xref: /src/sys/kern/kern_fork.c

/*	$NetBSD: kern_fork.c,v 1.228 2022/07/01 01:05:21 riastradh Exp $	*/

/*-
 * Copyright (c) 1999, 2001, 2004, 2006, 2007, 2008, 2019
 *     The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe of the Numerical Aerospace Simulation Facility,
 * NASA Ames Research Center, by Charles M. Hannum, and by Andrew Doran.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
 */

/*
 * 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.
 * 3. 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	8.8 (Berkeley) 2/14/95
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_fork.c,v 1.228 2022/07/01 01:05:21 riastradh Exp $");

#include "opt_ktrace.h"
#include "opt_dtrace.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/filedesc.h>
#include <sys/kernel.h>
#include <sys/pool.h>
#include <sys/mount.h>
#include <sys/proc.h>
#include <sys/ras.h>
#include <sys/resourcevar.h>
#include <sys/vnode.h>
#include <sys/file.h>
#include <sys/acct.h>
#include <sys/ktrace.h>
#include <sys/sched.h>
#include <sys/signalvar.h>
#include <sys/syscall.h>
#include <sys/kauth.h>
#include <sys/atomic.h>
#include <sys/syscallargs.h>
#include <sys/uidinfo.h>
#include <sys/sdt.h>
#include <sys/ptrace.h>

/*
 * DTrace SDT provider definitions
 */
SDT_PROVIDER_DECLARE(proc);
SDT_PROBE_DEFINE3(proc, kernel, , create,
    "struct proc *", /* new process */
    "struct proc *", /* parent process */
    "int" /* flags */);

u_int	nprocs __cacheline_aligned = 1;		/* process 0 */

/*
 * Number of ticks to sleep if fork() would fail due to process hitting
 * limits. Exported in miliseconds to userland via sysctl.
 */
int	forkfsleep = 0;

int
sys_fork(struct lwp *l, const void *v, register_t *retval)
{

	return fork1(l, 0, SIGCHLD, NULL, 0, NULL, NULL, retval);
}

/*
 * vfork(2) system call compatible with 4.4BSD (i.e. BSD with Mach VM).
 * Address space is not shared, but parent is blocked until child exit.
 */
int
sys_vfork(struct lwp *l, const void *v, register_t *retval)
{

	return fork1(l, FORK_PPWAIT, SIGCHLD, NULL, 0, NULL, NULL,
	    retval);
}

/*
 * New vfork(2) system call for NetBSD, which implements original 3BSD vfork(2)
 * semantics.  Address space is shared, and parent is blocked until child exit.
 */
int
sys___vfork14(struct lwp *l, const void *v, register_t *retval)
{

	return fork1(l, FORK_PPWAIT|FORK_SHAREVM, SIGCHLD, NULL, 0,
	    NULL, NULL, retval);
}

/*
 * Linux-compatible __clone(2) system call.
 */
int
sys___clone(struct lwp *l, const struct sys___clone_args *uap,
    register_t *retval)
{
	/* {
		syscallarg(int) flags;
		syscallarg(void *) stack;
	} */
	int flags, sig;

	/*
	 * We don't support the CLONE_PTRACE flag.
	 */
	if (SCARG(uap, flags) & (CLONE_PTRACE))
		return EINVAL;

	/*
	 * Linux enforces CLONE_VM with CLONE_SIGHAND, do same.
	 */
	if (SCARG(uap, flags) & CLONE_SIGHAND
	    && (SCARG(uap, flags) & CLONE_VM) == 0)
		return EINVAL;

	flags = 0;

	if (SCARG(uap, flags) & CLONE_VM)
		flags |= FORK_SHAREVM;
	if (SCARG(uap, flags) & CLONE_FS)
		flags |= FORK_SHARECWD;
	if (SCARG(uap, flags) & CLONE_FILES)
		flags |= FORK_SHAREFILES;
	if (SCARG(uap, flags) & CLONE_SIGHAND)
		flags |= FORK_SHARESIGS;
	if (SCARG(uap, flags) & CLONE_VFORK)
		flags |= FORK_PPWAIT;

	sig = SCARG(uap, flags) & CLONE_CSIGNAL;
	if (sig < 0 || sig >= _NSIG)
		return EINVAL;

