From d35f6a6ee6da0c11063b6a88a3ce0f88346e8142 Mon Sep 17 00:00:00 2001
From: Mitya Selivanov <automainint@guattari.tech>
Date: Sun, 9 Jun 2024 04:41:24 +0200
Subject: Writing dummy ELF executable

---
 binary_code_generation.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 78 insertions(+), 3 deletions(-)

diff --git a/binary_code_generation.c b/binary_code_generation.c
index 3735b2c..e0853ec 100755
--- a/binary_code_generation.c
+++ b/binary_code_generation.c
@@ -40,6 +40,8 @@ exit $?
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 
 typedef signed char        i8;
 typedef signed short       i16;
@@ -341,6 +343,12 @@ void unit_set_entry_point(entity_pool_t *pool, i64 unit, i64 entry_point_proc) {
   pool->entities[unit].unit.entry_point = entry_point_proc;
 }
 
+//  ================================================================
+//
+//    Code generation proc
+//
+//  ================================================================
+
 void unit_write(entity_pool_t *pool, i64 unit, i16 target, FILE *out) {
   assert(pool != NULL);
   assert(pool->entities[unit].is_enabled);
@@ -348,10 +356,76 @@ void unit_write(entity_pool_t *pool, i64 unit, i16 target, FILE *out) {
   assert(out != NULL);
   assert(target == (FORMAT_ELF | ARCH_X86_64));
 
-  //  TODO
-  printf("[TODO] Writing ELF x86_64 executable...");
+  printf("Writing ELF x86_64 executable...\n");
+
+  u16 ehs   = 64;
+  u16 shs   = 0;
+  u16 phs   = 56;
+  u64 align = 8;
+
+  u8 code[16] = {
+    0xb8,             // mov rax
+    0x3c, 0, 0, 0,    // 60            // exit
+    0x48, 0x31, 0xff, // xor rdx, rdx  // rdx = 0
+    0x0f, 0x05,       // syscall
+  };
+
+  u64 code_offset = (ehs + phs + (align - 1)) & (~(align - 1));
+  u64 code_size   = sizeof code;
+
+  assert((code_offset % align) == 0);
+  assert((code_size % align) == 0);
+
+  u64 entry = 0x400000 + ehs + phs;
+
+  //  ELF header
+  //
+
+  fwrite( (u8[4]) { 0x7f, 'E', 'L', 'F' }, 1, 4, out); // magic
+
+  fwrite(&(u8)    {  2 }, 1, 1, out); // elf64
+  fwrite(&(u8)    {  1 }, 1, 1, out); // 2's complement, little endian
+  fwrite(&(u8)    {  1 }, 1, 1, out); // current version
+  fwrite(&(u8)    {  0 }, 1, 1, out); // SysV
+  fwrite(&(u8)    {  0 }, 1, 1, out); // ABI version
+  fwrite( (u8[7]) {  0 }, 1, 7, out); // padding
+
+  fwrite(&(u16) {      2 }, 2, 1, out); // executable
+  fwrite(&(u16) {     62 }, 2, 1, out); // x86_64
+  fwrite(&(u32) {      1 }, 4, 1, out); // current version
+  fwrite(&(u64) { entry  }, 8, 1, out); // entry point address
+  fwrite(&(u64) { ehs    }, 8, 1, out); // program header offset
+  fwrite(&(u64) {      0 }, 8, 1, out); // section header offset
+  fwrite(&(u32) {      0 }, 4, 1, out); // flags
+  fwrite(&(u16) { ehs    }, 2, 1, out); // ELF header size
+  fwrite(&(u16) { phs    }, 2, 1, out); // program header size
+  fwrite(&(u16) {      1 }, 2, 1, out); // program header count
+  fwrite(&(u16) { shs    }, 2, 1, out); // section header size
+  fwrite(&(u16) {      0 }, 2, 1, out); // section header count
+  fwrite(&(u16) {      0 }, 2, 1, out); // string table section header index
+
+  //  Program header
+  //
+
+  fwrite(&(u32) {           1 }, 4, 1, out); // type   (PT_LOAD)
+  fwrite(&(u32) {           5 }, 4, 1, out); // flags  (PF_X | PF_R)
+  fwrite(&(u64) { code_offset }, 8, 1, out); // offset
+  fwrite(&(u64) { entry       }, 8, 1, out); // vaddr
+  fwrite(&(u64) { entry       }, 8, 1, out); // paddr
+  fwrite(&(u64) { code_size   }, 8, 1, out); // filesz
+  fwrite(&(u64) { code_size   }, 8, 1, out); // memsz
+  fwrite(&(u64) {           8 }, 8, 1, out); // align
+
+  //  Code
+  //
+
+  for (i64 i = code_offset - ehs - phs; i > 0; --i)
+    fwrite(&(u8) { 0 }, 1, 1, out);
+
+  fwrite(code, 1, code_size, out);
 }
 
+
 //  ================================================================
 //
 //    Helpers
@@ -401,11 +475,12 @@ void u_elf_x86_64(i64 unit, c8 const *output_file_name) {
   assert(f != NULL);
   unit_write(&g_pool, unit, FORMAT_ELF | ARCH_X86_64, f);
   fclose(f);
+  chmod(output_file_name, 0775);
 }
 
 //  ================================================================
 //
-//    Code generation proc
+//    Example
 //
 //  ================================================================
 
-- 
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