I do not know where to begin.
So basically, I have a file called yea.s , which contains some simple assembler instructions. I then try to compile it using the command as yea.s -o yea.o and then link is using ld yea.o -o yea. When running ld, I get this weird message:ld: warning: cannot find entry symbol _start; defaulting to 000000440000.
This is the program im trying to run, very simple and doesn't really do anything.
resMsg: .asciz "xxxxxxxx"
.text
.global main
main:
pushq $0
ret
I just cannot figure out what's going on. Obviously, this is for school homework. I'm not looking for the answer to the homework, obviously, but this is the starting point to where I can actually start the coding. And I just cant figure out how to simple run the program, which it doesn't say in the assignment. Anyway, thanks in advance guys!
Linux executables require an entry point to be specified. The entry point is the address of the first instruction to be executed in your program. If not specified otherwise, the link editor looks for a symbol named _start to use as an entry point. Your program does not contain such a symbol, thus the linker complains and picks the beginning of the .text section as the entry point. To fix this problem, rename main to _start.
Note further that unlike on DOS, there is nothing to return to from _start. So your attempt to return is going to cause a crash. Instead, call the system call sys_exit to exit the program:
mov $0, %edi # exit status
mov $60, %eax # system call number
syscall # perform exit call
Alternatively, if you want to use the C runtime environment and call functions from the C library, leave your program as is and instead assemble and link using the C compiler driver cc:
cc -o yea yea.s
If you do so, the C runtime environment provides the entry point for you and eventually tries to call a function main which is where your code comes in. This approach is required if you want to call functions from the C library. If you do it this way, make sure that main follows the SysV ABI (calling convention).
Note that even then your code is incorrect. The return value of a function is given in the eax (resp. rax) register and not pushed on the stack. To return zero from main, write
mov $0, %eax # exit status
ret # return from function
In all currently supported versions of Ubuntu open the terminal and type:
sudo apt install as31 nasm
as31: Intel 8031/8051 assembler
This is a fast, simple, easy to use Intel 8031/8051 assembler.
nasm: General-purpose x86 assembler
Netwide Assembler. NASM will currently output flat-form binary files, a.out, COFF and ELF Unix object files, and Microsoft 16-bit DOS and Win32 object files.
If you are using NASM in Ubuntu 18.04, the commands to compile and run an .asm file named example.asm are:
nasm -f elf64 example.asm # assemble the program
ld -s -o example example.o # link the object file nasm produced into an executable file
./example # example is an executable file

Getting undefined reference to "_printf" error for assembly code despite using gcc linker

I am trying to follow the exercise in the book PC Assembly by Paul Carter. http://pacman128.github.io/pcasm/
I'm trying to run the program from 1.4 page 23 on Ubuntu 18. The files are all available on the github site above.
Since original code is for 32bit I compile using
nasm -f elf32
for first.asm and asm_io.asm to get the object files. I also compile driver.c
I use the linker from gcc and run
gcc -m32 -o first first.o asm_io.o driver.o
but it keeps giving me a bun of errors like
undefined reference to '_scanf'
undefined reference to '_printf'
(note _printf appears instead of printf because some conversion is done in the file asm_io.asm to maintain compatibility between windows and linux OS's)
I don't know why these errors are appearing. I also try running using linker directly
ld -m elf_i386 -e main -o first -first.o driver.o asm_io.o -I /lib/i386-linux-gnu/ld-linux.so.2
and many variations since it seems that its not linking with the C libraries.
Any help? Stuck on this for a while and couldn't find a solution on similar questions
Linux doesn't prepend _ to names when mapping from C to asm symbol names in ELF object files1.
So call printf, not _printf, because there is no _printf in libc.
Whatever "compatibility" code did that is doing it wrong. Only Windows and OS X use _printf, Linux uses printf.
So either you've misconfigured something or defined the wrong setting, or it requires updating / porting to Linux.
Footnote 1: In ancient history (like over 20 years ago), Linux with the a.out file format did use leading underscores on symbol names.
Update: the library uses the NASM preprocessor to %define _scanf scanf and so on, but it requires you to manually define ELF_TYPE by assembling with nasm -d ELF_TYPE.
They could have detected ELF32 or ELF64 output formats on their own, because NASM pre-defines __OUTPUT_FORMAT__. Someone should submit a pull-request to make this detection automatic with code something like this:
%ifidn __OUTPUT_FORMAT__, elf32
%define ELF_TYPE 32
%elifidn __OUTPUT_FORMAT__, elf64
%define ELF_TYPE 64
%endif
%ifdef ELF_TYPE
...
%endif

Is it possible to assemble and run raw CPU instructions using `as`?

