title: Problems with asm code inside a c script (at linking with other asm script)
body: <p>i will explain my problem...<br>
  I'm coding a kernel in c + assembly, and when I coded some asm in-line in the main c script with gcc as compiler I had this problem:<br>
  All looked fine, the script compiled as always, without errors or warnings, but when I tried to build the version of my program (the kernel I was building) had a linker problem, pointing to a script failure.<br>
  The installation is simple, it is like:<br>
  Step 1: <code>nasm -f elf32 loader.asm -o kasm.o</code><br>
  Step 2: <code>gcc -m32 -c kernel.c -o kc.o</code><br>
  Step 3: <code>ld -m elf_i386 -T linker.ld -o kernel kasm.o kc.o</code><br>
  And you get a executable than can be run at qemu as kernel.<br>
  But when it came to the last step (the step 3) I had this problem:  </p>
  
  <pre><code>kc.o: In function `enablePaging':
  kernel.c:(.text+0x379): undefined reference to `__stack_chk_fail_local'
  ld: kernel: hidden symbol `__stack_chk_fail_local' isn't defined
  ld: final link failed: Bad value
  </code></pre>
  
  <p>I put here the 3 scripts involved in this process:<br>
  1: kernel.c, the main kernel file  </p>
  
  <pre><code>/*
   *
   * kernel.c - version 0.0.1
   * This script is under the license of the distributed package, this license
   * can be found in the package itself
   * Script coded by Cristian Simón for the CKA Proyect
   * ----
   * License: GNU GPL v3
   * Coder: Cristian Simón
   * Proyect: CKA 
   * 
   */
  
  #define BLACK_BGROUND 0X07 /* black background */
  #define WHITE_TXT 0x07 /* light gray on black text */
  #define GREEN_TXT 0x02 /* light green on black text */
  #define RED_TXT 0x04 /* light red on black text*/
  #include &lt;stddef.h&gt;
  #include &lt;stdint.h&gt;
  
  void k_clear_screen();
  void k_sleep_3sec();
  unsigned int k_printf(char *message, unsigned int line, float color);
  
  /* k_clear_screen : to clear the entire text screen */
  void k_clear_screen()
  {
      char *vidmem = (char *) 0xC00B8000;
      unsigned int i=0;
      while(i &lt; (80*25*2))
      {
          vidmem[i]=' ';
          i++;
          vidmem[i]=BLACK_BGROUND;
          i++;
      };
  };
  
  /* k_printf : the message and the line # */
  unsigned int k_printf(char *message, unsigned int line, float color)
  {
      char *vidmem = (char *) 0xC00B8000;
      unsigned int i=0;
  
      i=(line*80*2);
  
      while(*message!=0)
      {
          if(*message=='\n') // check for a new line
          {
              line++;
              i=(line*80*2);
              *message++;
          } else {
              vidmem[i]=*message;
              *message++;
              i++;
              vidmem[i]=color;
              i++;
          };
      };
  
      return(1);
  }
  
  /* k_sleep_10sec : to make a simple delay of aprox 10 sec */
  void k_sleep_3sec()
  {
      int c = 1, d = 1;
      for ( c = 1 ; c &lt;= 20000 ; c++ )
      for ( d = 1 ; d &lt;= 20000 ; d++ )
      {}
  }  
  extern void loadPageDirectory(unsigned int*);
  extern void enablePaging();
  /*main function*/
  void k_main() 
  {
      k_clear_screen();
      k_printf(" Wellcome to", 0, WHITE_TXT);
      k_printf(" CKA!", 1, GREEN_TXT);
      k_printf("==============&gt;", 2, WHITE_TXT);
      k_printf(" CKA stands for C Kernel with Assembly", 3, WHITE_TXT);
      k_printf(" Version 0.0.1, =&gt; based in the job of Debashis Barman", 4, WHITE_TXT);
      k_printf(" Contact =&gt; assemblyislaw@gmail.com / blueshell@mail2tor.com", 5, WHITE_TXT);
      k_printf("           or in the github repository page", 6, WHITE_TXT);
      k_sleep_3sec();
      k_clear_screen();
      //here start the magic
      k_printf(" Starting Checkup", 0, RED_TXT);
  