	/*
	 * Note that the Linux API does not provide a portable way of
	 * specifying the stack area; the caller must know if the stack
	 * grows up or down.  So, we pass a stack size of 0, so that the
	 * code that makes this adjustment is a noop.
	 */
	return fork1(l, flags, sig, SCARG(uap, stack), 0,
	    NULL, NULL, retval);
}

/*
 * Print the 'table full' message once per 10 seconds.
 */
static struct timeval fork_tfmrate = { 10, 0 };

/*
 * Check if a process is traced and shall inform about FORK events.
 */
static inline bool
tracefork(struct proc *p, int flags)
{

	return (p->p_slflag & (PSL_TRACEFORK|PSL_TRACED)) ==
	    (PSL_TRACEFORK|PSL_TRACED) && (flags & FORK_PPWAIT) == 0;
}

/*
 * Check if a process is traced and shall inform about VFORK events.
 */
static inline bool
tracevfork(struct proc *p, int flags)
{

	return (p->p_slflag & (PSL_TRACEVFORK|PSL_TRACED)) ==
	    (PSL_TRACEVFORK|PSL_TRACED) && (flags & FORK_PPWAIT) != 0;
}

/*
 * Check if a process is traced and shall inform about VFORK_DONE events.
 */
static inline bool
tracevforkdone(struct proc *p, int flags)
{

	return (p->p_slflag & (PSL_TRACEVFORK_DONE|PSL_TRACED)) ==
	    (PSL_TRACEVFORK_DONE|PSL_TRACED) && (flags & FORK_PPWAIT);
}

/*
 * General fork call.  Note that another LWP in the process may call exec()
 * or exit() while we are forking.  It's safe to continue here, because
 * neither operation will complete until all LWPs have exited the process.
 */
int
fork1(struct lwp *l1, int flags, int exitsig, void *stack, size_t stacksize,
    void (*func)(void *), void *arg, register_t *retval)
{
	struct proc	*p1, *p2, *parent;
	struct plimit   *p1_lim;
	uid_t		uid;
	struct lwp	*l2;
	int		count;
	vaddr_t		uaddr;
	int		tnprocs;
	int		error = 0;

	p1 = l1->l_proc;
	uid = kauth_cred_getuid(l1->l_cred);
	tnprocs = atomic_inc_uint_nv(&nprocs);

	/*
	 * Although process entries are dynamically created, we still keep
	 * a global limit on the maximum number we will create.
	 */
	if (__predict_false(tnprocs >= maxproc))
		error = -1;
	else
		error = kauth_authorize_process(l1->l_cred,
		    KAUTH_PROCESS_FORK, p1, KAUTH_ARG(tnprocs), NULL, NULL);

	if (error) {
		static struct timeval lasttfm;
		atomic_dec_uint(&nprocs);
		if (ratecheck(&lasttfm, &fork_tfmrate))
			tablefull("proc", "increase kern.maxproc or NPROC");
		if (forkfsleep)
			kpause("forkmx", false, forkfsleep, NULL);
		return EAGAIN;
	}

	/*
	 * Enforce limits.
	 */
	count = chgproccnt(uid, 1);
	if (__predict_false(count > p1->p_rlimit[RLIMIT_NPROC].rlim_cur)) {
		if (kauth_authorize_process(l1->l_cred, KAUTH_PROCESS_RLIMIT,
		    p1, KAUTH_ARG(KAUTH_REQ_PROCESS_RLIMIT_BYPASS),
		    &p1->p_rlimit[RLIMIT_NPROC], KAUTH_ARG(RLIMIT_NPROC)) != 0) {
			(void)chgproccnt(uid, -1);
			atomic_dec_uint(&nprocs);
			if (forkfsleep)
				kpause("forkulim", false, forkfsleep, NULL);
			return EAGAIN;
		}
	}

	/*
	 * Allocate virtual address space for the U-area now, while it
	 * is still easy to abort the fork operation if we're out of
	 * kernel virtual address space.
	 */
	uaddr = uvm_uarea_alloc();
	if (__predict_false(uaddr == 0)) {
		(void)chgproccnt(uid, -1);
		atomic_dec_uint(&nprocs);
		return ENOMEM;
	}

