freebsd-skq/sys/kern/kern_exec.c

/*-
 * Copyright (c) 1993, David Greenman
 * All rights reserved.
 *
 * 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 AUTHOR 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 AUTHOR 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.
 */

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

#include "opt_capsicum.h"
#include "opt_hwpmc_hooks.h"
#include "opt_ktrace.h"
#include "opt_vm.h"

#include <sys/param.h>
#include <sys/capsicum.h>
#include <sys/systm.h>
#include <sys/eventhandler.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/sysproto.h>
#include <sys/signalvar.h>
#include <sys/kernel.h>
#include <sys/mount.h>
#include <sys/filedesc.h>
#include <sys/fcntl.h>
#include <sys/acct.h>
#include <sys/exec.h>
#include <sys/imgact.h>
#include <sys/imgact_elf.h>
#include <sys/wait.h>
#include <sys/malloc.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/pioctl.h>
#include <sys/ptrace.h>
#include <sys/namei.h>
#include <sys/resourcevar.h>
#include <sys/rwlock.h>
#include <sys/sched.h>
#include <sys/sdt.h>
#include <sys/sf_buf.h>
#include <sys/syscallsubr.h>
#include <sys/sysent.h>
#include <sys/shm.h>
#include <sys/sysctl.h>
#include <sys/vnode.h>
#include <sys/stat.h>
#ifdef KTRACE
#include <sys/ktrace.h>
#endif

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/pmap.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_kern.h>
#include <vm/vm_extern.h>
#include <vm/vm_object.h>
#include <vm/vm_pager.h>

#ifdef	HWPMC_HOOKS
#include <sys/pmckern.h>
#endif

#include <machine/reg.h>

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

#ifdef KDTRACE_HOOKS
#include <sys/dtrace_bsd.h>
dtrace_execexit_func_t	dtrace_fasttrap_exec;
#endif

SDT_PROVIDER_DECLARE(proc);
SDT_PROBE_DEFINE1(proc, , , exec, "char *");
SDT_PROBE_DEFINE1(proc, , , exec__failure, "int");
SDT_PROBE_DEFINE1(proc, , , exec__success, "char *");

MALLOC_DEFINE(M_PARGS, "proc-args", "Process arguments");

int coredump_pack_fileinfo = 1;
SYSCTL_INT(_kern, OID_AUTO, coredump_pack_fileinfo, CTLFLAG_RWTUN,
    &coredump_pack_fileinfo, 0,
    "Enable file path packing in 'procstat -f' coredump notes");

int coredump_pack_vmmapinfo = 1;
SYSCTL_INT(_kern, OID_AUTO, coredump_pack_vmmapinfo, CTLFLAG_RWTUN,
    &coredump_pack_vmmapinfo, 0,
    "Enable file path packing in 'procstat -v' coredump notes");

static int sysctl_kern_ps_strings(SYSCTL_HANDLER_ARGS);
static int sysctl_kern_usrstack(SYSCTL_HANDLER_ARGS);
static int sysctl_kern_stackprot(SYSCTL_HANDLER_ARGS);
static int do_execve(struct thread *td, struct image_args *args,
    struct mac *mac_p);

/* XXX This should be vm_size_t. */
SYSCTL_PROC(_kern, KERN_PS_STRINGS, ps_strings, CTLTYPE_ULONG|CTLFLAG_RD,
    NULL, 0, sysctl_kern_ps_strings, "LU", "");

/* XXX This should be vm_size_t. */
SYSCTL_PROC(_kern, KERN_USRSTACK, usrstack, CTLTYPE_ULONG|CTLFLAG_RD|
    CTLFLAG_CAPRD, NULL, 0, sysctl_kern_usrstack, "LU", "");

