History

pommicket d052391270 stage 00 readme done		2021-08-31 02:10:17 -04:00
..
.gitignore	start 00 readme	2021-08-30 19:09:20 -07:00
A	start 00 readme	2021-08-30 19:09:20 -07:00
hexcompile	stage 00 readme done	2021-08-31 02:10:17 -04:00
README.md	stage 00 readme done	2021-08-31 02:10:17 -04:00

README.md

stage 00

This directory contains the file hexcompile, a handwritten executable. It takes input file A containing space/newline/[any character]-separated hexadecimal numbers and outputs them as bytes to the file B. On 64-bit Linux, try running ./hexcompile from this directory (I've already provided an A file), and you will get a file named B containing the text Hello, world!. This stage is needed so that you can use your favorite text editor to write executables by hand (which have bytes outside of ASCII/UTF-8). I wrote it with a program called hexedit, which can be found on most Linux distributions. Only 64-bit Linux is supported, because each OS/architecture combination would need its own separate executable. The executable is 632 bytes long, and you could definitely make it smaller if you wanted to, especially if you didn't limit it to the set of instructions I've decided on. Let's take a look at what's inside (od -t x1 -An hexcompile):

7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
02 00 3e 00 01 00 00 00 78 00 40 00 00 00 00 00
40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 40 00 38 00 01 00 00 00 00 00 00 00
01 00 00 00 07 00 00 00 78 00 00 00 00 00 00 00
78 00 40 00 00 00 00 00 00 00 00 00 00 00 00 00
00 02 00 00 00 00 00 00 00 02 00 00 00 00 00 00
00 10 00 00 00 00 00 00 48 b8 74 02 40 00 00 00
00 00 48 89 c7 48 b8 00 00 00 00 00 00 00 00 48
89 c6 48 89 c2 48 b8 02 00 00 00 00 00 00 00 0f
05 48 89 c5 48 b8 76 02 40 00 00 00 00 00 48 89
c7 48 b8 41 00 00 00 00 00 00 00 48 89 c6 48 b8
a4 01 00 00 00 00 00 00 48 89 c2 48 b8 02 00 00
00 00 00 00 00 0f 05 48 89 ef 48 b8 68 02 40 00
00 00 00 00 48 89 c6 48 b8 03 00 00 00 00 00 00
00 48 89 c2 48 b8 00 00 00 00 00 00 00 00 0f 05
48 89 c3 48 b8 03 00 00 00 00 00 00 00 48 39 d8
0f 8f 37 01 00 00 48 b8 68 02 40 00 00 00 00 00
48 89 c3 48 8b 03 48 89 c3 48 89 c7 48 b8 ff 00
00 00 00 00 00 00 48 21 d8 48 89 c6 48 b8 39 00
00 00 00 00 00 00 48 89 c3 48 89 f0 48 39 d8 0f
8f 1e 00 00 00 48 b8 30 00 00 00 00 00 00 00 48
f7 d8 48 89 f3 48 01 d8 e9 26 00 00 00 00 00 00
00 00 00 48 b8 a9 ff ff ff ff ff ff ff 48 89 f3
48 01 d8 e9 0b 00 00 00 00 00 00 00 00 00 00 00
00 00 00 48 89 c2 48 b8 ff 00 00 00 00 00 00 00
48 89 c3 48 89 f8 48 c1 e8 08 48 21 d8 48 93 48
b8 39 00 00 00 00 00 00 00 48 93 48 39 d8 0f 8f
1f 00 00 00 48 89 c3 48 b8 d0 ff ff ff ff ff ff
ff 48 01 d8 e9 2a 00 00 00 00 00 00 00 00 00 00
00 00 00 48 89 c3 48 b8 a9 ff ff ff ff ff ff 48
01 d8 e9 0c 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 48 89 c7 48 89 d0 48 c1 e0 04 48 89 fb
48 09 d8 48 93 48 b8 68 02 40 00 00 00 00 00 48
93 48 89 03 48 89 de 48 b8 04 00 00 00 00 00 00
00 48 89 c7 48 b8 01 00 00 00 00 00 00 00 48 89
c2 0f 05 e9 8f fe ff ff 00 00 00 00 00 48 b8 3c
00 00 00 00 00 00 00 0f 05 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 41 00 42 00

Okay, that doesn't tell us much. I'll annotate it below. You might notice that all the numbers are backwards, e.g. 3e 00 for the number 0x003e (62 decimal). This is because almost all modern architectures (including x86-64) are little-endian, meaning that the least significant byte goes first, and the most significant byte goes last. There are various reasons why this is easier to deal with, but I won't explain that here.

