path: root/src/userprog/load.c

#include <inttypes.h>  /* Elf32_word etc. */
#include <round.h>     /* ROUND_UP */
#include <stdio.h>
#include <string.h>    /* memcmp */

#include "userprog/process.h"
#include "userprog/load.h"
#include "userprog/pagedir.h"
#include "filesys/file.h"
#include "filesys/filesys.h"
#include "threads/palloc.h" /* PAL_* constants */
#include "threads/thread.h"
#include "threads/vaddr.h"  /* PGSIZE */

/* We load ELF binaries.  The following definitions are taken
   from the ELF specification, [ELF1], more-or-less verbatim.  */

/* ELF types.  See [ELF1] 1-2. */
typedef uint32_t Elf32_Word, Elf32_Addr, Elf32_Off;
typedef uint16_t Elf32_Half;

/* For use with ELF types in printf(). */
#define PE32Wx PRIx32   /* Print Elf32_Word in hexadecimal. */
#define PE32Ax PRIx32   /* Print Elf32_Addr in hexadecimal. */
#define PE32Ox PRIx32   /* Print Elf32_Off in hexadecimal. */
#define PE32Hx PRIx16   /* Print Elf32_Half in hexadecimal. */

/* Executable header.  See [ELF1] 1-4 to 1-8.
   This appears at the very beginning of an ELF binary. */
struct Elf32_Ehdr
  {
    unsigned char e_ident[16];
    Elf32_Half    e_type;
    Elf32_Half    e_machine;
    Elf32_Word    e_version;
    Elf32_Addr    e_entry;
    Elf32_Off     e_phoff;
    Elf32_Off     e_shoff;
    Elf32_Word    e_flags;
    Elf32_Half    e_ehsize;
    Elf32_Half    e_phentsize;
    Elf32_Half    e_phnum;
    Elf32_Half    e_shentsize;
    Elf32_Half    e_shnum;
    Elf32_Half    e_shstrndx;
  };

/* Program header.  See [ELF1] 2-2 to 2-4.
   There are e_phnum of these, starting at file offset e_phoff
   (see [ELF1] 1-6). */
struct Elf32_Phdr
  {
    Elf32_Word p_type;
    Elf32_Off  p_offset;
    Elf32_Addr p_vaddr;
    Elf32_Addr p_paddr;
    Elf32_Word p_filesz;
    Elf32_Word p_memsz;
    Elf32_Word p_flags;
    Elf32_Word p_align;
  };

/* Values for p_type.  See [ELF1] 2-3. */
#define PT_NULL    0            /* Ignore. */
#define PT_LOAD    1            /* Loadable segment. */
#define PT_DYNAMIC 2            /* Dynamic linking info. */
#define PT_INTERP  3            /* Name of dynamic loader. */
#define PT_NOTE    4            /* Auxiliary info. */
#define PT_SHLIB   5            /* Reserved. */
#define PT_PHDR    6            /* Program header table. */
#define PT_STACK   0x6474e551   /* Stack segment. */

/* Flags for p_flags.  See [ELF3] 2-3 and 2-4. */
#define PF_X 1          /* Executable. */
#define PF_W 2          /* Writable. */
#define PF_R 4          /* Readable. */

static bool setup_stack (void **esp);
static bool validate_segment (const struct Elf32_Phdr *, struct file *);
static bool load_segment (struct file *file, off_t ofs, uint8_t *upage,
                          uint32_t read_bytes, uint32_t zero_bytes,
                          bool writable);

/* Loads an ELF executable from FILE_NAME into the current thread.
   Stores the executable's entry point into *EIP
   and its initial stack pointer into *ESP.
   Returns true if successful, false otherwise. */
bool
load (const char *file_name, void (**eip) (void), void **esp) 
{
  struct thread *t = thread_current ();
  struct Elf32_Ehdr ehdr;
  struct file *file = NULL;
  off_t file_ofs;
  bool success = false;
  int i;

  /* Allocate and activate page directory. */
  t->pagedir = pagedir_create ();
  if (t->pagedir == NULL) 
    goto done;
  process_activate ();

  /* Set up stack. */
  if (!setup_stack (esp)){
    goto done;
  }

  /* Open executable file. */
  file = filesys_open (file_name);
  if (file == NULL) 
    {
      printf ("load: %s: open failed\n", file_name);
      goto done; 
    }

