pommicket 2288e47516 readme tweaks, mainly

2021-11-10 12:55:41 -05:00

17 KiB

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stage 00

This directory contains the file hexcompile, a handwritten executable. It takes input file in00 containing space/newline/(any character)-separated hexadecimal digit pairs (e.g. 3f) and outputs them as bytes to the file out00. On 64-bit Linux, try running ./hexcompile from this directory (I've already provided an in00 file, which you can take a look at), and you will get a file named out00 containing the text Hello, world!. This stage lets you use your favorite text editor to write executables (which have bytes outside of ASCII/UTF-8). I made hexcompile with a program called hexedit, which can be found in most Linux package managers. The executable is just 632 bytes long. Let's take a look at what's inside (od -t x1 -An -v 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 6d 02 40 00 00 00
00 00 48 89 c7 31 c0 48 89 c6 48 b8 02 00 00 00
00 00 00 00 0f 05 48 b8 72 02 40 00 00 00 00 00
48 89 c7 48 b8 41 02 00 00 00 00 00 00 48 89 c6
48 b8 ed 01 00 00 00 00 00 00 48 89 c2 48 b8 02
00 00 00 00 00 00 00 0f 05 48 b8 03 00 00 00 00
00 00 00 48 89 c7 48 89 c2 48 b8 6a 02 40 00 00
00 00 00 48 89 c6 31 c0 0f 05 48 89 c3 48 b8 03
00 00 00 00 00 00 00 48 39 d8 0f 8f 50 01 00 00
48 b8 6a 02 40 00 00 00 00 00 48 89 c3 31 c0 8a
03 48 89 c3 48 b8 39 00 00 00 00 00 00 00 48 39
d8 0f 8c 0f 00 00 00 48 b8 d0 ff ff ff ff ff ff
ff e9 0a 00 00 00 48 b8 a9 ff ff ff ff ff ff ff
48 01 d8 48 c1 e0 04 48 89 c7 48 b8 6b 02 40 00
00 00 00 00 48 89 c3 31 c0 8a 03 48 89 c3 48 b8
39 00 00 00 00 00 00 00 48 39 d8 0f 8c 0f 00 00
00 48 b8 d0 ff ff ff ff ff ff ff e9 0a 00 00 00
48 b8 a9 ff ff ff ff ff ff ff 48 01 d8 48 89 fb
48 09 d8 48 89 c3 48 b8 6c 02 40 00 00 00 00 00
48 93 88 03 48 b8 04 00 00 00 00 00 00 00 48 89
c7 48 b8 6c 02 40 00 00 00 00 00 48 89 c6 48 b8
01 00 00 00 00 00 00 00 48 89 c2 0f 05 e9 f7 fe
ff ff 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 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 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 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 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
31 c0 48 89 c7 48 b8 3c 00 00 00 00 00 00 00 0f
05 00 00 00 00 00 00 00 00 00 00 00 00 69 6e 30
30 00 6f 75 74 30 30 00

Okay, that doesn't tell us much. I'll annotate it below.

ELF header

This header has a bunch of metadata about the executable. Instead of reading my annotations, you can also run readelf -a --wide hexcompile to get this information in a compact form.

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
40 00 00 00 00 00 00 00 Program header table offset in bytes from start of file
00 00 00 00 00 00 00 00 Section header table offset (we're not using sections)
00 00 00 00 Flags (not important to us)
40 00 The size of this header, in bytes = 64
38 00 Size of the program header = 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)

You might notice that all the numbers are backwards, e.g. 38 00 for the number 0x0038 (56 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 reasons for this (see here, for example, if you're interested).

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 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 segment 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 bytes
78 00 40 00 00 00 00 00 Virtual address = 0x400078

wait a minute, what's that?

This is the virtual memory address that the segment will be loaded to. Nowadays, computers use virtual memory, meaning that addresses in our program don't actually correspond to where the memory is physically stored in RAM (the CPU translates between virtual and physical addresses). There are many reasons for this: making sure each process has its own memory space, memory protection, 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 segment's data is loaded to address 0x400078. The actual page of memory that the OS will allocate for our segment 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.

the code

Now we get to the actual code in our executable. 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:

48 b8 6d 02 40 00 00 00 00 00 mov rax, 0x40026d
48 89 c7 mov rdi, rax
31 c0 xor eax, eax (shorter form of mov rax, 0)
48 89 c6 mov rsi, rax
48 b8 02 00 00 00 00 00 00 00 mov rax, 2
0f 05 syscall

Here we open our input file, in00.