	/* Allocate new proc. */
	p2 = proc_alloc();
	if (p2 == NULL) {
		/* We were unable to allocate a process ID. */
		uvm_uarea_free(uaddr);
		mutex_enter(p1->p_lock);
		cred = p1->p_cred;
		uid = kauth_cred_getuid(cred);
		(void)chgproccnt(uid, -1);
		mutex_exit(p1->p_lock);
		atomic_dec_uint(&nprocs);
		return EAGAIN;
	}

	/*
	 * We are now committed to the fork.  From here on, we may
	 * block on resources, but resource allocation may NOT fail.
	 */

	/*
	 * 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.
	 */
	memset(&p2->p_startzero, 0,
	    (unsigned) ((char *)&p2->p_endzero - (char *)&p2->p_startzero));
	memcpy(&p2->p_startcopy, &p1->p_startcopy,
	    (unsigned) ((char *)&p2->p_endcopy - (char *)&p2->p_startcopy));

	TAILQ_INIT(&p2->p_sigpend.sp_info);

	LIST_INIT(&p2->p_lwps);
	LIST_INIT(&p2->p_sigwaiters);

	/*
	 * Duplicate sub-structures as needed.
	 * Increase reference counts on shared objects.
	 * Inherit flags we want to keep.  The flags related to SIGCHLD
	 * handling are important in order to keep a consistent behaviour
	 * for the child after the fork.  If we are a 32-bit process, the
	 * child will be too.
	 */
	p2->p_flag =
	    p1->p_flag & (PK_SUGID | PK_NOCLDWAIT | PK_CLDSIGIGN | PK_32);
	p2->p_emul = p1->p_emul;
	p2->p_execsw = p1->p_execsw;

	if (flags & FORK_SYSTEM) {
		/*
		 * Mark it as a system process.  Set P_NOCLDWAIT so that
		 * children are reparented to init(8) when they exit.
		 * init(8) can easily wait them out for us.
		 */
		p2->p_flag |= (PK_SYSTEM | PK_NOCLDWAIT);
	}

	mutex_init(&p2->p_stmutex, MUTEX_DEFAULT, IPL_HIGH);
	mutex_init(&p2->p_auxlock, MUTEX_DEFAULT, IPL_NONE);
	rw_init(&p2->p_reflock);
	cv_init(&p2->p_waitcv, "wait");
	cv_init(&p2->p_lwpcv, "lwpwait");

	/*
	 * Share a lock between the processes if they are to share signal
	 * state: we must synchronize access to it.
	 */
	if (flags & FORK_SHARESIGS) {
		p2->p_lock = p1->p_lock;
		mutex_obj_hold(p1->p_lock);
	} else
		p2->p_lock = mutex_obj_alloc(MUTEX_DEFAULT, IPL_NONE);

	kauth_proc_fork(p1, p2);

	p2->p_raslist = NULL;
#if defined(__HAVE_RAS)
	ras_fork(p1, p2);
#endif

	/* bump references to the text vnode (for procfs) */
	p2->p_textvp = p1->p_textvp;
	if (p2->p_textvp)
		vref(p2->p_textvp);
	if (p1->p_path)
		p2->p_path = kmem_strdupsize(p1->p_path, NULL, KM_SLEEP);
	else
		p2->p_path = NULL;

	if (flags & FORK_SHAREFILES)
		fd_share(p2);
	else if (flags & FORK_CLEANFILES)
		p2->p_fd = fd_init(NULL);
	else
		p2->p_fd = fd_copy();

	/* XXX racy */
	p2->p_mqueue_cnt = p1->p_mqueue_cnt;

	if (flags & FORK_SHARECWD)
		cwdshare(p2);
	else
		p2->p_cwdi = cwdinit();