  /* Read and verify executable header. */
  if (file_read (file, &ehdr, sizeof ehdr) != sizeof ehdr
      || memcmp (ehdr.e_ident, "\177ELF\1\1\1", 7)
      || ehdr.e_type != 2
      || ehdr.e_machine != 3
      || ehdr.e_version != 1
      || ehdr.e_phentsize != sizeof (struct Elf32_Phdr)
      || ehdr.e_phnum > 1024) 
    {
      printf ("load: %s: error loading executable\n", file_name);
      goto done; 
    }

  /* Read program headers. */
  file_ofs = ehdr.e_phoff;
  for (i = 0; i < ehdr.e_phnum; i++) 
    {
      struct Elf32_Phdr phdr;

      if (file_ofs < 0 || file_ofs > file_length (file))
        goto done;
      file_seek (file, file_ofs);

      if (file_read (file, &phdr, sizeof phdr) != sizeof phdr)
        goto done;
      file_ofs += sizeof phdr;
      switch (phdr.p_type) 
        {
        case PT_NULL:
        case PT_NOTE:
        case PT_PHDR:
        case PT_STACK:
        default:
          /* Ignore this segment. */
          break;
        case PT_DYNAMIC:
        case PT_INTERP:
        case PT_SHLIB:
          goto done;
        case PT_LOAD:
          if (validate_segment (&phdr, file)) 
            {
              bool writable = (phdr.p_flags & PF_W) != 0;
              uint32_t file_page = phdr.p_offset & ~PGMASK;
              uint32_t mem_page = phdr.p_vaddr & ~PGMASK;
              uint32_t page_offset = phdr.p_vaddr & PGMASK;
              uint32_t read_bytes, zero_bytes;
              if (phdr.p_filesz > 0)
                {
                  /* Normal segment.
                     Read initial part from disk and zero the rest. */
                  read_bytes = page_offset + phdr.p_filesz;
                  zero_bytes = (ROUND_UP (page_offset + phdr.p_memsz, PGSIZE)
                                - read_bytes);
                }
              else 
                {
                  /* Entirely zero.
                     Don't read anything from disk. */
                  read_bytes = 0;
                  zero_bytes = ROUND_UP (page_offset + phdr.p_memsz, PGSIZE);
                }
              if (!load_segment (file, file_page, (void *) mem_page,
                                 read_bytes, zero_bytes, writable))
                goto done;
            }
          else
            goto done;
          break;
        }
    }

  /* Start address. */
  *eip = (void (*) (void)) ehdr.e_entry;

  success = true;

 done:
  /* We arrive here whether the load is successful or not. */
  file_close (file);
  return success;
}

/* load() helpers. */

static bool install_page (void *upage, void *kpage, bool writable);

/* Checks whether PHDR describes a valid, loadable segment in
   FILE and returns true if so, false otherwise. */
static bool
validate_segment (const struct Elf32_Phdr *phdr, struct file *file) 
{
  /* p_offset and p_vaddr must have the same page offset. */
  if ((phdr->p_offset & PGMASK) != (phdr->p_vaddr & PGMASK)) 
    return false; 

  /* p_offset must point within FILE. */
  if (phdr->p_offset > (Elf32_Off) file_length (file)) 
    return false;

  /* p_memsz must be at least as big as p_filesz. */
  if (phdr->p_memsz < phdr->p_filesz) 
    return false; 

  /* The segment must not be empty. */
  if (phdr->p_memsz == 0)
    return false;
  
  /* The virtual memory region must both start and end within the
     user address space range. */
  if (!is_user_vaddr ((void *) phdr->p_vaddr))
    return false;
  if (!is_user_vaddr ((void *) (phdr->p_vaddr + phdr->p_memsz)))
    return false;

  /* The region cannot "wrap around" across the kernel virtual
     address space. */
  if (phdr->p_vaddr + phdr->p_memsz < phdr->p_vaddr)
    return false;

  /* Disallow mapping page 0.
     Not only is it a bad idea to map page 0, but if we allowed
     it then user code that passed a null pointer to system calls
     could quite likely panic the kernel by way of null pointer
     assertions in memcpy(), etc. */
  if (phdr->p_vaddr < PGSIZE)
    return false;

  /* It's okay. */
  return true;
}

/* Loads a segment starting at offset OFS in FILE at address
   UPAGE.  In total, READ_BYTES + ZERO_BYTES bytes of virtual
   memory are initialized, as follows:

        - READ_BYTES bytes at UPAGE must be read from FILE
          starting at offset OFS.