These instructions execute syscall 2 with arguments 0x40026d, 0. If you're familiar with C code, this is open("in00", O_RDONLY). 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. You can also install strace (e.g. with sudo apt install strace) and run strace ./hexcompile to see all the syscalls our program does. Syscall #2, on 64-bit Linux, is open. It's used to open a file. You can read about it with man 2 open. The first argument, 0x40026d, is a pointer to some data at the very end of this segment (see further down). Specifically, it holds the bytes 69 6e 30 30 00, the null-terminated ASCII string "in00". This indicates the name of the file. The second argument, 0, specifies that we will (only) be reading from this file. There is a third argument to this syscall (we'll get to it later), but it's not applicable here so we don't set it.

This call gives us back a file descriptor, a number which we can use to read from the file, in register rax. But we don't actually need to look at what file descriptor Linux gave us. This is because Linux assigns file descriptor numbers sequentially, starting from 0 for stdin, 1 for stdout, 2 for stderr, and then 3, 4, 5, ... for any files our program opens. So this file, the first one our program opens, will have descriptor 3.

Now we open our output file:

48 b8 72 02 40 00 00 00 00 00 mov rax, 0x400272
48 89 c7 mov rdi, rax
48 b8 41 02 00 00 00 00 00 00 mov rax, 0x241
48 89 c6 mov rsi, rax
48 b8 ed 01 00 00 00 00 00 00 mov rax, 0o755
48 89 c2 mov rdx, rax
48 b8 02 00 00 00 00 00 00 00 mov rax, 2
0f 05 syscall

In C, this is open("out00", O_WRONLY|O_CREAT|O_TRUNC, 0755). This is quite similar to our first call, with two important differences: first, we specify 0x241 as the second argument. This tells Linux that we are writing to the file (O_WRONLY = 0x01), that we want to create it if it doesn't exist (O_CREAT = 0x40), and that we want to delete any previous contents it had (O_TRUNC = 0x200). Secondly, we're setting the third argument this time. It specifies the permissions our file is created with (0o755 means user read/write/execute, group/other read/execute). This is not very important to the actual execution of the program, so don't worry if you don't know about UNIX permissions. Note that the output file's descriptor will be 4.

Now we can start reading from the file. We're going to loop back to this part of the code every time we want to read a new hexadecimal number from the input file.

48 b8 03 00 00 00 00 00 00 00 mov rax, 3
48 89 c7 mov rdi, rax
48 89 c2 mov rdx, rax
48 b8 6a 02 40 00 00 00 00 00 mov rax, 0x40026a
48 89 c6 mov rsi, rax
31 c0 mov rax, 0
0f 05 syscall

In C, this is read(3, 0x40026a, 3). Here we call syscall #0, read, with three arguments:

fd = 3 This is the descriptor number of our input file.
buf = 0x40026a This is the memory address we want Linux to output the data to.
count = 3 This is the number of bytes we want to read.

We're telling Linux to output to 0x40026a, which is just a part of this segment (see further down). Normally you would read to a different segment of the program from where the code is, but we want this to be as simple as possible. The number of bytes actually read, taking into account that we might have reached the end of the file, is stored in rax.

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 50 01 00 00 jg +0x150 (0x400250)

This tells the CPU to jump to a later part of the code (address 0x400250) if 3 is greater than the number of bytes we got, in other words, if we reached the end of the file. Note that we don't specifiy the address to jump to, but instead the relative address, relative to the first byte after the jump instruction (so here we're saying to jump 0x150 bytes forward). There are reasons for this which I won't get into here.

48 b8 6a 02 40 00 00 00 00 00 mov rax, 0x40026a
48 89 c3 mov rbx, rax
31 c0 mov rax, 0
8a 03 mov al, byte [rbx]

Here we put the ASCII code of the first character read from the file into rax. But now we need to turn the ASCII character code into the actual numerical value of the hex digit.

48 89 c3 mov rbx, rax
48 b8 39 00 00 00 00 00 00 00 mov rax, 0x39 ('9')
48 39 d8 cmp rax, rbx
0f 8c 0f 00 00 00 jl 0x400136

This checks if the character code is greater than the character code for the digit 9, and jumps to a different part of the code if so. This different part of the code will handle the case of the hex digits a through f.