SYSCTL_PROC(_kern, OID_AUTO, stackprot, CTLTYPE_INT|CTLFLAG_RD,
    NULL, 0, sysctl_kern_stackprot, "I", "");

u_long ps_arg_cache_limit = PAGE_SIZE / 16;
SYSCTL_ULONG(_kern, OID_AUTO, ps_arg_cache_limit, CTLFLAG_RW, 
    &ps_arg_cache_limit, 0, "");

static int disallow_high_osrel;
SYSCTL_INT(_kern, OID_AUTO, disallow_high_osrel, CTLFLAG_RW,
    &disallow_high_osrel, 0,
    "Disallow execution of binaries built for higher version of the world");

static int map_at_zero = 0;
SYSCTL_INT(_security_bsd, OID_AUTO, map_at_zero, CTLFLAG_RWTUN, &map_at_zero, 0,
    "Permit processes to map an object at virtual address 0.");

static int
sysctl_kern_ps_strings(SYSCTL_HANDLER_ARGS)
{
	struct proc *p;
	int error;

	p = curproc;
#ifdef SCTL_MASK32
	if (req->flags & SCTL_MASK32) {
		unsigned int val;
		val = (unsigned int)p->p_sysent->sv_psstrings;
		error = SYSCTL_OUT(req, &val, sizeof(val));
	} else
#endif
		error = SYSCTL_OUT(req, &p->p_sysent->sv_psstrings,
		   sizeof(p->p_sysent->sv_psstrings));
	return error;
}

static int
sysctl_kern_usrstack(SYSCTL_HANDLER_ARGS)
{
	struct proc *p;
	int error;

	p = curproc;
#ifdef SCTL_MASK32
	if (req->flags & SCTL_MASK32) {
		unsigned int val;
		val = (unsigned int)p->p_sysent->sv_usrstack;
		error = SYSCTL_OUT(req, &val, sizeof(val));
	} else
#endif
		error = SYSCTL_OUT(req, &p->p_sysent->sv_usrstack,
		    sizeof(p->p_sysent->sv_usrstack));
	return error;
}

static int
sysctl_kern_stackprot(SYSCTL_HANDLER_ARGS)
{
	struct proc *p;

	p = curproc;
	return (SYSCTL_OUT(req, &p->p_sysent->sv_stackprot,
	    sizeof(p->p_sysent->sv_stackprot)));
}

/*
 * Each of the items is a pointer to a `const struct execsw', hence the
 * double pointer here.
 */
static const struct execsw **execsw;

#ifndef _SYS_SYSPROTO_H_
struct execve_args {
	char    *fname; 
	char    **argv;
	char    **envv; 
};
#endif

int
sys_execve(struct thread *td, struct execve_args *uap)
{
	struct image_args args;
	struct vmspace *oldvmspace;
	int error;

	error = pre_execve(td, &oldvmspace);
	if (error != 0)
		return (error);
	error = exec_copyin_args(&args, uap->fname, UIO_USERSPACE,
	    uap->argv, uap->envv);
	if (error == 0)
		error = kern_execve(td, &args, NULL);
	post_execve(td, error, oldvmspace);
	return (error);
}

#ifndef _SYS_SYSPROTO_H_
struct fexecve_args {
	int	fd;
	char	**argv;
	char	**envv;
}
#endif
int
sys_fexecve(struct thread *td, struct fexecve_args *uap)
{
	struct image_args args;
	struct vmspace *oldvmspace;
	int error;

	error = pre_execve(td, &oldvmspace);
	if (error != 0)
		return (error);
	error = exec_copyin_args(&args, NULL, UIO_SYSSPACE,
	    uap->argv, uap->envv);
	if (error == 0) {
		args.fd = uap->fd;
		error = kern_execve(td, &args, NULL);
	}
	post_execve(td, error, oldvmspace);
	return (error);
}

#ifndef _SYS_SYSPROTO_H_
struct __mac_execve_args {
	char	*fname;
	char	**argv;
	char	**envv;
	struct mac	*mac_p;
};
#endif

int
sys___mac_execve(struct thread *td, struct __mac_execve_args *uap)
{
#ifdef MAC
	struct image_args args;
	struct vmspace *oldvmspace;
	int error;

	error = pre_execve(td, &oldvmspace);
	if (error != 0)
		return (error);
	error = exec_copyin_args(&args, uap->fname, UIO_USERSPACE,
	    uap->argv, uap->envv);
	if (error == 0)
		error = kern_execve(td, &args, uap->mac_p);
	post_execve(td, error, oldvmspace);
	return (error);
#else
	return (ENOSYS);
#endif
}

int
pre_execve(struct thread *td, struct vmspace **oldvmspace)
{
	struct proc *p;
	int error;