        - ZERO_BYTES bytes at UPAGE + READ_BYTES must be zeroed.

   The pages initialized by this function must be writable by the
   user process if WRITABLE is true, read-only otherwise.

   Return true if successful, false if a memory allocation error
   or disk read error occurs. */
static bool
load_segment (struct file *file, off_t ofs, uint8_t *upage,
              uint32_t read_bytes, uint32_t zero_bytes, bool writable) 
{
  ASSERT ((read_bytes + zero_bytes) % PGSIZE == 0);
  ASSERT (pg_ofs (upage) == 0);
  ASSERT (ofs % PGSIZE == 0);

  file_seek (file, ofs);
  while (read_bytes > 0 || zero_bytes > 0) 
    {
      /* Calculate how to fill this page.
         We will read PAGE_READ_BYTES bytes from FILE
         and zero the final PAGE_ZERO_BYTES bytes. */
      size_t page_read_bytes = read_bytes < PGSIZE ? read_bytes : PGSIZE;
      size_t page_zero_bytes = PGSIZE - page_read_bytes;

      /* Get a page of memory. */
      uint8_t *kpage = palloc_get_page (PAL_USER);
      if (kpage == NULL)
        return false;

      /* Load this page. */
      if (file_read (file, kpage, page_read_bytes) != (int) page_read_bytes)
        {
          palloc_free_page (kpage);
          return false; 
        }
      memset (kpage + page_read_bytes, 0, page_zero_bytes);

      /* Add the page to the process's address space. */
      if (!install_page (upage, kpage, writable)) 
        {
          palloc_free_page (kpage);
          return false; 
        }

      /* Advance. */
      read_bytes -= page_read_bytes;
      zero_bytes -= page_zero_bytes;
      upage += PGSIZE;
    }
  return true;
}

/* Create a minimal stack by mapping a zeroed page at the top of
   user virtual memory. */
static bool
setup_stack (void **esp) 
{
  uint8_t *kpage;
  bool success = false;

  kpage = palloc_get_page (PAL_USER | PAL_ZERO);
  if (kpage != NULL) 
    {
      success = install_page (((uint8_t *) PHYS_BASE) - PGSIZE, kpage, true);
      if (success)
        *esp = PHYS_BASE;
      else
        palloc_free_page (kpage);
    }
  return success;
}

/* Adds a mapping from user virtual address UPAGE to kernel
   virtual address KPAGE to the page table.
   If WRITABLE is true, the user process may modify the page;
   otherwise, it is read-only.
   UPAGE must not already be mapped.
   KPAGE should probably be a page obtained from the user pool
   with palloc_get_page().
   Returns true on success, false if UPAGE is already mapped or
   if memory allocation fails. */
static bool
install_page (void *upage, void *kpage, bool writable)
{
  struct thread *t = thread_current ();

  /* Verify that there's not already a page at that virtual
     address, then map our page there. */
  return (pagedir_get_page (t->pagedir, upage) == NULL
          && pagedir_set_page (t->pagedir, upage, kpage, writable));
}

/* A function that dumps 'size' bytes of memory starting at 'ptr'
 * it will dump the higher adress first letting the stack grow down.
 */
void dump_stack(void* ptr, int size)
{
  unsigned from = (unsigned)ptr;
  unsigned to = (unsigned)(ptr - size);
  unsigned adr;
  
  printf("# Adress  \thex-data \tchar-data\n");
  
  for ( adr = from; adr > to; --adr )
  {
    unsigned* uadr = (unsigned*)( adr );
    unsigned char* byte = (unsigned char*)( adr );

    printf("# %08x\t", adr); /* address */
      
    if ( (adr % 4) == 0 )
      printf("%08x\t", *uadr); /* content interpreted as address */
    else
      printf("        \t"); /* fill */
        
    if ( *byte >= 32 && *byte < 127 )
      printf("%c\n", *byte); /* content interpreted as character */
    else
      printf("\\0%o\n", *byte); /* octal character code */
    
    if ( adr == (unsigned)PHYS_BASE )
      printf("# -- ^ KERNEL SPACE ABOVE ^v USER SPACE BELOW v --\n");
    else if ( (adr % 4) == 0 )
      printf("# ------------------------------------------------\n");
  }
}