There are a couple of related questions here.
Consider a program consisting only of the following two instructions
movq 1, %rax
cpuid
If I throw this into a file called Foo.asm, and run as Foo.asm, where as is the portable GNU assembler, I will get a file called a.out, of size 665 bytes on my system.
If I then chmod 700 a.out and try ./a.out, I will get an error saying cannot execute binary file.
Why is the file so large, if I am merely trying to translate two asm instructions into binary?
Why can the binary not be executed? I am providing valid instructions, so I would expect the CPU to be able to execute them.
How can I get exactly the binary opcodes for the asm instructions in my input file, instead of a bunch of extra stuff?
Once I have the answer to 3, how can I get my processor to execute them? (Assuming that I am not running privileged instructions.)
Why is the file so large, if I am merely trying to translate two asm instructions into binary?
Because the assembler creates a relocatable object file which includes additional information, like memory Sections and Symbol tables.
Why can the binary not be executed?
Because it is an (relocatable) object file, not a loadable file. You need to link it in order to make it executable so that it can be loaded by the operating system:
$ ld -o Foo a.out
You also need to give the linker a hint about where your program starts, by specifying the _start symbol.
But then, still, the Foo executable is larger than you might expect since it still contains additional information (e.g. the elf header) required by the operating system to actually launch the program.
Also, if you launch the executable now, it will result in a segmentation fault, since you are loading the contents of address 1, which is not mapped into your address space, into rax. Still, if you fix this, the program will run into undefined code at the end - you need to make sure to gracefully exit the program through a syscall.
A minimal running example (assumed x86_64 architecture) would look like
.globl _start
_start:
movq $1, %rax
cpuid
mov $60, %rax # System-call "sys_exit"
mov $0, %rdi # exit code 0
syscall
How can I get exactly the binary opcodes for the asm instructions in my input file, instead of a bunch of extra stuff?
You can use objcopy to generate a raw binary image from an object file:
$ objcopy -O binary a.out Foo.bin
Then, Foo.bin will only contain the instruction opcodes.
nasm has a -f bin option which creates a binary-only representation of your assembly code. I used this to implement a bare boot loader for VirtualBox (warning: undocumented, protoype only!) to directly launch binary code inside a VirtualBox image without operating system.
Once I have the answer to 3, how can I get my processor to execute them?
You will not be able to directly execute the raw binary file under Linux. You will need to write your own loader for that or not use an operating system at all. For an example, see my bare boot loader link above - this writes the opcodes into the boot loader of a VirtualBox disc image, so that the instructions are getting executed when launching the VirtualBox machine.
The old MS-DOS COM file format does not include a header. It really only contains the binary executable code. The code size can, however, not exceed 64kb. I don't know whether Linux can execute these.
You can write the opcodes into a file using a hexeditor. Then you just need to surround it with an elf header that Linux knows how to execute it.
Here's an example:
hexedit myfile.bin
Now just write your opcodes inside the file using the hexeditor.
After that you need to add the elf header. You could do this by hand and write the elf header into your .bin file, but that a bit tricky. Easiest method is to use a few commands (In this example for 64 bit).
objcopy --input-target=binary --output-target=elf64-x86-64 myfile.bin myfile.o
ld -o myfile myfile.o -T binary.ld
You will also need a linker script. I called this for example binary.ld.
And that are the contents of binary.ld:
ENTRY(_start);
SECTIONS
{
_start = 0x0;
}
Now you can execute your program: ./myfile
Perhaps there's something like exe2bin utility for the GNU tool set. I've used various versions of exe2bin with Microsoft tools, and the ARM toolkit has the ability to produce binaries, but I don't recall if it was directly from the linked output or something like exe2bin.

starting point of ELF executable file?

I compile following C program on lubuntu 12.10 with anjuta
int main()
{
return 0;
}
the file name is foobar
then I open up terminal and write command
ndisasm foobar -b 32 1>asm.txt
(disassemble foobar with 32 bit instruction option and save disassembled result to asm.txt)
I open up asm.txt
there are many 0x0000 and miss-understandable code.
the instruction jg 0x47(0x7F45) on 0x00000000 and dec esp(0x4C) on 0x00000002
seems ELF file format signature.
(because the hex code 0x454c46 is 'ELF' in ascii)
the Linux might load this code to memory and don't jump to 0x00000000 because there is no executable code.
I have questions here.
how do I know the address of starting address?
which code is OK to ignore?(maybe many 0x0000 would be OK to ignore but what else?)
Even for the simplest program like yours, gcc is linking some libraries and some object files (notably crt0.o which calls your main and contains _start, the ELF starting point). And your binary is probably dynamically linked to some libc.so.6 so needs the dynamic linker (use ldd foobar to find out). Use gcc -v to understand what gcc is doing. And objdump has a lot of interesting flags or options.
You may also want to read the Assembly Howto, the X86 calling conventions, this question, the X86-64 ABI, these notes on X86-64 programming, etc

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