	KASSERT(td == curthread, ("non-current thread %p", td));
	error = 0;
	p = td->td_proc;
	if ((p->p_flag & P_HADTHREADS) != 0) {
		PROC_LOCK(p);
		if (thread_single(p, SINGLE_BOUNDARY) != 0)
			error = ERESTART;
		PROC_UNLOCK(p);
	}
	KASSERT(error != 0 || (td->td_pflags & TDP_EXECVMSPC) == 0,
	    ("nested execve"));
	*oldvmspace = p->p_vmspace;
	return (error);
}

void
post_execve(struct thread *td, int error, struct vmspace *oldvmspace)
{
	struct proc *p;

	KASSERT(td == curthread, ("non-current thread %p", td));
	p = td->td_proc;
	if ((p->p_flag & P_HADTHREADS) != 0) {
		PROC_LOCK(p);
		/*
		 * If success, we upgrade to SINGLE_EXIT state to
		 * force other threads to suicide.
		 */
		if (error == 0)
			thread_single(p, SINGLE_EXIT);
		else
			thread_single_end(p, SINGLE_BOUNDARY);
		PROC_UNLOCK(p);
	}
	if ((td->td_pflags & TDP_EXECVMSPC) != 0) {
		KASSERT(p->p_vmspace != oldvmspace,
		    ("oldvmspace still used"));
		vmspace_free(oldvmspace);
		td->td_pflags &= ~TDP_EXECVMSPC;
	}
}

/*
 * XXX: kern_execve has the astonishing property of not always returning to
 * the caller.  If sufficiently bad things happen during the call to
 * do_execve(), it can end up calling exit1(); as a result, callers must
 * avoid doing anything which they might need to undo (e.g., allocating
 * memory).
 */
int
kern_execve(struct thread *td, struct image_args *args, struct mac *mac_p)
{

	AUDIT_ARG_ARGV(args->begin_argv, args->argc,
	    args->begin_envv - args->begin_argv);
	AUDIT_ARG_ENVV(args->begin_envv, args->envc,
	    args->endp - args->begin_envv);
	return (do_execve(td, args, mac_p));
}

/*
 * In-kernel implementation of execve().  All arguments are assumed to be
 * userspace pointers from the passed thread.
 */
static int
do_execve(td, args, mac_p)
	struct thread *td;
	struct image_args *args;
	struct mac *mac_p;
{
	struct proc *p = td->td_proc;
	struct nameidata nd;
	struct ucred *oldcred;
	struct uidinfo *euip = NULL;
	register_t *stack_base;
	int error, i;
	struct image_params image_params, *imgp;
	struct vattr attr;
	int (*img_first)(struct image_params *);
	struct pargs *oldargs = NULL, *newargs = NULL;
	struct sigacts *oldsigacts = NULL, *newsigacts = NULL;
#ifdef KTRACE
	struct vnode *tracevp = NULL;
	struct ucred *tracecred = NULL;
#endif
	struct vnode *oldtextvp = NULL, *newtextvp;
	cap_rights_t rights;
	int credential_changing;
	int textset;
#ifdef MAC
	struct label *interpvplabel = NULL;
	int will_transition;
#endif
#ifdef HWPMC_HOOKS
	struct pmckern_procexec pe;
#endif
	static const char fexecv_proc_title[] = "(fexecv)";

	imgp = &image_params;

	/*
	 * Lock the process and set the P_INEXEC flag to indicate that
	 * it should be left alone until we're done here.  This is
	 * necessary to avoid race conditions - e.g. in ptrace() -
	 * that might allow a local user to illicitly obtain elevated
	 * privileges.
	 */
	PROC_LOCK(p);
	KASSERT((p->p_flag & P_INEXEC) == 0,
	    ("%s(): process already has P_INEXEC flag", __func__));
	p->p_flag |= P_INEXEC;
	PROC_UNLOCK(p);

