kernel/fork.c

/* */
This source file includes following definitions.
find_empty_process
copy_fd
dup_mmap
copy_files
copy_mm
copy_fs
sys_fork
   1 /*
   2  *  linux/kernel/fork.c
   3  *
   4  *  Copyright (C) 1991, 1992  Linus Torvalds
   5  */
   6 
   7 /*
   8  *  'fork.c' contains the help-routines for the 'fork' system call
   9  * (see also system_call.s).
  10  * Fork is rather simple, once you get the hang of it, but the memory
  11  * management can be a bitch. See 'mm/mm.c': 'copy_page_tables()'
  12  */
  13 
  14 #include <linux/errno.h>
  15 #include <linux/sched.h>
  16 #include <linux/kernel.h>
  17 #include <linux/mm.h>
  18 #include <linux/stddef.h>
  19 #include <linux/unistd.h>
  20 #include <linux/ptrace.h>
  21 #include <linux/malloc.h>
  22 #include <linux/ldt.h>
  23 
  24 #include <asm/segment.h>
  25 #include <asm/system.h>
  26 
  27 /* These should maybe be in <linux/tasks.h> */
  28 
  29 #define MAX_TASKS_PER_USER (NR_TASKS/2)
  30 #define MIN_TASKS_LEFT_FOR_ROOT 4
  31 
  32 long last_pid=0;
  33 
  34 static int find_empty_process(void)
     /*  */
  35 {
  36         int free_task;
  37         int i, tasks_free;
  38         int this_user_tasks;
  39 
  40 repeat:
  41         if ((++last_pid) & 0xffff8000)
  42                 last_pid=1;
  43         this_user_tasks = 0;
  44         tasks_free = 0;
  45         free_task = -EAGAIN;
  46         i = NR_TASKS;
  47         while (--i > 0) {
  48                 if (!task[i]) {
  49                         free_task = i;
  50                         tasks_free++;
  51                         continue;
  52                 }
  53                 if (task[i]->uid == current->uid)
  54                         this_user_tasks++;
  55                 if (task[i]->pid == last_pid || task[i]->pgrp == last_pid ||
  56                     task[i]->session == last_pid)
  57                         goto repeat;
  58         }
  59         if (tasks_free <= MIN_TASKS_LEFT_FOR_ROOT ||
  60             this_user_tasks > MAX_TASKS_PER_USER)
  61                 if (current->uid)
  62                         return -EAGAIN;
  63         return free_task;
  64 }
  65 
  66 static struct file * copy_fd(struct file * old_file)
     /*  */
  67 {
  68         struct file * new_file = get_empty_filp();
  69         int error;
  70 
  71         if (new_file) {
  72                 memcpy(new_file,old_file,sizeof(struct file));
  73                 new_file->f_count = 1;
  74                 if (new_file->f_inode)
  75                         new_file->f_inode->i_count++;
  76                 if (new_file->f_op && new_file->f_op->open) {
  77                         error = new_file->f_op->open(new_file->f_inode,new_file);
  78                         if (error) {
  79                                 iput(new_file->f_inode);
  80                                 new_file->f_count = 0;
  81                                 new_file = NULL;
  82                         }
  83                 }
  84         }
  85         return new_file;
  86 }
  87 
  88 static int dup_mmap(struct task_struct * tsk)
     /*  */
  89 {
  90         struct vm_area_struct * mpnt, **p, *tmp;
  91 
  92         tsk->mm->mmap = NULL;
  93         p = &tsk->mm->mmap;
  94         for (mpnt = current->mm->mmap ; mpnt ; mpnt = mpnt->vm_next) {
  95                 tmp = (struct vm_area_struct *) kmalloc(sizeof(struct vm_area_struct), GFP_KERNEL);
  96                 if (!tmp) {
  97                         exit_mmap(tsk);
  98                         return -ENOMEM;
  99                 }
 100                 *tmp = *mpnt;
 101                 tmp->vm_task = tsk;
 102                 tmp->vm_next = NULL;
 103                 if (tmp->vm_inode) {
 104                         tmp->vm_inode->i_count++;
 105                         /* insert tmp into the share list, just after mpnt */
 106                         tmp->vm_next_share->vm_prev_share = tmp;
 107                         mpnt->vm_next_share = tmp;
 108                         tmp->vm_prev_share = mpnt;
 109                 }
 110                 if (tmp->vm_ops && tmp->vm_ops->open)
 111                         tmp->vm_ops->open(tmp);
 112                 *p = tmp;
 113                 p = &tmp->vm_next;
 114         }
 115         build_mmap_avl(tsk);
 116         return 0;
 117 }
 118 
 119 /*
 120  * SHAREFD not yet implemented..
