From d05239127034da65ca0225f839f0ae3dd9e698bc Mon Sep 17 00:00:00 2001 From: pommicket Date: Tue, 31 Aug 2021 02:10:17 -0400 Subject: stage 00 readme done --- 00/README.txt | 147 ---------------------------------------------------------- 1 file changed, 147 deletions(-) delete mode 100644 00/README.txt (limited to '00/README.txt') diff --git a/00/README.txt b/00/README.txt deleted file mode 100644 index 19e57d3..0000000 --- a/00/README.txt +++ /dev/null @@ -1,147 +0,0 @@ ---- stage 00 --- - -This directory contains the file 'hexcompile', a handwritten executable. -It takes an 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!". -I made this program 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. Let's take a -look at what's inside (see hexdump -C 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 c1 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 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, which I won't explain 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 - This is an executable file (not a dynamic library/etc) -3e 00 - Architecture x86-64 -01 00 00 00 - Version 1 of ELF (minor version or something) -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, one for -code, one for read-only data, and one for read-write data, perhaps, 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 -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 c1 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 41 00 42 00 -- cgit v1.2.3