kern_fork.c

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/*-
 * Copyright (c) 1982, 1986, 1989, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)kern_fork.c 8.6 (Berkeley) 4/8/94
 */

#include <sys/cdefs.h>
__FBSDID("$BSDSUniX$");

#include "opt_kdtrace.h"
#include "opt_ktrace.h"
#include "opt_kstack_pages.h"
#include "opt_procdesc.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/sysproto.h>
#include <sys/eventhandler.h>
#include <sys/fcntl.h>
#include <sys/filedesc.h>
#include <sys/jail.h>
#include <sys/kernel.h>
#include <sys/kthread.h>
#include <sys/sysctl.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/procdesc.h>
#include <sys/pioctl.h>
#include <sys/ptrace.h>
#include <sys/racct.h>
#include <sys/resourcevar.h>
#include <sys/sched.h>
#include <sys/syscall.h>
#include <sys/vmmeter.h>
#include <sys/vnode.h>
#include <sys/acct.h>
#include <sys/ktr.h>
#include <sys/ktrace.h>
#include <sys/unistd.h> 
#include <sys/sdt.h>
#include <sys/sx.h>
#include <sys/sysent.h>
#include <sys/signalvar.h>

#include <security/audit/audit.h>
#include <security/mac/mac_framework.h>

#include <vm/vm.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_extern.h>
#include <vm/uma.h>

#ifdef KDTRACE_HOOKS
#include <sys/dtrace_bsd.h>
dtrace_fork_func_t      dtrace_fasttrap_fork;
#endif

SDT_PROVIDER_DECLARE(proc);
SDT_PROBE_DEFINE3(proc, kernel, , create, "struct proc *",
    "struct proc *", "int");

#ifndef _SYS_SYSPROTO_H_
struct fork_args {
        int     dummy;
};
#endif

/* ARGSUSED */
int
sys_fork(struct thread *td, struct fork_args *uap)
{
        int error;
        struct proc *p2;

        error = fork1(td, RFFDG | RFPROC, 0, &p2, NULL, 0);
        if (error == 0) {
                td->td_retval[0] = p2->p_pid;
                td->td_retval[1] = 0;
        }
        return (error);
}

/* ARGUSED */
int
sys_pdfork(td, uap)
        struct thread *td;
        struct pdfork_args *uap;
{
#ifdef PROCDESC
        int error, fd;
        struct proc *p2;

        /*
         * It is necessary to return fd by reference because 0 is a valid file
         * descriptor number, and the child needs to be able to distinguish
         * itself from the parent using the return value.
         */
        error = fork1(td, RFFDG | RFPROC | RFPROCDESC, 0, &p2,
            &fd, uap->flags);
        if (error == 0) {
                td->td_retval[0] = p2->p_pid;
                td->td_retval[1] = 0;
                error = copyout(&fd, uap->fdp, sizeof(fd));
        }
        return (error);
#else
        return (ENOSYS);
#endif
}

/* ARGSUSED */
int
sys_vfork(struct thread *td, struct vfork_args *uap)
{
        int error, flags;
        struct proc *p2;

#ifdef XEN
        flags = RFFDG | RFPROC; /* validate that this is still an issue */
#else
        flags = RFFDG | RFPROC | RFPPWAIT | RFMEM;
#endif          
        error = fork1(td, flags, 0, &p2, NULL, 0);
        if (error == 0) {
                td->td_retval[0] = p2->p_pid;
                td->td_retval[1] = 0;
        }
        return (error);
}

int
sys_rfork(struct thread *td, struct rfork_args *uap)
{
        struct proc *p2;
        int error;

        /* Don't allow kernel-only flags. */
        if ((uap->flags & RFKERNELONLY) != 0)
                return (EINVAL);

        AUDIT_ARG_FFLAGS(uap->flags);
        error = fork1(td, uap->flags, 0, &p2, NULL, 0);
        if (error == 0) {
                td->td_retval[0] = p2 ? p2->p_pid : 0;
                td->td_retval[1] = 0;
        }
        return (error);
}

int     nprocs = 1;             /* process 0 */
int     lastpid = 0;
SYSCTL_INT(_kern, OID_AUTO, lastpid, CTLFLAG_RD, &lastpid, 0, 
    "Last used PID");

