kernel/fork.c

/* */
This source file includes following definitions.
find_empty_process
copy_fd
dup_mmap
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/segment.h>
  21 #include <linux/ptrace.h>
  22 #include <linux/malloc.h>
  23 #include <linux/ldt.h>
  24 
  25 #include <asm/segment.h>
  26 #include <asm/system.h>
  27 
  28 asmlinkage void ret_from_sys_call(void) __asm__("ret_from_sys_call");
  29 
  30 /* These should maybe be in <linux/tasks.h> */
  31 
  32 #define MAX_TASKS_PER_USER (NR_TASKS/2)
  33 #define MIN_TASKS_LEFT_FOR_ROOT 4
  34 
  35 extern int shm_fork(struct task_struct *, struct task_struct *);
  36 long last_pid=0;
  37 
  38 static int find_empty_process(void)
     /*  */
  39 {
  40         int free_task;
  41         int i, tasks_free;
  42         int this_user_tasks;
  43 
  44 repeat:
  45         if ((++last_pid) & 0xffff8000)
  46                 last_pid=1;
  47         this_user_tasks = 0;
  48         tasks_free = 0;
  49         free_task = -EAGAIN;
  50         i = NR_TASKS;
  51         while (--i > 0) {
  52                 if (!task[i]) {
  53                         free_task = i;
  54                         tasks_free++;
  55                         continue;
  56                 }
  57                 if (task[i]->uid == current->uid)
  58                         this_user_tasks++;
  59                 if (task[i]->pid == last_pid || task[i]->pgrp == last_pid)
  60                         goto repeat;
  61         }
  62         if (tasks_free <= MIN_TASKS_LEFT_FOR_ROOT ||
  63             this_user_tasks > MAX_TASKS_PER_USER)
  64                 if (current->uid)
  65                         return -EAGAIN;
  66         return free_task;
  67 }
  68 
  69 static struct file * copy_fd(struct file * old_file)
     /*  */
  70 {
  71         struct file * new_file = get_empty_filp();
  72         int error;
  73 
  74         if (new_file) {
  75                 memcpy(new_file,old_file,sizeof(struct file));
  76                 new_file->f_count = 1;
  77                 if (new_file->f_inode)
  78                         new_file->f_inode->i_count++;
  79                 if (new_file->f_op && new_file->f_op->open) {
  80                         error = new_file->f_op->open(new_file->f_inode,new_file);
  81                         if (error) {
  82                                 iput(new_file->f_inode);
  83                                 new_file->f_count = 0;
  84                                 new_file = NULL;
  85                         }
  86                 }
  87         }
  88         return new_file;
  89 }
  90 
  91 int dup_mmap(struct task_struct * tsk)
     /*  */
  92 {
  93         struct vm_area_struct * mpnt, **p, *tmp;
  94 
  95         tsk->mmap = NULL;
  96         tsk->stk_vma = NULL;
  97         p = &tsk->mmap;
  98         for (mpnt = current->mmap ; mpnt ; mpnt = mpnt->vm_next) {
  99                 tmp = (struct vm_area_struct *) kmalloc(sizeof(struct vm_area_struct), GFP_KERNEL);
 100                 if (!tmp)
 101                         return -ENOMEM;
 102                 *tmp = *mpnt;
 103                 tmp->vm_task = tsk;
 104                 tmp->vm_next = NULL;
 105                 if (tmp->vm_inode)
 106                         tmp->vm_inode->i_count++;
 107                 *p = tmp;
 108                 p = &tmp->vm_next;
 109                 if (current->stk_vma == mpnt)
 110                         tsk->stk_vma = tmp;
 111         }
 112         return 0;
 113 }
 114 
 115 #define IS_CLONE (regs.orig_eax == __NR_clone)
 116 #define copy_vm(p) ((clone_flags & COPYVM)?copy_page_tables(p):clone_page_tables(p))
 117 
 118 /*
 119  *  Ok, this is the main fork-routine. It copies the system process
 120  * information (task[nr]) and sets up the necessary registers. It
 121  * also copies the data segment in its entirety.