	/*
	 * Note: p_limit (rlimit stuff) is copy-on-write, so normally
	 * we just need increase pl_refcnt.
	 */
	p1_lim = p1->p_limit;
	if (!p1_lim->pl_writeable) {
		lim_addref(p1_lim);
		p2->p_limit = p1_lim;
	} else {
		p2->p_limit = lim_copy(p1_lim);
	}

	if (flags & FORK_PPWAIT) {
		/* Mark ourselves as waiting for a child. */
		p2->p_lflag = PL_PPWAIT;
		l1->l_vforkwaiting = true;
		p2->p_vforklwp = l1;
	} else {
		p2->p_lflag = 0;
		l1->l_vforkwaiting = false;
	}
	p2->p_sflag = 0;
	p2->p_slflag = 0;
	parent = (flags & FORK_NOWAIT) ? initproc : p1;
	p2->p_pptr = parent;
	p2->p_ppid = parent->p_pid;
	LIST_INIT(&p2->p_children);

	p2->p_aio = NULL;

#ifdef KTRACE
	/*
	 * Copy traceflag and tracefile if enabled.
	 * If not inherited, these were zeroed above.
	 */
	if (p1->p_traceflag & KTRFAC_INHERIT) {
		mutex_enter(&ktrace_lock);
		p2->p_traceflag = p1->p_traceflag;
		if ((p2->p_tracep = p1->p_tracep) != NULL)
			ktradref(p2);
		mutex_exit(&ktrace_lock);
	}
#endif

	/*
	 * Create signal actions for the child process.
	 */
	p2->p_sigacts = sigactsinit(p1, flags & FORK_SHARESIGS);
	mutex_enter(p1->p_lock);
	p2->p_sflag |=
	    (p1->p_sflag & (PS_STOPFORK | PS_STOPEXEC | PS_NOCLDSTOP));
	sched_proc_fork(p1, p2);
	mutex_exit(p1->p_lock);

	p2->p_stflag = p1->p_stflag;

	/*
	 * p_stats.
	 * Copy parts of p_stats, and zero out the rest.
	 */
	p2->p_stats = pstatscopy(p1->p_stats);

	/*
	 * Set up the new process address space.
	 */
	uvm_proc_fork(p1, p2, (flags & FORK_SHAREVM) ? true : false);

	/*
	 * Finish creating the child process.
	 * It will return through a different path later.
	 */
	lwp_create(l1, p2, uaddr, (flags & FORK_PPWAIT) ? LWP_VFORK : 0,
	    stack, stacksize, (func != NULL) ? func : child_return, arg, &l2,
	    l1->l_class, &l1->l_sigmask, &l1->l_sigstk);

	/*
	 * Inherit l_private from the parent.
	 * Note that we cannot use lwp_setprivate() here since that
	 * also sets the CPU TLS register, which is incorrect if the
	 * process has changed that without letting the kernel know.
	 */
	l2->l_private = l1->l_private;

	/*
	 * If emulation has a process fork hook, call it now.
	 */
	if (p2->p_emul->e_proc_fork)
		(*p2->p_emul->e_proc_fork)(p2, l1, flags);

	/*
	 * ...and finally, any other random fork hooks that subsystems
	 * might have registered.
	 */
	doforkhooks(p2, p1);

	SDT_PROBE(proc, kernel, , create, p2, p1, flags, 0, 0);

	/*
	 * It's now safe for the scheduler and other processes to see the
	 * child process.
	 */
	mutex_enter(&proc_lock);

	if (p1->p_session->s_ttyvp != NULL && p1->p_lflag & PL_CONTROLT)
		p2->p_lflag |= PL_CONTROLT;

	LIST_INSERT_HEAD(&parent->p_children, p2, p_sibling);
	p2->p_exitsig = exitsig;		/* signal for parent on exit */

	/*
	 * Trace fork(2) and vfork(2)-like events on demand in a debugger.
	 */
	if (tracefork(p1, flags) || tracevfork(p1, flags)) {
		proc_changeparent(p2, p1->p_pptr);
		SET(p2->p_slflag, PSL_TRACEDCHILD);
	}

	p2->p_oppid = p1->p_pid; /* Remember the original parent id. */

	LIST_INSERT_AFTER(p1, p2, p_pglist);
	LIST_INSERT_HEAD(&allproc, p2, p_list);

	p2->p_trace_enabled = trace_is_enabled(p2);
#ifdef __HAVE_SYSCALL_INTERN
	(*p2->p_emul->e_syscall_intern)(p2);
#endif

