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

DEFINITIONS
