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

DEFINITIONS