      //paging setup
      k_printf(" [Proc1] Setting up paging", 1, WHITE_TXT);
      uint32_t page_directory[1024] __attribute__((aligned(4096)));
      //set each entry to not present
      int i;
      for(i = 0; i &lt; 1024; i++)
      {
          // This sets the following flags to the pages:
          //   Supervisor: Only kernel-mode can access them
          //   Write Enabled: It can be both read from and written to
          //   Not Present: The page table is not present
          page_directory[i] = 0x00000002;
      }
      uint32_t first_page_table[1024] __attribute__((aligned(4096)));
      // holds the physical address where we want to start mapping these pages to.
      // in this case, we want to map these pages to the very beginning of memory.
      unsigned int w;
  
      //we will fill all 1024 entries in the table, mapping 4 megabytes
      for(w = 0; w &lt; 1024; w++)
      {
          // As the address is page aligned, it will always leave 12 bits zeroed.
          // Those bits are used by the attributes ;)
          first_page_table[w] = (w * 0x1000) | 3; // attributes: supervisor level, read/write, present.
      }
      // attributes: supervisor level, read/write, present
      page_directory[0] = ((unsigned int)first_page_table) | 3;
      //paging asm
      asm(".text\n\t"
              ".globl loadPageDirectory\n\t" 
              "loadPageDirectory:\n\t" 
              "push %ebp\n\t"
              "mov %esp, %ebp\n\t"
              "mov 8(%esp), %eax\n\t"
              "mov %eax, %cr3\n\t"
              "mov %ebp, %esp\n\t"
              "pop %ebp\n\t"
              "ret"
      );
      asm(".text\n\t"
              ".globl enablePaging\n\t"
              "enablePaging:\n\t"
              "push %ebp\n\t"
              "mov %esp, %ebp\n\t"
              "mov %cr0, %eax\n\t"
              "or $0x80000000, %eax\n\t"
              "mov %eax, %cr0\n\t"
              "mov %ebp, %esp\n\t"
              "pop %ebp\n\t"
              "ret"
      );
      loadPageDirectory(page_directory);
      enablePaging();
      k_printf("[Proc1] Paging enabled!", 2, GREEN_TXT);
  }
  </code></pre>
  
  <p>As you can see the asm code that is my suspect of cause the problem is at the end, in the second <code>asm(...) block</code>  </p>
  
  <p>Script 2: linker, to link loader and kernel</p>
  
  <pre><code>ENTRY(loader)
  OUTPUT_FORMAT(elf32-i386)
  
  SECTIONS {
     /* The kernel will live at 3GB + 1MB in the virtual
        address space, which will be mapped to 1MB in the
        physical address space. */
     . = 0xC0100000;
  
     .text : AT(ADDR(.text) - 0xC0000000) {
         *(.text)
         *(.rodata*)
     }
  
     .data ALIGN (0x1000) : AT(ADDR(.data) - 0xC0000000) {
         *(.data)
     }
  
     .bss : AT(ADDR(.bss) - 0xC0000000) {
         _sbss = .;
         *(COMMON)
         *(.bss)
         _ebss = .;
     }
  }
  </code></pre>
  
  <p>Script 3: Loader, in assembly.</p>
  
  <pre><code>global _loader                          ; Make entry point visible to linker.
  extern k_main                           ; _main is defined elsewhere
  
  ; setting up the Multiboot header - see GRUB docs for details
  MODULEALIGN equ  1&lt;&lt;0             ; align loaded modules on page boundaries
  MEMINFO     equ  1&lt;&lt;1             ; provide memory map
  FLAGS       equ  MODULEALIGN | MEMINFO  ; this is the Multiboot 'flag' field
  MAGIC       equ    0x1BADB002     ; 'magic number' lets bootloader find the header
  CHECKSUM    equ -(MAGIC + FLAGS)  ; checksum required
  
