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diff --git a/00/README.md b/00/README.md new file mode 100644 index 0000000..d69bc63 --- /dev/null +++ b/00/README.md @@ -0,0 +1,383 @@ +# 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](https://filippo.io/linux-syscall-table/) 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. <s>I just wanted to +show two different ways of doing it</s> 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. |