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                     task[i]->session == last_pid)
  61                         goto repeat;
  62         }
  63         if (tasks_free <= MIN_TASKS_LEFT_FOR_ROOT ||
  64             this_user_tasks > MAX_TASKS_PER_USER)
  65                 if (current->uid)
  66                         return -EAGAIN;
  67         return free_task;
  68 }
  69 
  70 static struct file * copy_fd(struct file * old_file)
     /*  */
  71 {
  72         struct file * new_file = get_empty_filp();
  73         int error;
  74 
  75         if (new_file) {
  76                 memcpy(new_file,old_file,sizeof(struct file));
  77                 new_file->f_count = 1;
  78                 if (new_file->f_inode)
  79                         new_file->f_inode->i_count++;
  80                 if (new_file->f_op && new_file->f_op->open) {
  81                         error = new_file->f_op->open(new_file->f_inode,new_file);
  82                         if (error) {
  83                                 iput(new_file->f_inode);
  84                                 new_file->f_count = 0;
  85                                 new_file = NULL;
  86                         }
  87                 }
  88         }
  89         return new_file;
  90 }
  91 
  92 int dup_mmap(struct task_struct * tsk)
     /*  */
  93 {
  94         struct vm_area_struct * mpnt, **p, *tmp;
  95 
  96         tsk->mmap = NULL;
  97         tsk->stk_vma = NULL;
  98         p = &tsk->mmap;
  99         for (mpnt = current->mmap ; mpnt ; mpnt = mpnt->vm_next) {
 100                 tmp = (struct vm_area_struct *) kmalloc(sizeof(struct vm_area_struct), GFP_KERNEL);
 101                 if (!tmp)
 102                         return -ENOMEM;
 103                 *tmp = *mpnt;
 104                 tmp->vm_task = tsk;
 105                 tmp->vm_next = NULL;
 106                 if (tmp->vm_inode)
 107                         tmp->vm_inode->i_count++;
 108                 *p = tmp;
 109                 p = &tmp->vm_next;
 110                 if (current->stk_vma == mpnt)
 111                         tsk->stk_vma = tmp;
 112         }
 113         return 0;
 114 }
 115 
 116 #define IS_CLONE (regs.orig_eax == __NR_clone)
 117 #define copy_vm(p) ((clone_flags & COPYVM)?copy_page_tables(p):clone_page_tables(p))
 118 
 119 /*
 120  *  Ok, this is the main fork-routine. It copies the system process
 121  * information (task[nr]) and sets up the necessary registers. It
 122  * also copies the data segment in its entirety.
 123  */
 124 asmlinkage int sys_fork(struct pt_regs regs)
     /*  */
 125 {
 126         struct pt_regs * childregs;
 127         struct task_struct *p;
 128         int i,nr;
 129         struct file *f;
 130         unsigned long clone_flags = COPYVM | SIGCHLD;
 131 
 132         if(!(p = (struct task_struct*)__get_free_page(GFP_KERNEL)))
 133                 goto bad_fork;
 134         nr = find_empty_process();
 135         if (nr < 0)
 136                 goto bad_fork_free;
 137         task[nr] = p;
 138         *p = *current;
 139         p->kernel_stack_page = 0;
 140         p->state = TASK_UNINTERRUPTIBLE;
 141         p->flags &= ~(PF_PTRACED|PF_TRACESYS);
 142         p->pid = last_pid;
 143         p->swappable = 1;
 144         p->p_pptr = p->p_opptr = current;
 145         p->p_cptr = NULL;
 146         SET_LINKS(p);
 147         p->signal = 0;
 148         p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
 149         p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
 150         p->leader = 0;          /* process leadership doesn't inherit */
 151         p->utime = p->stime = 0;
 152         p->cutime = p->cstime = 0;
 153         p->min_flt = p->maj_flt = 0;
 154         p->cmin_flt = p->cmaj_flt = 0;
 155         p->start_time = jiffies;
 156 /*
 157  * set up new TSS and kernel stack
 158  */
 159         if (!(p->kernel_stack_page = __get_free_page(GFP_KERNEL)))
 160                 goto bad_fork_cleanup;
 161         p->tss.es = KERNEL_DS;
 162         p->tss.cs = KERNEL_CS;
 163         p->tss.ss = KERNEL_DS;
 164         p->tss.ds = KERNEL_DS;
 165         p->tss.fs = USER_DS;
 166         p->tss.gs = KERNEL_DS;
 167         p->tss.ss0 = KERNEL_DS;
 168         p->tss.esp0 = p->kernel_stack_page + PAGE_SIZE;
 169         p->tss.tr = _TSS(nr);
 170         childregs = ((struct pt_regs *) (p->kernel_stack_page + PAGE_SIZE)) - 1;
 171         p->tss.esp = (unsigned long) childregs;
 172         p->tss.eip = (unsigned long) ret_from_sys_call;
 173         *childregs = regs;
 174         childregs->eax = 0;
 175         p->tss.back_link = 0;
 176         p->tss.eflags = regs.eflags & 0xffffcfff;       /* iopl is always 0 for a new process */
 177         if (IS_CLONE) {
 178                 if (regs.ebx)
 179                         childregs->esp = regs.ebx;
 180                 clone_flags = regs.ecx;
 181                 if (childregs->esp == regs.esp)
 182                         clone_flags |= COPYVM;
 183         }
 184         p->exit_signal = clone_flags & CSIGNAL;
 185         p->tss.ldt = _LDT(nr);
 186         if (p->ldt) {
 187                 p->ldt = (struct desc_struct*) vmalloc(LDT_ENTRIES*LDT_ENTRY_SIZE);
 188                 if (p->ldt != NULL)
 189                         memcpy(p->ldt, current->ldt, LDT_ENTRIES*LDT_ENTRY_SIZE);
 190         }
 191         p->tss.bitmap = offsetof(struct tss_struct,io_bitmap);
 192         for (i = 0; i < IO_BITMAP_SIZE+1 ; i++) /* IO bitmap is actually SIZE+1 */
 193                 p->tss.io_bitmap[i] = ~0;
 194         if (last_task_used_math == current)
 195                 __asm__("clts ; fnsave %0 ; frstor %0":"=m" (p->tss.i387));
 196         p->semun = NULL; p->shm = NULL;
 197         if (copy_vm(p) || shm_fork(current, p))
 198                 goto bad_fork_cleanup;
 199         if (clone_flags & COPYFD) {
 200                 for (i=0; i<NR_OPEN;i++)
 201                         if ((f = p->filp[i]) != NULL)
 202                                 p->filp[i] = copy_fd(f);
 203         } else {
 204                 for (i=0; i<NR_OPEN;i++)
 205                         if ((f = p->filp[i]) != NULL)
 206                                 f->f_count++;
 207         }
 208         if (current->pwd)
 209                 current->pwd->i_count++;
 210         if (current->root)
 211                 current->root->i_count++;
 212         if (current->executable)
 213                 current->executable->i_count++;
 214         dup_mmap(p);
 215         set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));
 216         if (p->ldt)
 217                 set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,p->ldt, 512);
 218         else
 219                 set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&default_ldt, 1);
 220 
 221         p->counter = current->counter >> 1;
 222         p->state = TASK_RUNNING;        /* do this last, just in case */
 223         return p->pid;
 224 bad_fork_cleanup:
 225         task[nr] = NULL;
 226         REMOVE_LINKS(p);
 227         free_page(p->kernel_stack_page);
 228 bad_fork_free:
 229         free_page((long) p);
 230 bad_fork:
 231         return -EAGAIN;
 232 }
/* */
root/kernel/fork.c

DEFINITIONS
