From 506d1e30bf51b3c8d46759543bafb52e655d0a45 Mon Sep 17 00:00:00 2001
From: Eduard Urbach <admin@akyoto.dev>
Date: Wed, 18 Oct 2023 17:03:31 +0200
Subject: [PATCH] Implemented basic data access

---
 build/Build.go      | 12 ++++++++++--
 build/elf/Header.go |  3 ++-
 build/elf/elf.go    | 12 +++++-------
 build/elf/elf.md    | 24 +++++++++++++++++++-----
 4 files changed, 36 insertions(+), 15 deletions(-)

diff --git a/build/Build.go b/build/Build.go
index 0d0b156..ac3198e 100644
--- a/build/Build.go
+++ b/build/Build.go
@@ -38,9 +38,17 @@ func New(directory string) (*Build, error) {
 func (build *Build) Run() error {
 	if build.WriteExecutable {
 		sampleCode := []byte{
-			0xb8, 0x3c, 0x00, 0x00, 0x00, // mov rax, 60
-			0xbf, 0x00, 0x00, 0x00, 0x00, // mov rdi, 0
+			0xb8, 0x01, 0x00, 0x00, 0x00, // mov eax, 1
+			0xbf, 0x01, 0x00, 0x00, 0x00, // mov edi, 1
+			0xbe, 0x80 + 0x22, 0x00, 0x40, 0x00, // mov esi, 0x4000A2
+			0xba, 0x06, 0x00, 0x00, 0x00, // mov edx, 6
 			0x0f, 0x05, // syscall
+
+			0xb8, 0x3c, 0x00, 0x00, 0x00, // mov eax, 60
+			0xbf, 0x00, 0x00, 0x00, 0x00, // mov edi, 0
+			0x0f, 0x05, // syscall
+
+			'H', 'e', 'l', 'l', 'o', '\n',
 		}
 
 		return writeToDisk(build.ExecutablePath, sampleCode, nil)
diff --git a/build/elf/Header.go b/build/elf/Header.go
index bdb7e24..49b231c 100644
--- a/build/elf/Header.go
+++ b/build/elf/Header.go
@@ -4,8 +4,9 @@ const (
 	LittleEndian      = 1
 	TypeExecutable    = 2
 	ArchitectureAMD64 = 0x3E
-	Align             = 16
 	HeaderSize        = 64
+	CacheLineSize     = 64
+	Align             = CacheLineSize
 )
 
 // Header contains general information.
diff --git a/build/elf/elf.go b/build/elf/elf.go
index 740ab77..1da251e 100644
--- a/build/elf/elf.go
+++ b/build/elf/elf.go
@@ -6,15 +6,14 @@ import (
 )
 
 const (
-	baseAddress = 0x10000
+	baseAddress = 0x400000
 )
 
 // ELF64 represents an ELF 64-bit file.
 type ELF64 struct {
 	Header
 	ProgramHeader
-	Padding []byte
-	Code    []byte
+	Code []byte
 }
 
 // New creates a new 64-bit ELF binary.
@@ -43,7 +42,7 @@ func New(code []byte) *ELF64 {
 		},
 		ProgramHeader: ProgramHeader{
 			Type:            ProgramTypeLOAD,
-			Flags:           ProgramFlagsExecutable,
+			Flags:           ProgramFlagsExecutable | ProgramFlagsReadable,
 			Offset:          0x80,
 			VirtualAddress:  baseAddress + 0x80,
 			PhysicalAddress: baseAddress + 0x80,
@@ -51,8 +50,7 @@ func New(code []byte) *ELF64 {
 			SizeInMemory:    int64(len(code)),
 			Align:           Align,
 		},
-		Padding: []byte{0, 0, 0, 0, 0, 0, 0, 0},
-		Code:    code,
+		Code: code,
 	}
 
 	return elf
@@ -62,6 +60,6 @@ func New(code []byte) *ELF64 {
 func (elf *ELF64) Write(writer io.Writer) {
 	binary.Write(writer, binary.LittleEndian, &elf.Header)
 	binary.Write(writer, binary.LittleEndian, &elf.ProgramHeader)
-	writer.Write(elf.Padding)
+	writer.Write([]byte{0, 0, 0, 0, 0, 0, 0, 0})
 	writer.Write(elf.Code)
 }
diff --git a/build/elf/elf.md b/build/elf/elf.md
index dd0de12..83df35c 100644
--- a/build/elf/elf.md
+++ b/build/elf/elf.md
@@ -1,12 +1,26 @@
 # ELF
 
-## File contents
+## Basic structure
 
-- ELF header
-- Program headers
-- Sections
-- Section headers
+1. ELF header (0x00 - 0x40)
+2. Program header (0x40 - 0x78)
+3. Padding (0x78 - 0x80)
+4. Machine code (0x80)
 
 ## Entry point
 
 The entry point is defined in the first 64 bytes (ELF header).
+
+## Base address
+
+The minimum base address is controlled by the `mmap` settings:
+
+```shell
+sysctl vm.mmap_min_addr
+```
+
+Usually, this value is 65536 (0x1000).
+
+## Initialization in Linux
+
+See `/lib/modules/$(uname -r)/build/arch/x86/include/asm/elf.h`.