48 b8 d0 ff ff ff ff ff ff ff mov rax, -48

Set rax to the two's complement representation of -48. This will be added to the character code to get the numerical value of the digit (0 has ASCII code 48).

e9 0a 00 00 00 jmp 0x400140

This skips over the a-f handling code (coming up next).

48 b8 a9 ff ff ff ff ff ff ff mov rax, -87

If you add the ASCII code for a to -87 you get 10. Similarly, adding -87 to f gives you 15. So this will convert between a-f digits and numerical values.

48 01 d8 add rax, rbx

Okay, now we add -48 or -87 to the character code to get the numerical value of the digit in rax, whether it was one of 0123456789 or abcdef.

48 c1 e0 04 shl rax, 4
48 89 c7 mov rdi, rax

Now we shift it left by 4 bits (multiply it by 16), because it's the first hex digit, and store it away in rdi. The bottom 4 bits will be the second hex digit in the digit pair, which we'll read now, via a very similar process to the one above:

48 b8 6b 02 40 00 00 00 00 00 mov rax, 0x40026b
48 89 c3 mov rbx, rax
31 c0 mov rax, 0
8a 03 mov al, byte [rbx]
48 89 c3 mov rbx, rax
48 b8 39 00 00 00 00 00 00 00 mov rax, 0x39 ('9')
48 39 d8 cmp rax, rbx
0f 8c 0f 00 00 00 jl 0x400180
48 b8 d0 ff ff ff ff ff ff ff mov rax, -48
e9 0a 00 00 00 jmp 0x40018a
48 b8 a9 ff ff ff ff ff ff ff mov rax, -87
48 01 d8 add rax, rbx
48 89 fb mov rbx, rdi
48 09 d8 or rax, rbx

Okay, now rax contains the byte specified by the two hex digits we read.

48 89 c3 mov rbx, rax
48 b8 6c 02 40 00 00 00 00 00 mov rax, 0x40026c
48 93 xchg rax, rbx
88 03 mov byte [rbx], al

Put the byte in a specific memory location (address 0x40026c).

48 b8 04 00 00 00 00 00 00 00 mov rax, 4
48 89 c7 mov rdi, rax
48 b8 6c 02 40 00 00 00 00 00 mov rax, 0x40026c
48 89 c6 mov rsi, rax
48 b8 01 00 00 00 00 00 00 00 mov rax, 1
48 89 c2 mov rdx, rax
0f 05 syscall

In C, this is write(4, 0x40026c, 1). This calls syscall #1, write, with arguments:

fd = 4 The file descriptor to write to.
buf = 0x40026c Pointer to the data we want to write.
count = 1 The number of bytes to write.
e9 f7 fe ff ff jmp 0x4000c9

This jumps way back in the program, to read the next digit pair from the input file.

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 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 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 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 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

These bytes aren't actually used by our program, and could be set to anything. These are here because I wasn't sure how long the program would be when I started, so I just set the segment size to 512 bytes, which turned out to be more than enough. I could have cut these out and edited all the addresses to get a smaller executable, but really there's no point—modern computers can definitely handle 600-byte files.

31 c0 mov rax, 0
48 89 c7 mov rdi, rax
48 b8 3c 00 00 00 00 00 00 00 mov rax, 60
0f 05 syscall

This is where we conditionally jumped to way back when we determined if we reached the end of the file. This calls syscall #60, exit, with one argument, 0 (exit code 0, indicating we exited successfully).

Normally, you would close files descriptors (with syscall #3), to tell Linux you're done with them, but we don't need to. It'll automatically close all our open file descriptors when our program exits.

00 00 00 00 00 00 00 00 00 (more unused bytes)
00 00 00 this is where we read data to, and wrote data from
69 6e 30 30 00 input filename, "in00"
6f 75 74 30 30 00 output filename, "out00"

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.

limitations

There are many ways in which this is a bad program. It will only properly handle lowercase hexadecimal digit pairs, separated by exactly one character, with a terminating character. What's worse, a bad input file (maybe someone accidentally writes 3F instead of 3f) won't print out a nice error message, but instead continue processing as usual, without any indication that anything's gone wrong, giving you an unexpected result. Also, we only read in data three bytes at a time, and output one byte at a time. This is a very bad idea because syscalls (e.g. read) are slow. read might take ~3 microseconds, which doesn't sound like a lot, but it means that if we used code like this to process a 50 megabyte file, say, we'd be waiting for a while.

But these problems aren't really a big deal. We'll only be running this on little programs and we'll be sure to check that our input is in the right format. And with that, we are ready to move on to the next stage...

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