/*
 * Random component to lastpid generation.  We mix in a random factor to make
 * it a little harder to predict.  We sanity check the modulus value to avoid
 * doing it in critical paths.  Don't let it be too small or we pointlessly
 * waste randomness entropy, and don't let it be impossibly large.  Using a
 * modulus that is too big causes a LOT more process table scans and slows
 * down fork processing as the pidchecked caching is defeated.
 */
static int randompid = 0;

static int
sysctl_kern_randompid(SYSCTL_HANDLER_ARGS)
{
        int error, pid;

        error = sysctl_wire_old_buffer(req, sizeof(int));
        if (error != 0)
                return(error);
        sx_xlock(&allproc_lock);
        pid = randompid;
        error = sysctl_handle_int(oidp, &pid, 0, req);
        if (error == 0 && req->newptr != NULL) {
                if (pid < 0 || pid > pid_max - 100)     /* out of range */
                        pid = pid_max - 100;
                else if (pid < 2)                       /* NOP */
                        pid = 0;
                else if (pid < 100)                     /* Make it reasonable */
                        pid = 100;
                randompid = pid;
        }
        sx_xunlock(&allproc_lock);
        return (error);
}

SYSCTL_PROC(_kern, OID_AUTO, randompid, CTLTYPE_INT|CTLFLAG_RW,
    0, 0, sysctl_kern_randompid, "I", "Random PID modulus");

static int
fork_findpid(int flags)
{
        struct proc *p;
        int trypid;
        static int pidchecked = 0;

        /*
         * Requires allproc_lock in order to iterate over the list
         * of processes, and proctree_lock to access p_pgrp.
         */
        sx_assert(&allproc_lock, SX_LOCKED);
        sx_assert(&proctree_lock, SX_LOCKED);

        /*
         * Find an unused process ID.  We remember a range of unused IDs
         * ready to use (from lastpid+1 through pidchecked-1).
         *
         * If RFHIGHPID is set (used during system boot), do not allocate
         * low-numbered pids.
         */
        trypid = lastpid + 1;
        if (flags & RFHIGHPID) {
                if (trypid < 10)
                        trypid = 10;
        } else {
                if (randompid)
                        trypid += arc4random() % randompid;
        }
retry:
        /*
         * If the process ID prototype has wrapped around,
         * restart somewhat above 0, as the low-numbered procs
         * tend to include daemons that don't exit.
         */
        if (trypid >= pid_max) {
                trypid = trypid % pid_max;
                if (trypid < 100)
                        trypid += 100;
                pidchecked = 0;
        }
        if (trypid >= pidchecked) {
                int doingzomb = 0;

                pidchecked = PID_MAX;
                /*
                 * Scan the active and zombie procs to check whether this pid
                 * is in use.  Remember the lowest pid that's greater
                 * than trypid, so we can avoid checking for a while.
                 */
                p = LIST_FIRST(&allproc);
again:
                for (; p != NULL; p = LIST_NEXT(p, p_list)) {
                        while (p->p_pid == trypid ||
                            (p->p_pgrp != NULL &&
                            (p->p_pgrp->pg_id == trypid ||
                            (p->p_session != NULL &&
                            p->p_session->s_sid == trypid)))) {
                                trypid++;
                                if (trypid >= pidchecked)
                                        goto retry;
                        }
                        if (p->p_pid > trypid && pidchecked > p->p_pid)
                                pidchecked = p->p_pid;
                        if (p->p_pgrp != NULL) {
                                if (p->p_pgrp->pg_id > trypid &&
                                    pidchecked > p->p_pgrp->pg_id)
                                        pidchecked = p->p_pgrp->pg_id;
                                if (p->p_session != NULL &&
                                    p->p_session->s_sid > trypid &&
                                    pidchecked > p->p_session->s_sid)
                                        pidchecked = p->p_session->s_sid;
                        }
                }
                if (!doingzomb) {
                        doingzomb = 1;
                        p = LIST_FIRST(&zombproc);
                        goto again;
                }
        }

        /*
         * RFHIGHPID does not mess with the lastpid counter during boot.
         */
        if (flags & RFHIGHPID)
                pidchecked = 0;
        else
                lastpid = trypid;

        return (trypid);
}

static int
fork_norfproc(struct thread *td, int flags)
{
        int error;
        struct proc *p1;

