A simple elf sample code

Author: AndersSundman

.gitignore

@@ -1 +1 @@
-xmas
+

Makefile

@@ -1,2 +1,14 @@
 xmas: xmas.c xmas.ld Makefile
-	gcc -T xmas.ld -fno-asynchronous-unwind-tables -fno-unwind-tables -nostdlib -no-pie -o $@ $<
+	gcc -T xmas.ld -fno-asynchronous-unwind-tables -fno-unwind-tables -nostdlib -no-pie -o build/$@ $<
+
+hello: hello.c Makefile
+	gcc -o build/$@ $<
+
+hello-nostd: hello-syscall.c Makefile
+	gcc -Wl,-entry=startup -nostdlib -o build/$@ $<
+
+hello-nostd-nopie: hello-syscall.c Makefile
+	gcc -no-pie -nostdlib -Wl,-e,startup -Wl,--build-id=none -fcf-protection=none -fno-stack-protector -fno-asynchronous-unwind-tables -fno-unwind-tables -o build/$@ $<
+
+hello-data: hello-data.c Makefile
+	gcc -T hello.ld -no-pie -nostdlib -Wl,-e,startup -Wl,--build-id=none -fcf-protection=none -fno-stack-protector -fno-asynchronous-unwind-tables -fno-unwind-tables -o build/$@ $<

README.md

@@ -4,7 +4,7 @@ This repo contains sample code for my two blog posts: [A Simple ELF](https://4zm
 
 ## XMAS Special
 
-Let's have some fun with linker scripts, ANSI codes and emojis!
+Let's have some fun with linker scripts, ANSI codes, and emojis!
 
 ![objdump](xmas.gif)
 
@@ -25,6 +25,43 @@ SECTIONS
 ENTRY(xmas)
 ```
 
-If you want to dig in to some more details about how all this works, my other article "[A Simple ELF](https://4zm.org/2024/12/25/a-simple-elf.html)" might be helpful.
+If you want to dig in to some more details about how all this works, my blog post "[A Simple ELF](https://4zm.org/2024/12/25/a-simple-elf.html)" might be helpful.
 
 What other fun things can you come up with? Multi-line (ie. multi segment) ASCII art? Or hiding text using other ANSI codes? Let me know!
+
+## A Simple ELF
+
+These are a few examples of peeling away complexity from my blog post [A Simple ELF](https://4zm.org/2024/12/25/a-simple-elf.html):
+
+```
+$ make hello
+$ make hello-nostd
+$ make hello-nostd-nopie
+$ make hello-data
+```
+
+In the end, we arrive at:
+
+```
+$ objdump -t -h ./build/hello-data
+
+./build/hello-data:     file format elf64-x86-64
+
+Sections:
+Idx Name          Size      VMA               LMA               File off  Algn
+  0 📜 .text    0000005c  00000000c0de0000  00000000c0de0000  00001000  2**0
+                  CONTENTS, ALLOC, LOAD, READONLY, CODE
+  1 📦 .data    00000008  00000000feed0000  00000000feed0000  00003000  2**3
+                  CONTENTS, ALLOC, LOAD, DATA
+  2 📁 .bss     00000008  00000000feed0008  00000000feed0008  00003008  2**3
+                  ALLOC
+  3 🧊 .rodata  00000013  00000000dead0000  00000000dead0000  00002000  2**4
+                  CONTENTS, ALLOC, LOAD, READONLY, DATA
+SYMBOL TABLE:
+0000000000000000 l    df *ABS*	0000000000000000 hello-data.c
+00000000c0de002f g     F 📜 .text	000000000000002d startup
+00000000feed0000 g     O 📦 .data	0000000000000008 length
+00000000dead0000 g     O 🧊 .rodata	0000000000000013 message
+00000000c0de0000 g     F 📜 .text	000000000000002f main
+00000000feed0008 g     O 📁 .bss	0000000000000008 status
+```

build/.gitignore

@@ -0,0 +1 @@
+*

hello-data.c

@@ -0,0 +1,31 @@
+const char message[] = "Hello Simplicity!\n";   // .rodata
+unsigned long length = sizeof(message) - 1;     // .data
+unsigned long status;                           // .bss
+
+int main() {
+  // write(1, message, length)
+  asm volatile("mov $1, %%rax\n"                // write syscall number (0x01)
+               "mov $1, %%rdi\n"                // Stdout file descriptor (0x01)
+               "mov %0, %%rsi\n"                // Message buffer
+               "mov %1, %%rdx\n"                // Buffer length
+               "syscall"                        // Make the syscall
+               :                                // No output operands
+               : "r"(message), "r"(length)      // Input operands
+               : "%rax", "%rdi", "%rsi", "%rdx" // Clobbered registers
+  );
+
+  return 0;
+}
+
+void startup() {
+  status = main();
+
+  // exit(status)
+  asm volatile("mov $0x3c, %%rax\n" // exit syscall number (0x3c)
+               "mov %0, %%rdi\n"    // exit status
+               "syscall"            // Make the syscall
+               :                    // No output operands
+               : "r"(status)        // Input operands
+               : "%rax", "%rdi"     // Clobbered registers
+  );
+}

hello-syscall.c

@@ -0,0 +1,30 @@
+int main() {
+  volatile const char message[] = "Hello Simplicity!\n";
+  volatile const unsigned long length = sizeof(message) - 1;
+
+  // write(1, message, length)
+  asm volatile("mov $1, %%rax\n"                // write syscall number (0x01)
+               "mov $1, %%rdi\n"                // Stdout file descriptor (0x01)
+               "mov %0, %%rsi\n"                // Message buffer
+               "mov %1, %%rdx\n"                // Buffer length
+               "syscall"                        // Make the syscall
+               :                                // No output operands
+               : "r"(message), "r"(length)      // Input operands
+               : "%rax", "%rdi", "%rsi", "%rdx" // Clobbered registers
+  );
+
+  return 0;
+}
+
+void startup() {
+  volatile unsigned long status = main();
+
+  // exit(status)
+  asm volatile("mov $0x3c, %%rax\n" // exit syscall number (0x3c)
+               "mov %0, %%rdi\n"    // exit status
+               "syscall"            // Make the syscall
+               :                    // No output operands
+               : "r"(status)        // Input operands
+               : "%rax", "%rdi"     // Clobbered registers
+  );
+}

hello.c

@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+int main() {
+    printf("Hello Simplicity!\n");
+}

hello.ld

@@ -0,0 +1,28 @@
+MEMORY {
+  IRAM (rx) : ORIGIN = 0xC0DE0000, LENGTH = 0x1000
+  RAM  (rw) : ORIGIN = 0xFEED0000, LENGTH = 0x1000
+  ROM  (r)  : ORIGIN = 0xDEAD0000, LENGTH = 0x1000
+}
+
+SECTIONS
+{
+  "📜 .text" : {
+    *(.text*)
+  } > IRAM
+
+  "📦 .data" : {
+    *(.data*)
+  } > RAM
+
+  "📁 .bss" : {
+    *(.bss*)
+  } > RAM
+
+  "🧊 .rodata" : {
+    *(.rodata*)
+  }  > ROM
+
+  /DISCARD/ : { *(.comment) }
+}
+
+ENTRY(startup)