path: root/userprog/process.c

#include "userprog/process.h"
#include <debug.h>
#include <inttypes.h>
#include <round.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "userprog/gdt.h"
#include "userprog/pagedir.h"
#include "userprog/tss.h"
#include "filesys/directory.h"
#include "filesys/file.h"
#include "filesys/filesys.h"
#include "threads/flags.h"
#include "threads/init.h"
#include "threads/interrupt.h"
#include "threads/palloc.h"
#include "threads/synch.h"
#include "threads/thread.h"
#include "threads/vaddr.h"
#include "vm/page.h"

/* data structure to communicate with the thread initializing a new process */
struct start_aux_data {
  char *args;
  struct semaphore startup_sem;
  struct thread *parent_thread;
  struct process *new_process;
};

/* filesystem lock */
struct lock filesys_lock;

/* prototypes */
static thread_func start_process NO_RETURN;
static bool load (const char *args, void (**eip) (void), void **esp);
static bool setup_stack (uint32_t **esp, const char *args);
static bool init_fd_table (struct fd_table * table);

/* Initialize the filesystem lock */
void
process_init ()
{
  lock_init (&filesys_lock);
}

/* Get current process (only valid for processes) */
struct process*
process_current ()
{
    ASSERT (thread_current()->process != NULL);
    return thread_current()->process;
}

/* Starts a new thread running a user program loaded from
   `filename`.
   The new thread may be scheduled (and may even exit)
   before process_execute() returns.  Returns the new process's
   thread id, or TID_ERROR if the thread cannot be created.

   In the first assignment, you should change this to function to

     process_execute (const char *cmd)

   and support command strings such as "echo A B C". You
   will also need to change `load` and `setup_stack`. */
tid_t
process_execute (const char *cmd)
{
  tid_t tid = TID_ERROR;
  char *fn_copy = NULL;
  struct start_aux_data *aux_data = NULL;
  char *name, *args;

  /* Setup the auxiliary data for starting up the new process */
  fn_copy = palloc_get_page (0);
  aux_data = palloc_get_page (0);
  if (aux_data == NULL || fn_copy == NULL)
    goto done;
  strlcpy (fn_copy, cmd, PGSIZE);
  name = strtok_r (fn_copy, " ", &args);
  aux_data->args = args;
  aux_data->parent_thread = thread_current ();
  aux_data->new_process = NULL;
  sema_init (&aux_data->startup_sem, 0);

  /* Create a new thread to execute FILE_NAME. */
  tid = thread_create (name, PRI_DEFAULT, start_process, aux_data);
  if (tid == TID_ERROR)
    goto done;

  /* wait for startup */
  sema_down (&aux_data->startup_sem);
  if (aux_data->new_process == NULL) {
    tid = TID_ERROR;
    goto done;
  }
  /* register child process */
  list_push_back (&thread_current()->children, 
		  &aux_data->new_process->parentelem);

 done:
  palloc_free_page (fn_copy);
  palloc_free_page (aux_data);
  return tid;
}

/* A thread function that loads a user process and starts it
   running. */
static void
start_process (void *aux)
{
  struct intr_frame if_;
  struct start_aux_data *aux_data = (struct start_aux_data*) aux;
  struct thread *thread = thread_current ();

  /* Initialize Process */
  struct process *process = palloc_get_page (PAL_ZERO);
  if (process == NULL)
    goto signal;
  sema_init (&process->exit_sem, 0);
  lock_init (&process->exit_lock);
  process->exit_status = -1;
  if (! init_fd_table (&process->fd_table))
    goto signal;

  /* register process */
  process->thread_id  = thread->tid;
  process->parent_tid = aux_data->parent_thread->tid;
  thread->process = process;

  page_table_init (&thread->page_table);
  if (!mmap_table_init (&process->mmap_table))
    goto signal;