        KASSERT((flags & RFPROC) == 0,
            ("fork_norfproc called with RFPROC set"));
        p1 = td->td_proc;

        if (((p1->p_flag & (P_HADTHREADS|P_SYSTEM)) == P_HADTHREADS) &&
            (flags & (RFCFDG | RFFDG))) {
                PROC_LOCK(p1);
                if (thread_single(SINGLE_BOUNDARY)) {
                        PROC_UNLOCK(p1);
                        return (ERESTART);
                }
                PROC_UNLOCK(p1);
        }

        error = vm_forkproc(td, NULL, NULL, NULL, flags);
        if (error)
                goto fail;

        /*
         * Close all file descriptors.
         */
        if (flags & RFCFDG) {
                struct filedesc *fdtmp;
                fdtmp = fdinit(td->td_proc->p_fd);
                fdfree(td);
                p1->p_fd = fdtmp;
        }

        /*
         * Unshare file descriptors (from parent).
         */
        if (flags & RFFDG) 
                fdunshare(p1, td);

fail:
        if (((p1->p_flag & (P_HADTHREADS|P_SYSTEM)) == P_HADTHREADS) &&
            (flags & (RFCFDG | RFFDG))) {
                PROC_LOCK(p1);
                thread_single_end();
                PROC_UNLOCK(p1);
        }
        return (error);
}

static void
do_fork(struct thread *td, int flags, struct proc *p2, struct thread *td2,
    struct vmspace *vm2, int pdflags)
{
        struct proc *p1, *pptr;
        int p2_held, trypid;
        struct filedesc *fd;
        struct filedesc_to_leader *fdtol;
        struct sigacts *newsigacts;

        sx_assert(&proctree_lock, SX_SLOCKED);
        sx_assert(&allproc_lock, SX_XLOCKED);

        p2_held = 0;
        p1 = td->td_proc;

        /*
         * Increment the nprocs resource before blocking can occur.  There
         * are hard-limits as to the number of processes that can run.
         */
        nprocs++;

        trypid = fork_findpid(flags);

        sx_sunlock(&proctree_lock);

        p2->p_state = PRS_NEW;          /* protect against others */
        p2->p_pid = trypid;
        AUDIT_ARG_PID(p2->p_pid);
        LIST_INSERT_HEAD(&allproc, p2, p_list);
        LIST_INSERT_HEAD(PIDHASH(p2->p_pid), p2, p_hash);
        tidhash_add(td2);
        PROC_LOCK(p2);
        PROC_LOCK(p1);

        sx_xunlock(&allproc_lock);

        bcopy(&p1->p_startcopy, &p2->p_startcopy,
            __rangeof(struct proc, p_startcopy, p_endcopy));
        pargs_hold(p2->p_args);
        PROC_UNLOCK(p1);

        bzero(&p2->p_startzero,
            __rangeof(struct proc, p_startzero, p_endzero));
        p2->p_treeflag = 0;

        p2->p_ucred = crhold(td->td_ucred);

        /* Tell the prison that we exist. */
        prison_proc_hold(p2->p_ucred->cr_prison);

        PROC_UNLOCK(p2);

        /*
         * Malloc things while we don't hold any locks.
         */
        if (flags & RFSIGSHARE)
                newsigacts = NULL;
        else
                newsigacts = sigacts_alloc();

        /*
         * Copy filedesc.
         */
        if (flags & RFCFDG) {
                fd = fdinit(p1->p_fd);
                fdtol = NULL;
        } else if (flags & RFFDG) {
                fd = fdcopy(p1->p_fd);
                fdtol = NULL;
        } else {
                fd = fdshare(p1->p_fd);
                if (p1->p_fdtol == NULL)
                        p1->p_fdtol = filedesc_to_leader_alloc(NULL, NULL,
                            p1->p_leader);
                if ((flags & RFTHREAD) != 0) {
                        /*
                         * Shared file descriptor table, and shared
                         * process leaders.
                         */
                        fdtol = p1->p_fdtol;
                        FILEDESC_XLOCK(p1->p_fd);
                        fdtol->fdl_refcount++;
                        FILEDESC_XUNLOCK(p1->p_fd);
                } else {
                        /* 
                         * Shared file descriptor table, and different
                         * process leaders.
                         */
                        fdtol = filedesc_to_leader_alloc(p1->p_fdtol,
                            p1->p_fd, p2);
                }
        }
        /*
         * Make a proc table entry for the new process.
         * Start by zeroing the section of proc that is zero-initialized,
         * then copy the section that is copied directly from the parent.
         */

