From 9bcbd94e461db61aae4e6ebafb76082543ddf04a Mon Sep 17 00:00:00 2001 From: pommicket <pommicket@gmail.com> Date: Mon, 30 Aug 2021 19:09:20 -0700 Subject: start 00 readme --- 00/.gitignore | 1 - 00/A | 1 + 00/README.txt | 147 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 00/hexcompile | Bin 633 -> 632 bytes 4 files changed, 148 insertions(+), 1 deletion(-) create mode 100644 00/A create mode 100644 00/README.txt (limited to '00') diff --git a/00/.gitignore b/00/.gitignore index 35d242b..223b783 100644 --- a/00/.gitignore +++ b/00/.gitignore @@ -1,2 +1 @@ -A B diff --git a/00/A b/00/A new file mode 100644 index 0000000..c556a07 --- /dev/null +++ b/00/A @@ -0,0 +1 @@ +48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 0a diff --git a/00/README.txt b/00/README.txt new file mode 100644 index 0000000..19e57d3 --- /dev/null +++ b/00/README.txt @@ -0,0 +1,147 @@ +--- 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 diff --git a/00/hexcompile b/00/hexcompile index 66c462e..0c462c1 100755 Binary files a/00/hexcompile and b/00/hexcompile differ -- cgit v1.2.3