ELF header

This header has a bunch of metadata about the executable.

7f 45 4c 46 Special identifier saying that this is an ELF file (ELF is the format of almost all Linux executables)
02 64-bit
01 Little-endian
01 ELF version 1 (there is no version 2 yet)
00 00 00 00 00 00 00 00 00 Reserved (not important yet, but may be in a later version of ELF)
02 00 Object type = executable file (not a dynamic library/etc.)
3e 00 Architecture x86-64
01 00 00 00 Version 1 of ELF, again
78 00 40 00 00 00 00 00 Entry point of the executable = 0x400078 (explained later)
40 00 00 00 00 00 00 00 Program header table offset in bytes from start of file (see below)
00 00 00 00 00 00 00 00 Section header table offset (we're not using sections)
00 00 00 00 Flags (not important)
40 00 The size of this header, in bytes = 64
38 00 Size of the program header (see below) = 56
01 00 Number of program headers = 1
00 00 Size of each section header (unused)
00 00 Number of section headers (unused)
00 00 Index of special .shstrtab section (unused)

program header

The program header describes a segment of data that is loaded into memory when the program starts. Normally, you would have more than one of these, maybe one for code, one for read-only data, and one for read-write data, but to simplify things we've only got one, which we'll use for any code and any data we need. This means it'll have to be read-enabled, write-enabled, and execute-enabled. Normally people don't do this, for security, but we won't worry about that (don't compile any untrusted code with any compiler from this series!) Without further ado, here's the contents of the program header:

01 00 00 00 Segment type 1 (this should be loaded into memory)
07 00 00 00 Flags = RWE (readable, writeable, and executable)
78 00 00 00 00 00 00 00 Offset in file = 120
78 00 40 00 00 00 00 00 Virtual address = 0x400078

wait a minute, what's that?

We just specified the virtual address of this segment. This is the virtual memory address that the segment will be loaded to. Virtual memory means that memory addresses in our program do not actually correspond to where the memory is physically stored in RAM. There are many reasons for it, including allowing different processes to have overlapping memory addresses, making sure that some memory can't be read/written/executed, etc. You can read more about it elsewhere.

00 00 00 00 00 00 00 00 Physical address (not applicable)
00 02 00 00 00 00 00 00 Size of this segment in the executable file = 512 bytes
00 02 00 00 00 00 00 00 Size of this segment when loaded into memory = also 512 bytes
00 10 00 00 00 00 00 00 Segment alignment = 4096 bytes

That last field, segment alignment, is needed, because on default-settings Linux each page (block) of memory is 4096 bytes long, and has to start at an address that is a multiple of 4096. Our program needs to be loaded into a memory page, so its virtual address needs to be a multiple of 4096. We're using 0x400000. But wait! Didn't we use 0x400078 for the virtual address? Well, yes but that's because the data in the file is loaded to address 0x400078. The actual page of memory that the OS will allocate for our code will start at 0x400000. The reason we need to start 0x78 bytes in is that Linux expects the data in the file to be at the same position in the page as when it will be loaded, and it appears at offset 0x78 in our file. Don't worry if you didn't understand all of that.

the code

Now we get to the actual code in our executable (well there's a bit of data here too). We specified 0x400078 as the entry point of our executable, which means that the program will start executing from there. That virtual address corresponds to the start of the code right here:

The first thing we want to do is open our input file, A:

48 b8 74 02 40 00 00 00 00 00 mov rax, 0x400274
48 89 c7 mov rdi, rax
48 b8 00 00 00 00 00 00 00 00 mov rax, 0
48 89 c6 mov rsi, rax
48 89 c2 mov rdx, rax
48 b8 02 00 00 00 00 00 00 00 mov rax, 2
0f 05 syscall

These instructions execute syscall 2 with arguments 0x400274, 0, 0. If you're familiar with C code, this is open("A", O_RDONLY, 0). A syscall is the mechanism which lets software ask the kernel to do things. Here is a nice table of syscalls you can look through if you're interested. Syscall #2, on Linux, is open. It's used to open a file. On Linux, you can read about it by running man 2 open. The first argument, 0x400274, is a pointer to some data at the very end of this segment (scroll down). Specifically, it holds the byte 41 (ASCII A), followed by 00 (null byte). This indicates the name of the file, "A". The second argument (O_RDONLY, or 0) specifies that we will be reading from this file. The third is only really needed when creating new files, but I've just set it to 0, why not.