        PROC_LOCK(p2);
        PROC_LOCK(p1);

        bzero(&td2->td_startzero,
            __rangeof(struct thread, td_startzero, td_endzero));

        bcopy(&td->td_startcopy, &td2->td_startcopy,
            __rangeof(struct thread, td_startcopy, td_endcopy));

        bcopy(&p2->p_comm, &td2->td_name, sizeof(td2->td_name));
        td2->td_sigstk = td->td_sigstk;
        td2->td_sigmask = td->td_sigmask;
        td2->td_flags = TDF_INMEM;
        td2->td_lend_user_pri = PRI_MAX;
        td2->td_dbg_sc_code = td->td_dbg_sc_code;
        td2->td_dbg_sc_narg = td->td_dbg_sc_narg;

#ifdef VIMAGE
        td2->td_vnet = NULL;
        td2->td_vnet_lpush = NULL;
#endif

        /*
         * Allow the scheduler to initialize the child.
         */
        thread_lock(td);
        sched_fork(td, td2);
        thread_unlock(td);

        /*
         * Duplicate sub-structures as needed.
         * Increase reference counts on shared objects.
         */
        p2->p_flag = P_INMEM;
        p2->p_flag2 = 0;
        p2->p_swtick = ticks;
        if (p1->p_flag & P_PROFIL)
                startprofclock(p2);
        td2->td_ucred = crhold(p2->p_ucred);

        if (flags & RFSIGSHARE) {
                p2->p_sigacts = sigacts_hold(p1->p_sigacts);
        } else {
                sigacts_copy(newsigacts, p1->p_sigacts);
                p2->p_sigacts = newsigacts;
        }

        if (flags & RFTSIGZMB)
                p2->p_sigparent = RFTSIGNUM(flags);
        else if (flags & RFLINUXTHPN)
                p2->p_sigparent = SIGUSR1;
        else
                p2->p_sigparent = SIGCHLD;

        p2->p_textvp = p1->p_textvp;
        p2->p_fd = fd;
        p2->p_fdtol = fdtol;

        if (p1->p_flag2 & P2_INHERIT_PROTECTED) {
                p2->p_flag |= P_PROTECTED;
                p2->p_flag2 |= P2_INHERIT_PROTECTED;
        }

        /*
         * p_limit is copy-on-write.  Bump its refcount.
         */
        lim_fork(p1, p2);

        pstats_fork(p1->p_stats, p2->p_stats);

        PROC_UNLOCK(p1);
        PROC_UNLOCK(p2);

        /* Bump references to the text vnode (for procfs). */
        if (p2->p_textvp)
                vref(p2->p_textvp);

        /*
         * Set up linkage for kernel based threading.
         */
        if ((flags & RFTHREAD) != 0) {
                mtx_lock(&ppeers_lock);
                p2->p_peers = p1->p_peers;
                p1->p_peers = p2;
                p2->p_leader = p1->p_leader;
                mtx_unlock(&ppeers_lock);
                PROC_LOCK(p1->p_leader);
                if ((p1->p_leader->p_flag & P_WEXIT) != 0) {
                        PROC_UNLOCK(p1->p_leader);
                        /*
                         * The task leader is exiting, so process p1 is
                         * going to be killed shortly.  Since p1 obviously
                         * isn't dead yet, we know that the leader is either
                         * sending SIGKILL's to all the processes in this
                         * task or is sleeping waiting for all the peers to
                         * exit.  We let p1 complete the fork, but we need
                         * to go ahead and kill the new process p2 since
                         * the task leader may not get a chance to send
                         * SIGKILL to it.  We leave it on the list so that
                         * the task leader will wait for this new process
                         * to commit suicide.
                         */
                        PROC_LOCK(p2);
                        kern_psignal(p2, SIGKILL);
                        PROC_UNLOCK(p2);
                } else
                        PROC_UNLOCK(p1->p_leader);
        } else {
                p2->p_peers = NULL;
                p2->p_leader = p2;
        }

        sx_xlock(&proctree_lock);
        PGRP_LOCK(p1->p_pgrp);
        PROC_LOCK(p2);
        PROC_LOCK(p1);

