feat(app): nolibc hello world

app/entry.s

@@ -0,0 +1,29 @@
+.global _start, syscall5
+
+
+_start: 
+    /* Load argc -> rdi, argv[0] -> rsi */
+    xor %rbp, %rbp
+    pop %rdi
+    mov %rsp, %rsi
+
+    and $0xFFFFFFFFFFFFFFF0, %rsp
+    
+    call main
+
+    /* Exit with ret val of main */
+    mov %rax, %rdi
+    mov $0x3C, %rax
+    syscall
+
+syscall5:
+    mov    %rdi, %rax /* %rax (syscall number)  = func param 1 (%rdi)    */
+    mov    %rsi, %rdi /* %rdi (syscall param 1) = func param 2 (%rsi)    */
+    mov    %rdx, %rsi /* %rsi (syscall param 2) = func param 3 (%rdx)    */
+    mov    %rcx, %rdx /* %rdx (syscall param 3) = func param 4 (%rcx)    */
+    mov    %r8,  %r10 /* %r10 (syscall param 4) = func param 5 (%r8)     */
+    mov    %r9,  %r8  /* %r8  (syscall param 5) = func param 6 (%r9)     */
+    syscall           /* Enter a syscall (return value in %rax)          */
+    ret               /* Return value is already in %rax, we can return. */
+    
+.section .note.GNU-stack,"",@progbits

app/nolibc.c

@@ -0,0 +1,18 @@
+void *syscall5(void *number, void *arg1, void *arg2, void *arg3, void *arg4,
+               void *arg5);
+
+typedef unsigned long int size_t;
+typedef long int ssize_t;
+
+static ssize_t write(int fd, void const *data, size_t nbytes) {
+  return (ssize_t)syscall5((void *)1, /* SYS_write, call number 1 */
+                           (void *)(size_t)fd, (void *)data, (void *)nbytes,
+                           0, /* Ignored */
+                           0  /* Ignored */
+  );
+}
+
+int main() {
+  write(0, "Hello World!\n", 13);
+  return 0;
+}

app/nolibc.ld

@@ -0,0 +1,38 @@
+ENTRY(_start)
+
+PHDRS
+{
+    text PT_LOAD FLAGS(5); /* R + X */
+    data PT_LOAD FLAGS(6); /* R + W */
+}
+
+SECTIONS
+{
+    . = 0x400000;
+
+    .text : ALIGN(16) {
+        *(.text .text.*)
+    }:text
+
+    .rodata : {
+        *(.rodata .rodata.*)
+    }:text
+
+    .data : {
+        *(.data .data.*)
+    }:data
+
+    .bss : {
+        *(.bss .bss.*)
+        *(COMMON)
+    }:data
+
+    /DISCARD/ : {
+        *(.note*)
+        *(.comment*)
+        *(.eh_frame*)
+        *(.symtab*)
+        *(.strtab*)
+    }
+}
+