This call gives us back a file descriptor, used later to read from the file, in register rax.

48 89 c5 mov rbp, rax Store the file descriptor for later

Now we'll open the output file

48 b8 76 02 40 00 00 00 00 00 mov rax, 0x400276
48 89 c7 mov rdi, rax
48 b8 41 00 00 00 00 00 00 00 mov rax, 0x41
48 89 c6 mov rsi, rax
48 b8 a4 01 00 00 00 00 00 00 mov rax, 0o644
48 89 c2 mov rdx, rax
48 b8 02 00 00 00 00 00 00 00 mov rax, 2
0f 05 syscall

These instructions execute the syscall open("B", O_WRONLY|O_CREAT, 0644). This is similar to our first one, but with some important differences. First, the second argument specifies both that we are writing to a file 0x01, and that we want to create the file if it doesn't exist 0x40. Secondly, the third argument specifies the permissions that the file should be created with (644 - user read/write, group read). This here isn't particularly important to how the program works.

48 89 ef mov rdi, rbp
48 b8 68 02 40 00 00 00 00 00 mov rax, 0x400268
48 89 c6 mov rsi, rax
48 b8 03 00 00 00 00 00 00 00 mov rax, 3
48 89 c2 mov rdx, rax
48 b8 00 00 00 00 00 00 00 00 mov rax, 0
0f 05 syscall

Here we call syscall #0 (read) to read from a file. The arguments are:

fd (rdi) = rbp read from the file descriptor we stored away earlier
buf (rsi) = 0x400268 output to a part of this segment I've left empty
count (rdx) = 3 read 3 bytes

The number of bytes actually read (taking into account the fact that we might have reached the end of the file) is stored in rax.

Note that we read the entire file 3 bytes at a time, which is a terrible idea for performance. syscalls take quite a while (3 microseconds or so, which would make this very slow for a several-megabyte file), so modern programs tend to read ~4KB at a time. But our programs will be small, and we don't care a lot about performance, so it's okay.

48 89 c3 mov rbx, rax
48 b8 03 00 00 00 00 00 00 00 mov rax, 3
48 39 d8 cmp rax, rbx
0f 8f 37 01 00 00 jg 0x40024d

Together, these instructions say to jump to a different part of the code (explained later), if we ended up reading less than 3 bytes, i.e. we reached the end of the file. Note that rather than specifying the address to jump to, we specify the relative address (it's relative to the address of the first byte after the jump instruction). In other words, we're adding 0x137 to the program counter, rip. This has many reasons including saving space.

48 b8 68 02 40 00 00 00 00 00 mov rax, 0x400268
48 89 c3 mov rbx, rax
48 8b 03 mov rax, qword [rbx]

This copies out 8 bytes of the data that was just read into the 64-bit register rax. We only read 3 bytes of data from the file, but the rest will just be zeros (because that's what we put at offset 0x268 of the file).

48 89 c3 mov rbx, rax
48 89 c7 mov rdi, rax

Here we copy away this data for later use.

48 b8 ff 00 00 00 00 00 00 00 mov rax, 0xff
48 21 d8 and rax, rbx

This grabs the first byte of data we read and stores it in rax. This will be the code of the first ASCII character of the hexadecimal number in our input file.

48 89 c6 mov rsi, rax
48 b8 39 00 00 00 00 00 00 00 mov rax, 0x39 ('9')
48 89 c3 mov rax, rbx
48 89 f0 mov rax, rsi
48 39 d8 cmp rax, rbx
0f 8f 1e 00 00 00 jg 0x400173

These instructions compare that character code against the character code for 9. If it's greater, then it's one of the hex digits a through f, which are handled separately later.

48 b8 30 00 00 00 00 00 00 00 mov rax, 0x30 ('0')
48 f7 d8 neg rax
48 89 f3 mov rbx, rsi
48 01 d8 add rax, rbx

Subtract the character code for 0 from the character code we read in, to get the number corresponding to the first hex digit in the pair.

e9 26 00 00 00 jmp 0x400193

Go to a different part of the program (we'll get there later).