        /*
         * Preserve some more flags in subprocess.  P_PROFIL has already
         * been preserved.
         */
        p2->p_flag |= p1->p_flag & P_SUGID;
        td2->td_pflags |= td->td_pflags & TDP_ALTSTACK;
        SESS_LOCK(p1->p_session);
        if (p1->p_session->s_ttyvp != NULL && p1->p_flag & P_CONTROLT)
                p2->p_flag |= P_CONTROLT;
        SESS_UNLOCK(p1->p_session);
        if (flags & RFPPWAIT)
                p2->p_flag |= P_PPWAIT;

        p2->p_pgrp = p1->p_pgrp;
        LIST_INSERT_AFTER(p1, p2, p_pglist);
        PGRP_UNLOCK(p1->p_pgrp);
        LIST_INIT(&p2->p_children);
        LIST_INIT(&p2->p_orphans);

        callout_init_mtx(&p2->p_itcallout, &p2->p_mtx, 0);

        /*
         * If PF_FORK is set, the child process inherits the
         * procfs ioctl flags from its parent.
         */
        if (p1->p_pfsflags & PF_FORK) {
                p2->p_stops = p1->p_stops;
                p2->p_pfsflags = p1->p_pfsflags;
        }

        /*
         * This begins the section where we must prevent the parent
         * from being swapped.
         */
        _PHOLD(p1);
        PROC_UNLOCK(p1);

        /*
         * Attach the new process to its parent.
         *
         * If RFNOWAIT is set, the newly created process becomes a child
         * of init.  This effectively disassociates the child from the
         * parent.
         */
        if (flags & RFNOWAIT)
                pptr = initproc;
        else
                pptr = p1;
        p2->p_pptr = pptr;
        LIST_INSERT_HEAD(&pptr->p_children, p2, p_sibling);
        sx_xunlock(&proctree_lock);

        /* Inform accounting that we have forked. */
        p2->p_acflag = AFORK;
        PROC_UNLOCK(p2);

#ifdef KTRACE
        ktrprocfork(p1, p2);
#endif

        /*
         * Finish creating the child process.  It will return via a different
         * execution path later.  (ie: directly into user mode)
         */
        vm_forkproc(td, p2, td2, vm2, flags);

        if (flags == (RFFDG | RFPROC)) {
                PCPU_INC(cnt.v_forks);
                PCPU_ADD(cnt.v_forkpages, p2->p_vmspace->vm_dsize +
                    p2->p_vmspace->vm_ssize);
        } else if (flags == (RFFDG | RFPROC | RFPPWAIT | RFMEM)) {
                PCPU_INC(cnt.v_vforks);
                PCPU_ADD(cnt.v_vforkpages, p2->p_vmspace->vm_dsize +
                    p2->p_vmspace->vm_ssize);
        } else if (p1 == &proc0) {
                PCPU_INC(cnt.v_kthreads);
                PCPU_ADD(cnt.v_kthreadpages, p2->p_vmspace->vm_dsize +
                    p2->p_vmspace->vm_ssize);
        } else {
                PCPU_INC(cnt.v_rforks);
                PCPU_ADD(cnt.v_rforkpages, p2->p_vmspace->vm_dsize +
                    p2->p_vmspace->vm_ssize);
        }

#ifdef PROCDESC
        /*
         * Associate the process descriptor with the process before anything
         * can happen that might cause that process to need the descriptor.
         * However, don't do this until after fork(2) can no longer fail.
         */
        if (flags & RFPROCDESC)
                procdesc_new(p2, pdflags);
#endif

        /*
         * Both processes are set up, now check if any loadable modules want
         * to adjust anything.
         */
        EVENTHANDLER_INVOKE(process_fork, p1, p2, flags);

        /*
         * Set the child start time and mark the process as being complete.
         */
        PROC_LOCK(p2);
        PROC_LOCK(p1);
        microuptime(&p2->p_stats->p_start);
        PROC_SLOCK(p2);
        p2->p_state = PRS_NORMAL;
        PROC_SUNLOCK(p2);