	/*
	 * Update stats now that we know the fork was successful.
	 */
	KPREEMPT_DISABLE(l1);
	CPU_COUNT(CPU_COUNT_FORKS, 1);
	if (flags & FORK_PPWAIT)
		CPU_COUNT(CPU_COUNT_FORKS_PPWAIT, 1);
	if (flags & FORK_SHAREVM)
		CPU_COUNT(CPU_COUNT_FORKS_SHAREVM, 1);
	KPREEMPT_ENABLE(l1);

	if (ktrpoint(KTR_EMUL))
		p2->p_traceflag |= KTRFAC_TRC_EMUL;

	/*
	 * Notify any interested parties about the new process.
	 */
	if (!SLIST_EMPTY(&p1->p_klist)) {
		mutex_exit(&proc_lock);
		knote_proc_fork(p1, p2);
		mutex_enter(&proc_lock);
	}

	/*
	 * Make child runnable, set start time, and add to run queue except
	 * if the parent requested the child to start in SSTOP state.
	 */
	mutex_enter(p2->p_lock);

	/*
	 * Start profiling.
	 */
	if ((p2->p_stflag & PST_PROFIL) != 0) {
		mutex_spin_enter(&p2->p_stmutex);
		startprofclock(p2);
		mutex_spin_exit(&p2->p_stmutex);
	}

	getmicrotime(&p2->p_stats->p_start);
	p2->p_acflag = AFORK;
	lwp_lock(l2);
	KASSERT(p2->p_nrlwps == 1);
	KASSERT(l2->l_stat == LSIDL);
	if (p2->p_sflag & PS_STOPFORK) {
		p2->p_nrlwps = 0;
		p2->p_stat = SSTOP;
		p2->p_waited = 0;
		p1->p_nstopchild++;
		l2->l_stat = LSSTOP;
		KASSERT(l2->l_wchan == NULL);
		lwp_unlock(l2);
	} else {
		p2->p_nrlwps = 1;
		p2->p_stat = SACTIVE;
		setrunnable(l2);
		/* LWP now unlocked */
	}

	/*
	 * Return child pid to parent process,
	 * marking us as parent via retval[1].
	 */
	if (retval != NULL) {
		retval[0] = p2->p_pid;
		retval[1] = 0;
	}

	mutex_exit(p2->p_lock);

	/*
	 * Let the parent know that we are tracing its child.
	 */
	if (tracefork(p1, flags) || tracevfork(p1, flags)) {
		mutex_enter(p1->p_lock);
		eventswitch(TRAP_CHLD,
		    tracefork(p1, flags) ? PTRACE_FORK : PTRACE_VFORK,
		    retval[0]);
		mutex_enter(&proc_lock);
	}

	/*
	 * Preserve synchronization semantics of vfork.  If waiting for
	 * child to exec or exit, sleep until it clears p_vforkwaiting.
	 */
	while (l1->l_vforkwaiting)
		cv_wait(&l1->l_waitcv, &proc_lock);

	/*
	 * Let the parent know that we are tracing its child.
	 */
	if (tracevforkdone(p1, flags)) {
		mutex_enter(p1->p_lock);
		eventswitch(TRAP_CHLD, PTRACE_VFORK_DONE, retval[0]);
	} else
		mutex_exit(&proc_lock);

	return 0;
}

/*
 * MI code executed in each newly spawned process before returning to userland.
 */
void
child_return(void *arg)
{
	struct lwp *l = curlwp;
	struct proc *p = l->l_proc;

	if ((p->p_slflag & (PSL_TRACED|PSL_TRACEDCHILD)) ==
	    (PSL_TRACED|PSL_TRACEDCHILD)) {
		eventswitchchild(p, TRAP_CHLD,
		    ISSET(p->p_lflag, PL_PPWAIT) ? PTRACE_VFORK : PTRACE_FORK);
	}

	md_child_return(l);

	/*
	 * Return SYS_fork for all fork types, including vfork(2) and clone(2).
	 *
	 * This approach simplifies the code and avoids extra locking.
	 */
	ktrsysret(SYS_fork, 0, 0);
}