  /* Initialize interrupt frame and load executable. */
  memset (&if_, 0, sizeof if_);
  if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
  if_.cs = SEL_UCSEG;
  if_.eflags = FLAG_IF | FLAG_MBS;
  if (! load (aux_data->args, &if_.eip, &if_.esp)) {
    thread->process = NULL;
  } else {
    aux_data->new_process = thread->process;
  }

  signal:
  /* Signal the parent process that loading is finished */
  sema_up (&aux_data->startup_sem); /* transfer ownership of aux_data */

  /* If process startup failed, quit. */
  if (thread->process == NULL)  {
    if (process != NULL) {
      /* close/free memory mapped files */
      mmap_table_free (&process->mmap_table);

      /* free page table */
      page_table_free (&thread->page_table);

      if (process->fd_table.fds != NULL)
        palloc_free_page (process->fd_table.fds);
      palloc_free_page (process);
    }
    thread_exit ();
  }

  /* Start the user process by simulating a return from an
     interrupt, implemented by intr_exit (in
     threads/intr-stubs.S).  Because intr_exit takes all of its
     arguments on the stack in the form of a `struct intr_frame',
     we just point the stack pointer (%esp) to our stack frame
     and jump to it. */
  asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
  NOT_REACHED ();
}

/* Waits for thread TID to die and returns its exit status.  If
   it was terminated by the kernel (i.e. killed due to an
   exception), returns -1.  If TID is invalid or if it was not a
   child of the calling process, or if process_wait() has already
   been successfully called for the given TID, returns -1
   immediately, without waiting. */
int
process_wait (tid_t child_tid)
{
  struct thread *cur = thread_current ();
  struct process *child = NULL;

  /* iterate over child processes */
  struct list_elem *e = list_head (&cur->children);
  while ((e = list_next (e)) != list_end (&cur->children)) {
    struct process* t = list_entry (e, struct process, parentelem);
    if (t->thread_id == child_tid) {
      list_remove (e);
      child = t;
      break;
    }
  }
  if (child == NULL) {
    return -1;
  }
  sema_down (&child->exit_sem);
  int exit_status = child->exit_status;
  palloc_free_page (child);
  return exit_status;
}

/* Free the current process's resources. */
void
process_exit (void)
{
  struct thread *thread = thread_current ();
  ASSERT (thread != NULL);

  /* remove (and if necessary clean up) child processes */
  struct list_elem *e = list_head (&thread->children);
  while ((e = list_next (e)) != list_end (&thread->children)) {
    struct process *p = list_entry (e, struct process, parentelem);
    bool process_dying;
    lock_acquire (&p->exit_lock);
    process_dying = p->parent_tid < 0;
    p->parent_tid = -1;
    lock_release (&p->exit_lock);
    if (process_dying)
      palloc_free_page (p);
  }

  if (thread->process == NULL)
    return; /* not a process, nothing else left to do */

  struct process *proc = thread->process;
  uint32_t *pd;

  printf ("%s: exit(%d)\n", thread->name, proc->exit_status);

  /* close executable, allow write */
  if (proc->executable != NULL) {
    lock_acquire (&filesys_lock);
    file_close (proc->executable);
    lock_release (&filesys_lock);
  }

  /* close/free memory mapped files */
  mmap_table_free (&proc->mmap_table);

  int fd;
  for (fd = 2; fd <= proc->fd_table.fd_max; fd++) {
    process_close_file (fd);
  }
  palloc_free_page (proc->fd_table.fds);

  /* Destroy the current process's page directory and switch back
     to the kernel-only page directory. */
  pd = thread->pagedir;
  if (pd != NULL) {
    /* Correct ordering here is crucial.  We must set
         cur->pagedir to NULL before switching page directories,
         so that a timer interrupt can't switch back to the
         process page directory.  We must activate the base page
         directory before destroying the process's page
         directory, or our active page directory will be one
         that's been freed (and cleared). */
    thread->pagedir = NULL;
    pagedir_activate (NULL);
    pagedir_destroy (pd);
  }

  /* free page table */
  page_table_free (&thread->page_table);