#ifdef KDTRACE_HOOKS
        /*
         * Tell the DTrace fasttrap provider about the new process so that any
         * tracepoints inherited from the parent can be removed. We have to do
         * this only after p_state is PRS_NORMAL since the fasttrap module will
         * use pfind() later on.
         */
        if ((flags & RFMEM) == 0 && dtrace_fasttrap_fork)
                dtrace_fasttrap_fork(p1, p2);
#endif
        if ((p1->p_flag & (P_TRACED | P_FOLLOWFORK)) == (P_TRACED |
            P_FOLLOWFORK)) {
                /*
                 * Arrange for debugger to receive the fork event.
                 *
                 * We can report PL_FLAG_FORKED regardless of
                 * P_FOLLOWFORK settings, but it does not make a sense
                 * for runaway child.
                 */
                td->td_dbgflags |= TDB_FORK;
                td->td_dbg_forked = p2->p_pid;
                td2->td_dbgflags |= TDB_STOPATFORK;
                _PHOLD(p2);
                p2_held = 1;
        }
        if (flags & RFPPWAIT) {
                td->td_pflags |= TDP_RFPPWAIT;
                td->td_rfppwait_p = p2;
        }
        PROC_UNLOCK(p2);
        if ((flags & RFSTOPPED) == 0) {
                /*
                 * If RFSTOPPED not requested, make child runnable and
                 * add to run queue.
                 */
                thread_lock(td2);
                TD_SET_CAN_RUN(td2);
                sched_add(td2, SRQ_BORING);
                thread_unlock(td2);
        }

        /*
         * Now can be swapped.
         */
        _PRELE(p1);
        PROC_UNLOCK(p1);

        /*
         * Tell any interested parties about the new process.
         */
        knote_fork(&p1->p_klist, p2->p_pid);
        SDT_PROBE3(proc, kernel, , create, p2, p1, flags);

        /*
         * Wait until debugger is attached to child.
         */
        PROC_LOCK(p2);
        while ((td2->td_dbgflags & TDB_STOPATFORK) != 0)
                cv_wait(&p2->p_dbgwait, &p2->p_mtx);
        if (p2_held)
                _PRELE(p2);
        PROC_UNLOCK(p2);
}

int
fork1(struct thread *td, int flags, int pages, struct proc **procp,
    int *procdescp, int pdflags)
{
        struct proc *p1;
        struct proc *newproc;
        int ok;
        struct thread *td2;
        struct vmspace *vm2;
        vm_ooffset_t mem_charged;
        int error;
        static int curfail;
        static struct timeval lastfail;
#ifdef PROCDESC
        struct file *fp_procdesc = NULL;
#endif

        /* Check for the undefined or unimplemented flags. */
        if ((flags & ~(RFFLAGS | RFTSIGFLAGS(RFTSIGMASK))) != 0)
                return (EINVAL);

        /* Signal value requires RFTSIGZMB. */
        if ((flags & RFTSIGFLAGS(RFTSIGMASK)) != 0 && (flags & RFTSIGZMB) == 0)
                return (EINVAL);

        /* Can't copy and clear. */
        if ((flags & (RFFDG|RFCFDG)) == (RFFDG|RFCFDG))
                return (EINVAL);

        /* Check the validity of the signal number. */
        if ((flags & RFTSIGZMB) != 0 && (u_int)RFTSIGNUM(flags) > _SIG_MAXSIG)
                return (EINVAL);

#ifdef PROCDESC
        if ((flags & RFPROCDESC) != 0) {
                /* Can't not create a process yet get a process descriptor. */
                if ((flags & RFPROC) == 0)
                        return (EINVAL);

                /* Must provide a place to put a procdesc if creating one. */
                if (procdescp == NULL)
                        return (EINVAL);
        }
#endif

        p1 = td->td_proc;

        /*
         * Here we don't create a new process, but we divorce
         * certain parts of a process from itself.
         */
        if ((flags & RFPROC) == 0) {
                *procp = NULL;
                return (fork_norfproc(td, flags));
        }

