Added a linker

docs/elf.md

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+# ELF
+
+## Basic structure
+
+1. ELF header [0x00 : 0x40]
+2. Program headers [0x40 : 0xB0]
+3. Padding
+4. Executable code [0x1000 : ...]
+5. Padding
+6. Read-only data [0x2000 : ...]
+7. String table
+8. Section headers
+
+## Entry point
+
+The executables are compiled as position-independent executables (PIE). Therefore the entry point is defined as a file offset instead of a static virtual address.
+
+## Padding
+
+To ensure that execution permissions are properly applied,
+the code section and the data section are aligned on page boundaries. Permissions like read, write and execute can only be applied to an entire page in memory.
+
+## Initialization in Linux
+
+ELF loader:
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/binfmt_elf.c
+
+ELF register definitions:
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/include/asm/elf.h
+
+## Links
+
+- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/elf.h
+- https://lwn.net/Articles/631631/
+- https://en.wikipedia.org/wiki/Executable_and_Linkable_Format
+- https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
+- https://nathanotterness.com/2021/10/tiny_elf_modernized.html