  ; This is the virtual base address of kernel space. It must be used to convert virtual
  ; addresses into physical addresses until paging is enabled. Note that this is not
  ; the virtual address where the kernel image itself is loaded -- just the amount that must
  ; be subtracted from a virtual address to get a physical address.
  KERNEL_VIRTUAL_BASE equ 0xC0000000                  ; 3GB
  KERNEL_PAGE_NUMBER equ (KERNEL_VIRTUAL_BASE &gt;&gt; 22)  ; Page directory index of kernel's 4MB PTE.
  
  
  section .data
  align 0x1000
  BootPageDirectory:
      ; This page directory entry identity-maps the first 4MB of the 32-bit physical address space.
      ; All bits are clear except the following:
      ; bit 7: PS The kernel page is 4MB.
      ; bit 1: RW The kernel page is read/write.
      ; bit 0: P  The kernel page is present.
      ; This entry must be here -- otherwise the kernel will crash immediately after paging is
      ; enabled because it can't fetch the next instruction! It's ok to unmap this page later.
      dd 0x00000083
      times (KERNEL_PAGE_NUMBER - 1) dd 0                 ; Pages before kernel space.
      ; This page directory entry defines a 4MB page containing the kernel.
      dd 0x00000083
      times (1024 - KERNEL_PAGE_NUMBER - 1) dd 0  ; Pages after the kernel image.
  
  
  section .text
  align 4
  MultiBootHeader:
      dd MAGIC
      dd FLAGS
      dd CHECKSUM
  
  ; reserve initial kernel stack space -- that's 16k.
  STACKSIZE equ 0x4000
  
  ; setting up entry point for linker
  loader equ (_loader - 0xC0000000)
  global loader
  
  _loader:
      ; NOTE: Until paging is set up, the code must be position-independent and use physical
      ; addresses, not virtual ones!
      mov ecx, (BootPageDirectory - KERNEL_VIRTUAL_BASE)
      mov cr3, ecx                                        ; Load Page Directory Base Register.
  
      mov ecx, cr4
      or ecx, 0x00000010                          ; Set PSE bit in CR4 to enable 4MB pages.
      mov cr4, ecx
  
      mov ecx, cr0
      or ecx, 0x80000000                          ; Set PG bit in CR0 to enable paging.
      mov cr0, ecx
  
      ; Start fetching instructions in kernel space.
      ; Since eip at this point holds the physical address of this command (approximately 0x00100000)
      ; we need to do a long jump to the correct virtual address of StartInHigherHalf which is
      ; approximately 0xC0100000.
      lea ecx, [StartInHigherHalf]
      jmp ecx                                                     ; NOTE: Must be absolute jump!
  
  StartInHigherHalf:
      ; Unmap the identity-mapped first 4MB of physical address space. It should not be needed
      ; anymore.
      mov dword [BootPageDirectory], 0
      invlpg [0]
  
      ; NOTE: From now on, paging should be enabled. The first 4MB of physical address space is
      ; mapped starting at KERNEL_VIRTUAL_BASE. Everything is linked to this address, so no more
      ; position-independent code or funny business with virtual-to-physical address translation
      ; should be necessary. We now have a higher-half kernel.
      mov esp, stack+STACKSIZE           ; set up the stack
      push eax                           ; pass Multiboot magic number
  
      ; pass Multiboot info structure -- WARNING: This is a physical address and may not be
      ; in the first 4MB!
      push ebx
  
      call  k_main                 ; call kernel proper
      hlt                          ; halt machine should kernel return
  
  
  section .bss
  align 32
  stack:
      resb STACKSIZE      ; reserve 16k stack on a uint64_t boundary
  </code></pre>
  
  <p>Honestly I dont think the last one have nothing wrong, but is Freestanding coding with a language that I dont domine, so well, ensure is nothing bad at all.<br>
  Some specs of the kernel that could be helpfull:<br>
  The part that does error is related to paging.<br>
  It works in "Higher Half", but I dont thing this disturb anything.</p>