#ifdef PROCDESC
        /*
         * If required, create a process descriptor in the parent first; we
         * will abandon it if something goes wrong. We don't finit() until
         * later.
         */
        if (flags & RFPROCDESC) {
                error = falloc(td, &fp_procdesc, procdescp, 0);
                if (error != 0)
                        return (error);
        }
#endif

        mem_charged = 0;
        vm2 = NULL;
        if (pages == 0)
                pages = KSTACK_PAGES;
        /* Allocate new proc. */
        newproc = uma_zalloc(proc_zone, M_WAITOK);
        td2 = FIRST_THREAD_IN_PROC(newproc);
        if (td2 == NULL) {
                td2 = thread_alloc(pages);
                if (td2 == NULL) {
                        error = ENOMEM;
                        goto fail1;
                }
                proc_linkup(newproc, td2);
        } else {
                if (td2->td_kstack == 0 || td2->td_kstack_pages != pages) {
                        if (td2->td_kstack != 0)
                                vm_thread_dispose(td2);
                        if (!thread_alloc_stack(td2, pages)) {
                                error = ENOMEM;
                                goto fail1;
                        }
                }
        }

        if ((flags & RFMEM) == 0) {
                vm2 = vmspace_fork(p1->p_vmspace, &mem_charged);
                if (vm2 == NULL) {
                        error = ENOMEM;
                        goto fail1;
                }
                if (!swap_reserve(mem_charged)) {
                        /*
                         * The swap reservation failed. The accounting
                         * from the entries of the copied vm2 will be
                         * substracted in vmspace_free(), so force the
                         * reservation there.
                         */
                        swap_reserve_force(mem_charged);
                        error = ENOMEM;
                        goto fail1;
                }
        } else
                vm2 = NULL;

        /*
         * XXX: This is ugly; when we copy resource usage, we need to bump
         *      per-cred resource counters.
         */
        newproc->p_ucred = p1->p_ucred;

        /*
         * Initialize resource accounting for the child process.
         */
        error = racct_proc_fork(p1, newproc);
        if (error != 0) {
                error = EAGAIN;
                goto fail1;
        }

#ifdef MAC
        mac_proc_init(newproc);
#endif
        knlist_init_mtx(&newproc->p_klist, &newproc->p_mtx);
        STAILQ_INIT(&newproc->p_ktr);

        /* We have to lock the process tree while we look for a pid. */
        sx_slock(&proctree_lock);

        /*
         * Although process entries are dynamically created, we still keep
         * a global limit on the maximum number we will create.  Don't allow
         * a nonprivileged user to use the last ten processes; don't let root
         * exceed the limit. The variable nprocs is the current number of
         * processes, maxproc is the limit.
         */
        sx_xlock(&allproc_lock);
        if ((nprocs >= maxproc - 10 && priv_check_cred(td->td_ucred,
            PRIV_MAXPROC, 0) != 0) || nprocs >= maxproc) {
                error = EAGAIN;
                goto fail;
        }

        /*
         * Increment the count of procs running with this uid. Don't allow
         * a nonprivileged user to exceed their current limit.
         *
         * XXXRW: Can we avoid privilege here if it's not needed?
         */
        error = priv_check_cred(td->td_ucred, PRIV_PROC_LIMIT, 0);
        if (error == 0)
                ok = chgproccnt(td->td_ucred->cr_ruidinfo, 1, 0);
        else {
                PROC_LOCK(p1);
                ok = chgproccnt(td->td_ucred->cr_ruidinfo, 1,
                    lim_cur(p1, RLIMIT_NPROC));
                PROC_UNLOCK(p1);
        }
        if (ok) {
                do_fork(td, flags, newproc, td2, vm2, pdflags);

                /*
                 * Return child proc pointer to parent.
                 */
                *procp = newproc;
#ifdef PROCDESC
                if (flags & RFPROCDESC)
                        procdesc_finit(newproc->p_procdesc, fp_procdesc);
#endif
                racct_proc_fork_done(newproc);
                return (0);
        }

        error = EAGAIN;
fail:
        sx_sunlock(&proctree_lock);
        if (ppsratecheck(&lastfail, &curfail, 1))
                printf("maxproc limit exceeded by uid %u (pid %d); see tuning(7) and login.conf(5)\n",
                    td->td_ucred->cr_ruid, p1->p_pid);
        sx_xunlock(&allproc_lock);
#ifdef MAC
        mac_proc_destroy(newproc);
#endif
        racct_proc_exit(newproc);
fail1:
        if (vm2 != NULL)
                vmspace_free(vm2);
        uma_zfree(proc_zone, newproc);
#ifdef PROCDESC
        if (((flags & RFPROCDESC) != 0) && (fp_procdesc != NULL))
                fdrop(fp_procdesc, td);
#endif
        pause("fork", hz / 2);
        return (error);
}