 121  */
 122 static void copy_files(unsigned long clone_flags, struct task_struct * p)
     /*  */
 123 {
 124         int i;
 125         struct file * f;
 126 
 127         if (clone_flags & COPYFD) {
 128                 for (i=0; i<NR_OPEN;i++)
 129                         if ((f = p->files->fd[i]) != NULL)
 130                                 p->files->fd[i] = copy_fd(f);
 131         } else {
 132                 for (i=0; i<NR_OPEN;i++)
 133                         if ((f = p->files->fd[i]) != NULL)
 134                                 f->f_count++;
 135         }
 136 }
 137 
 138 /*
 139  * CLONEVM not yet correctly implemented: needs to clone the mmap
 140  * instead of duplicating it..
 141  */
 142 static int copy_mm(unsigned long clone_flags, struct task_struct * p)
     /*  */
 143 {
 144         if (clone_flags & COPYVM) {
 145                 p->mm->swappable = 1;
 146                 p->mm->min_flt = p->mm->maj_flt = 0;
 147                 p->mm->cmin_flt = p->mm->cmaj_flt = 0;
 148                 if (copy_page_tables(p))
 149                         return 1;
 150                 return dup_mmap(p);
 151         } else {
 152                 if (clone_page_tables(p))
 153                         return 1;
 154                 return dup_mmap(p);             /* wrong.. */
 155         }
 156 }
 157 
 158 static void copy_fs(unsigned long clone_flags, struct task_struct * p)
     /*  */
 159 {
 160         if (current->fs->pwd)
 161                 current->fs->pwd->i_count++;
 162         if (current->fs->root)
 163                 current->fs->root->i_count++;
 164 }
 165 
 166 /*
 167  *  Ok, this is the main fork-routine. It copies the system process
 168  * information (task[nr]) and sets up the necessary registers. It
 169  * also copies the data segment in its entirety.
 170  */
 171 asmlinkage int sys_fork(struct pt_regs regs)
     /*  */
 172 {
 173         int nr;
 174         struct task_struct *p;
 175         unsigned long new_stack;
 176         unsigned long clone_flags = COPYVM | SIGCHLD;
 177 
 178         if(!(p = (struct task_struct*)__get_free_page(GFP_KERNEL)))
 179                 goto bad_fork;
 180         new_stack = get_free_page(GFP_KERNEL);
 181         if (!new_stack)
 182                 goto bad_fork_free;
 183         nr = find_empty_process();
 184         if (nr < 0)
 185                 goto bad_fork_free;
 186 
 187         *p = *current;
 188 
 189         if (p->exec_domain && p->exec_domain->use_count)
 190                 (*p->exec_domain->use_count)++;
 191         if (p->binfmt && p->binfmt->use_count)
 192                 (*p->binfmt->use_count)++;
 193 
 194         p->did_exec = 0;
 195         p->kernel_stack_page = new_stack;
 196         *(unsigned long *) p->kernel_stack_page = STACK_MAGIC;
 197         p->state = TASK_UNINTERRUPTIBLE;
 198         p->flags &= ~(PF_PTRACED|PF_TRACESYS);
 199         p->pid = last_pid;
 200         p->p_pptr = p->p_opptr = current;
 201         p->p_cptr = NULL;
 202         SET_LINKS(p);
 203         p->signal = 0;
 204         p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
 205         p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
 206         p->leader = 0;          /* process leadership doesn't inherit */
 207         p->tty_old_pgrp = 0;
 208         p->utime = p->stime = 0;
 209         p->cutime = p->cstime = 0;
 210         p->start_time = jiffies;
 211         task[nr] = p;
 212 
 213         /* copy all the process information */
 214         clone_flags = copy_thread(nr, COPYVM | SIGCHLD, p, &regs);
 215         if (copy_mm(clone_flags, p))
 216                 goto bad_fork_cleanup;
 217         p->semundo = NULL;
 218         copy_files(clone_flags, p);
 219         copy_fs(clone_flags, p);
 220 
 221         /* ok, now we should be set up.. */
 222         p->exit_signal = clone_flags & CSIGNAL;
 223         p->counter = current->counter >> 1;
 224         p->state = TASK_RUNNING;        /* do this last, just in case */
 225         return p->pid;
 226 bad_fork_cleanup:
 227         task[nr] = NULL;
 228         REMOVE_LINKS(p);
 229 bad_fork_free:
 230         free_page(new_stack);
 231         free_page((long) p);
 232 bad_fork:
 233         return -EAGAIN;
 234 }
/* */
root/kernel/fork.c

DEFINITIONS