  /* Destroy the process structure if the parent is not alive
   * any more. Atomic test and set would be sufficient here.
   */
  bool parent_dead = false;
  lock_acquire (&proc->exit_lock);
  parent_dead = proc->parent_tid < 0;
  proc->parent_tid = -1;
  lock_release (&proc->exit_lock);
  if (parent_dead) {
    palloc_free_page (proc);
  } else {
    sema_up (&proc->exit_sem);
  }
}

/* Sets up the CPU for running user code in the current
   thread.
   This function is called on every context switch. */
void
process_activate (void)
{
  struct thread *t = thread_current ();
  /* Activate thread's page tables. */
  pagedir_activate (t->pagedir);

  /* Set thread's kernel stack for use in processing
     interrupts. */
  tss_update ();
}

/* 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 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 t->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 *args, 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)
    return false;
  process_activate ();

  /* Coarse grained filesystem lock for loading */
  lock_acquire (&filesys_lock);

  /* Open executable file. */
  file = filesys_open (t->name);
  if (file == NULL)
    goto done;

  /* Deny writes to the file during loading */
  file_deny_write (file);

  /* 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", t->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;
    if (phdr.p_vaddr < PGSIZE)
      continue; /* Ignore build-id segment */
    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 (phdr.p_vaddr == 0)
        break; // Ignore the .note.gnu.build-i segment
      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;
    }
  }

  /* Set up stack. */
  if (!setup_stack ((uint32_t **) esp, args))
    goto done;

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

  success = true;

  done:
  /* We arrive here whether the load is successful or not. */
  if (success) {
    process_current()->executable = file;
  } else {
    file_close (file);
  }
  lock_release (&filesys_lock);
  return success;
}

/* load() helpers. */

/* 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;

      /* add segment to page table for on demand loading */
      if (!page_table_insert_segment (&thread_current ()->page_table, file, ofs,
            upage, page_read_bytes, writable))
        return false;

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

/* Create a minimal stack by mapping a zeroed page at the top of
   user virtual memory.
   You will implement this function in the Project 0.
   Consider using `hex_dump` for debugging purposes */
static bool
setup_stack (uint32_t **esp, const char *args)
{
  uint8_t *kpage = NULL;
  const char *name = thread_current ()->name;
  char *argv_cur;
  uint32_t argc = 0;
  unsigned namelen, argslen = 0;
  uint32_t *stack_end;

  kpage = palloc_get_page (PAL_USER | PAL_ZERO);
  if (kpage == NULL)
      return false;

  if (! process_install_page (((uint8_t *) PHYS_BASE) - PGSIZE, kpage, true)) {
      palloc_free_page (kpage);
      return false;
  }

  *esp = PHYS_BASE;

  /* calculate end of stack which we'll enforce,
     we need at least 4 entries for a correct stack and an
     additional entry due to the way our argv[]-entries-loop works */
  stack_end = PHYS_BASE - PGSIZE;
  stack_end += 4 + 1;

  /* copy arguments to stack */
  argslen = strlen(args);
  if (argslen > 0)
    {
      argslen += 1; /* add the trailing \0 */
      *(char **) esp -= argslen;
      memcpy(*esp, args, argslen);
    }

  /* copy executable name to stack */
  namelen = strlen(name) + 1;
  *(char **) esp -= namelen;
  memcpy(*esp, name, namelen);

  /* align our currend address by word-size (thanks to thomas & edy) */
  *(char **) esp -= (sizeof(uint32_t) - (PHYS_BASE - *(void **) esp) % sizeof(uint32_t));

  /* terminate argv[] array by NULL ptr */
  (*esp)--;
  **esp = '\0';

  /* push the argv[] entries */
  if (argslen > 0)
    {
      /* we walk through our already pushed arguments in reverse order and
         replace every occurrence of the space-character with '\0'-character.
         due to the fact we walk in reverse order we can easily push
         the pointers to the elements of argv[] too. */
      for(argv_cur = PHYS_BASE - 1; argv_cur >= (char *)PHYS_BASE - argslen; argv_cur--)
        {
          /* check for space-character and replace if necessary */
          if (*argv_cur == ' ')
            {
              *argv_cur = '\0';