/*
 * Handle the return of a child process from fork1().  This function
 * is called from the MD fork_trampoline() entry point.
 */
void
fork_exit(void (*callout)(void *, struct trapframe *), void *arg,
    struct trapframe *frame)
{
        struct proc *p;
        struct thread *td;
        struct thread *dtd;

        td = curthread;
        p = td->td_proc;
        KASSERT(p->p_state == PRS_NORMAL, ("executing process is still new"));

        CTR4(KTR_PROC, "fork_exit: new thread %p (td_sched %p, pid %d, %s)",
                td, td->td_sched, p->p_pid, td->td_name);

        sched_fork_exit(td);
        /*
        * Processes normally resume in mi_switch() after being
        * cpu_switch()'ed to, but when children start up they arrive here
        * instead, so we must do much the same things as mi_switch() would.
        */
        if ((dtd = PCPU_GET(deadthread))) {
                PCPU_SET(deadthread, NULL);
                thread_stash(dtd);
        }
        thread_unlock(td);

        /*
         * cpu_set_fork_handler intercepts this function call to
         * have this call a non-return function to stay in kernel mode.
         * initproc has its own fork handler, but it does return.
         */
        KASSERT(callout != NULL, ("NULL callout in fork_exit"));
        callout(arg, frame);

        /*
         * Check if a kernel thread misbehaved and returned from its main
         * function.
         */
        if (p->p_flag & P_KTHREAD) {
                printf("Kernel thread \"%s\" (pid %d) exited prematurely.\n",
                    td->td_name, p->p_pid);
                kthread_exit();
        }
        mtx_assert(&Giant, MA_NOTOWNED);

        if (p->p_sysent->sv_schedtail != NULL)
                (p->p_sysent->sv_schedtail)(td);
}

/*
 * Simplified back end of syscall(), used when returning from fork()
 * directly into user mode.  Giant is not held on entry, and must not
 * be held on return.  This function is passed in to fork_exit() as the
 * first parameter and is called when returning to a new userland process.
 */
void
fork_return(struct thread *td, struct trapframe *frame)
{
        struct proc *p, *dbg;

        p = td->td_proc;
        if (td->td_dbgflags & TDB_STOPATFORK) {
                sx_xlock(&proctree_lock);
                PROC_LOCK(p);
                if ((p->p_pptr->p_flag & (P_TRACED | P_FOLLOWFORK)) ==
                    (P_TRACED | P_FOLLOWFORK)) {
                        /*
                         * If debugger still wants auto-attach for the
                         * parent's children, do it now.
                         */
                        dbg = p->p_pptr->p_pptr;
                        p->p_flag |= P_TRACED;
                        p->p_oppid = p->p_pptr->p_pid;
                        CTR2(KTR_PTRACE,
                    "fork_return: attaching to new child pid %d: oppid %d",
                            p->p_pid, p->p_oppid);
                        proc_reparent(p, dbg);
                        sx_xunlock(&proctree_lock);
                        td->td_dbgflags |= TDB_CHILD | TDB_SCX;
                        ptracestop(td, SIGSTOP);
                        td->td_dbgflags &= ~(TDB_CHILD | TDB_SCX);
                } else {
                        /*
                         * ... otherwise clear the request.
                         */
                        sx_xunlock(&proctree_lock);
                        td->td_dbgflags &= ~TDB_STOPATFORK;
                        cv_broadcast(&p->p_dbgwait);
                }
                PROC_UNLOCK(p);
        } else if (p->p_flag & P_TRACED) {
                /*
                 * This is the start of a new thread in a traced
                 * process.  Report a system call exit event.
                 */
                PROC_LOCK(p);
                td->td_dbgflags |= TDB_SCX;
                _STOPEVENT(p, S_SCX, td->td_dbg_sc_code);
                if ((p->p_stops & S_PT_SCX) != 0)
                        ptracestop(td, SIGTRAP);
                td->td_dbgflags &= ~TDB_SCX;
                PROC_UNLOCK(p);
        }

        userret(td, frame);

#ifdef KTRACE
        if (KTRPOINT(td, KTR_SYSRET))
                ktrsysret(SYS_fork, 0, 0);
#endif
        mtx_assert(&Giant, MA_NOTOWNED);
}