00 00 00 00 00 00

Unneeded 0 bytes I left in, to make room in case I needed it.

Now we get to the a-f handling code:

48 b8 a9 ff ff ff ff ff ff ff mov rax, -87
48 89 f3 mov rbx, rsi
48 01 d8 add rax, rbx
e9 0b 00 00 00 jmp 0x400193
00 00 00 00 00 00 00 00 00 00 00 (unused)

If our character code is one of abcdef, we add -87 (subtract 87) from it, to convert the character code to the numerical value of the digit. Here I decided to just set rax to the two's complement encoding for -87, but you could also use the neg instruction, like I did last time. ~~I just wanted to show two different ways of doing it~~ I thought of the better way the second time around.

Now we get to 0x400193, the common place we jumped to from both branches.

48 89 c2 mov rdx, rax

Store away the first digit in the pair into rdx.

48 b8 ff 00 00 00 00 00 00 00 mov rax, 0xff
48 89 c3 mov rbx, rax
48 89 f8 mov rax, rdi
48 c1 e8 08 shr rax, 8
48 21 d8 and rax, rbx

Now we extract the second character code we read from the file. The entire character code to number conversion is rewritten here, but slightly differently this time because I came up with some new ideas.

48 93 xchg rax, rbx
48 b8 39 00 00 00 00 00 00 00 mov rax, 0x39 ('9')
48 93 xchg rax, rbx
48 39 d8 cmp rax, rbx
0f 8f 1f 00 00 00 jg 0x4001e3 ('a'-'f' handling code)
48 89 c3 mov rbx, rax
48 b8 d0 ff ff ff ff ff ff ff mov rax, -48
48 01 d8 add rax, rbx
e9 2a 00 00 00 jmp 0x400203
00 00 00 00 00 00 00 00 00 00 (unused)

('a'-'f' handling)

48 89 c3 mov rbx, rax
48 b8 a9 ff ff ff ff ff ff mov rax, -87
48 01 d8 add rax, rbx
e9 0c 00 00 jmp 0x400203
00 00 00 00 00 00 00 00 00 00 00 00 00 (unused)

(common code)

48 89 c7 mov rdi, rax

Okay now we've read the first hex digit into rdx, and the second into rdi.

48 89 d0 mov rax, rdx
48 c1 e0 04 shl rax, 4
48 89 fb mov rbx, rsi
48 09 d8 or rax, rbx

Okay, now we have the full hexadecimal number in rax!

48 93 xchg rax, rbx
48 b8 68 02 40 00 00 00 00 00 mov rax, 0x400268
48 93 xchg rax, rbx
48 89 03 mov qword [rbx], rax

This stores the byte we want to write to the file at address 0x400268. This is the same address we used to read in the input text; again, it's just part of this segment I've left blank.

48 89 de mov rsi, rbx
48 b8 04 00 00 00 00 00 00 00 mov rax, 4
48 89 c7 mov rdi, rax
48 b8 01 00 00 00 00 00 00 00 mov rax, 1
48 89 c2 mov rdx, rax
0f 05 syscall

Here we call syscall #1, write, with arguments:

fd = 4 we could have stored away the file descriptor we got before for the output file, like we did with the input file, but I was out of easy-to-use registers! Instead, we can use the fact that Linux assigns file descriptors sequentially starting from 3 (0, 1, and 2 are standard input, output, and error), so we know our output file, the second file we opened, will have descriptor 4.
buf = 0x400268 where we put our data
count = 1 write 1 byte
e9 8f fe ff ff jmp 0x4000d7
00 00 00 00 00 (unused)

Now we go back to read in the next pair of digits! Finally...

48 b8 3c 00 00 00 00 00 00 00 mov rax, 0x3c
0f 05 syscall

This is where we conditionally jumped to way back when we determined if we reached the end of the file. This just calls syscall #60, exit, to exit our program nicely. We didn't specify the exit code, but that's okay for our purposes. And we could close the files (syscall #3), to tell Linux we're done with them, but we don't need to. It'll close all our open file descriptors when our program exits.

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 Unused bytes (I wasn't sure exactly how long the program would be)
00 00 00 00 00 00 00 00 This is where we read/wrote the file data!
41 00 Input file name, "A"
42 00 Output file name, "B"

That's quite a lot to take in for such a simple program, but here we are! We now have something that will let us write individual bytes with an ordinary text editor and get them translated into a binary file.