 122  */
 123 asmlinkage int sys_fork(struct pt_regs regs)
     /*  */
 124 {
 125         struct pt_regs * childregs;
 126         struct task_struct *p;
 127         int i,nr;
 128         struct file *f;
 129         unsigned long clone_flags = COPYVM | SIGCHLD;
 130 
 131         if(!(p = (struct task_struct*)__get_free_page(GFP_KERNEL)))
 132                 goto bad_fork;
 133         nr = find_empty_process();
 134         if (nr < 0)
 135                 goto bad_fork_free;
 136         task[nr] = p;
 137         *p = *current;
 138         p->kernel_stack_page = 0;
 139         p->state = TASK_UNINTERRUPTIBLE;
 140         p->flags &= ~(PF_PTRACED|PF_TRACESYS);
 141         p->pid = last_pid;
 142         p->swappable = 1;
 143         p->p_pptr = p->p_opptr = current;
 144         p->p_cptr = NULL;
 145         SET_LINKS(p);
 146         p->signal = 0;
 147         p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
 148         p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
 149         p->leader = 0;          /* process leadership doesn't inherit */
 150         p->utime = p->stime = 0;
 151         p->cutime = p->cstime = 0;
 152         p->min_flt = p->maj_flt = 0;
 153         p->cmin_flt = p->cmaj_flt = 0;
 154         p->start_time = jiffies;
 155 /*
 156  * set up new TSS and kernel stack
 157  */
 158         if (!(p->kernel_stack_page = __get_free_page(GFP_KERNEL)))
 159                 goto bad_fork_cleanup;
 160         p->tss.es = KERNEL_DS;
 161         p->tss.cs = KERNEL_CS;
 162         p->tss.ss = KERNEL_DS;
 163         p->tss.ds = KERNEL_DS;
 164         p->tss.fs = USER_DS;
 165         p->tss.gs = KERNEL_DS;
 166         p->tss.ss0 = KERNEL_DS;
 167         p->tss.esp0 = p->kernel_stack_page + PAGE_SIZE;
 168         p->tss.tr = _TSS(nr);
 169         childregs = ((struct pt_regs *) (p->kernel_stack_page + PAGE_SIZE)) - 1;
 170         p->tss.esp = (unsigned long) childregs;
 171         p->tss.eip = (unsigned long) ret_from_sys_call;
 172         *childregs = regs;
 173         childregs->eax = 0;
 174         p->tss.back_link = 0;
 175         p->tss.eflags = regs.eflags & 0xffffcfff;       /* iopl is always 0 for a new process */
 176         if (IS_CLONE) {
 177                 if (regs.ebx)
 178                         childregs->esp = regs.ebx;
 179                 clone_flags = regs.ecx;
 180                 if (childregs->esp == regs.esp)
 181                         clone_flags |= COPYVM;
 182         }
 183         p->exit_signal = clone_flags & CSIGNAL;
 184         p->tss.ldt = _LDT(nr);
 185         if (p->ldt) {
 186                 p->ldt = (struct desc_struct*) vmalloc(LDT_ENTRIES*LDT_ENTRY_SIZE);
 187                 if (p->ldt != NULL)
 188                         memcpy(p->ldt, current->ldt, LDT_ENTRIES*LDT_ENTRY_SIZE);
 189         }
 190         p->tss.bitmap = offsetof(struct tss_struct,io_bitmap);
 191         for (i = 0; i < IO_BITMAP_SIZE+1 ; i++) /* IO bitmap is actually SIZE+1 */
 192                 p->tss.io_bitmap[i] = ~0;
 193         if (last_task_used_math == current)
 194                 __asm__("clts ; fnsave %0 ; frstor %0":"=m" (p->tss.i387));
 195         p->semun = NULL; p->shm = NULL;
 196         if (copy_vm(p) || shm_fork(current, p))
 197                 goto bad_fork_cleanup;
 198         if (clone_flags & COPYFD) {
 199                 for (i=0; i<NR_OPEN;i++)
 200                         if ((f = p->filp[i]) != NULL)
 201                                 p->filp[i] = copy_fd(f);
 202         } else {
 203                 for (i=0; i<NR_OPEN;i++)
 204                         if ((f = p->filp[i]) != NULL)
 205                                 f->f_count++;
 206         }
 207         if (current->pwd)
 208                 current->pwd->i_count++;
 209         if (current->root)
 210                 current->root->i_count++;
 211         if (current->executable)
 212                 current->executable->i_count++;
 213         dup_mmap(p);
 214         set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));
 215         if (p->ldt)
 216                 set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,p->ldt, 512);
 217         else
 218                 set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&default_ldt, 1);
 219 
 220         p->counter = current->counter >> 1;
 221         p->state = TASK_RUNNING;        /* do this last, just in case */
 222         return p->pid;
 223 bad_fork_cleanup:
 224         task[nr] = NULL;
 225         REMOVE_LINKS(p);
 226         free_page(p->kernel_stack_page);
 227 bad_fork_free:
 228         free_page((long) p);
 229 bad_fork:
 230         return -EAGAIN;
 231 }
/* */
root/kernel/fork.c

DEFINITIONS