              /* since we walk in reverse order and already have replaced a
                 character we simply check the preceding character. if it's
                 a non-'\0'-character we know an argument starts here. */
              if (*(argv_cur + 1) != '\0')
                {
                  /* push pointer to argument */
                  (*esp)--;
                  **esp = (uint32_t) argv_cur + 1;
                  argc++;

                  /* check for possible stack overflow */
                  if (*esp <= stack_end)
                    return false;
                }
            }
        }

      /* our loop above doesn't check/push the first argument as it's the
         end of the loop. so let's do that */
      if (*(argv_cur + 1) != '\0')
        {
          (*esp)--;
          **esp = (uint32_t) argv_cur + 1;
          argc++;
        }
    }

  /* push argv[0] (executable name) */
  (*esp)--;
  **esp = (uint32_t) (PHYS_BASE - (argslen + namelen));
  argc++;

  /* push pointer to argv[]. this is simple as it's our last address */
  (*esp)--;
  **esp = (uint32_t) (*esp + 1);

  /* push argc */
  (*esp)--;
  **esp = (uint32_t) argc;

  /* push fake return address */
  (*esp)--;
  **esp = 0;

  return true;
}

/* expand the stack of the process by one new page
   which will be installed at the address of UPAGE */
bool
process_grow_stack (void *upage)
{
  uint8_t *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
  if (kpage == NULL)
      return false;

  if (!process_install_page (upage, kpage, true))
    {
      palloc_free_page (kpage);
      return false;
    }

  return true;
}

/* 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. */
bool
process_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));
}

static
bool
init_fd_table (struct fd_table *table)
{
  table->fds = palloc_get_page (PAL_ZERO);
  if (table->fds == NULL)
    return false;
  table->fd_cap = PGSIZE / sizeof (table->fds[0]);
  table->fd_free = 2;
  table->fd_max  = 1;
  return true;
}

/* Open the file with the given name; returns
   a file descriptor for this file if successful,
   and a negative value otherwise */
int
process_open_file (const char* fname)
{
  struct fd_table *fdt = &process_current()->fd_table;
  if (fdt->fd_free >= fdt->fd_cap)
    return -1;

  lock_acquire (&filesys_lock);
  struct file *f = filesys_open (fname);
  lock_release (&filesys_lock);

  if (f == NULL)
    return -1;

  int fd = fdt->fd_free++;
  fdt->fds[fd] = f;

  /* update index of free/max file descriptor index*/
  if (fd > fdt->fd_max) fdt->fd_max = fd;
  while (fdt->fds[fdt->fd_free] != NULL) {
    fdt->fd_free++;
    if (fdt->fd_free >= fdt->fd_cap)
      break;
  }
  return fd;
}

/* Get the file associated with the given file
   descriptor; return NULL if no file is associated
   with the given descriptor */
struct file*
process_get_file (int fd)
{
  struct fd_table *fdt = &process_current()->fd_table;
  if (fd < 2 || fd >= fdt->fd_cap || ! fdt->fds[fd])
    return NULL;
  return fdt->fds[fd];
}

/* Acquire global lock for the filesystem */
void process_lock_filesys (void)
{
  lock_acquire (&filesys_lock);
}

/* Release global filesystem lock */
void process_unlock_filesys (void)
{
  lock_release (&filesys_lock);
}

/* Close the file associated with the given file
   descriptor; returns true if close was successful */
bool
process_close_file (int fd)
{
  struct file *file = process_get_file (fd);
  if (file == NULL)
    return false;

  lock_acquire (&filesys_lock);
  file_close (file);
  lock_release (&filesys_lock);

  struct fd_table *fdt = &process_current()->fd_table;
  fdt->fds[fd] = NULL;

  /* update index of free/max file descriptor index*/
  if (fd < fdt->fd_free) fdt->fd_free = fd;
  while (fdt->fds[fdt->fd_max] == NULL) {
    fdt->fd_max--;
    if (fdt->fd_max < 2)
      break;
  }
  return true;
}