	/*
	 * Initialize part of the common data
	 */
	bzero(imgp, sizeof(*imgp));
	imgp->proc = p;
	imgp->attr = &attr;
	imgp->args = args;
	oldcred = p->p_ucred;

#ifdef MAC
	error = mac_execve_enter(imgp, mac_p);
	if (error)
		goto exec_fail;
#endif

	/*
	 * Translate the file name. namei() returns a vnode pointer
	 *	in ni_vp among other things.
	 *
	 * XXXAUDIT: It would be desirable to also audit the name of the
	 * interpreter if this is an interpreted binary.
	 */
	if (args->fname != NULL) {
		NDINIT(&nd, LOOKUP, ISOPEN | LOCKLEAF | FOLLOW | SAVENAME
		    | AUDITVNODE1, UIO_SYSSPACE, args->fname, td);
	}

	SDT_PROBE1(proc, , , exec, args->fname);

interpret:
	if (args->fname != NULL) {
#ifdef CAPABILITY_MODE
		/*
		 * While capability mode can't reach this point via direct
		 * path arguments to execve(), we also don't allow
		 * interpreters to be used in capability mode (for now).
		 * Catch indirect lookups and return a permissions error.
		 */
		if (IN_CAPABILITY_MODE(td)) {
			error = ECAPMODE;
			goto exec_fail;
		}
#endif
		error = namei(&nd);
		if (error)
			goto exec_fail;

		newtextvp = nd.ni_vp;
		imgp->vp = newtextvp;
	} else {
		AUDIT_ARG_FD(args->fd);
		/*
		 * Descriptors opened only with O_EXEC or O_RDONLY are allowed.
		 */
		error = fgetvp_exec(td, args->fd,
		    cap_rights_init(&rights, CAP_FEXECVE), &newtextvp);
		if (error)
			goto exec_fail;
		vn_lock(newtextvp, LK_EXCLUSIVE | LK_RETRY);
		AUDIT_ARG_VNODE1(newtextvp);
		imgp->vp = newtextvp;
	}

	/*
	 * Check file permissions (also 'opens' file)
	 */
	error = exec_check_permissions(imgp);
	if (error)
		goto exec_fail_dealloc;

	imgp->object = imgp->vp->v_object;
	if (imgp->object != NULL)
		vm_object_reference(imgp->object);

	/*
	 * Set VV_TEXT now so no one can write to the executable while we're
	 * activating it.
	 *
	 * Remember if this was set before and unset it in case this is not
	 * actually an executable image.
	 */
	textset = VOP_IS_TEXT(imgp->vp);
	VOP_SET_TEXT(imgp->vp);

	error = exec_map_first_page(imgp);
	if (error)
		goto exec_fail_dealloc;

	imgp->proc->p_osrel = 0;

	/*
	 * Implement image setuid/setgid.
	 *
	 * Determine new credentials before attempting image activators
	 * so that it can be used by process_exec handlers to determine
	 * credential/setid changes.
	 *
	 * Don't honor setuid/setgid if the filesystem prohibits it or if
	 * the process is being traced.
	 *
	 * We disable setuid/setgid/etc in capability mode on the basis
	 * that most setugid applications are not written with that
	 * environment in mind, and will therefore almost certainly operate
	 * incorrectly. In principle there's no reason that setugid
	 * applications might not be useful in capability mode, so we may want
	 * to reconsider this conservative design choice in the future.
	 *
	 * XXXMAC: For the time being, use NOSUID to also prohibit
	 * transitions on the file system.
	 */
	credential_changing = 0;
	credential_changing |= (attr.va_mode & S_ISUID) &&
	    oldcred->cr_uid != attr.va_uid;
	credential_changing |= (attr.va_mode & S_ISGID) &&
	    oldcred->cr_gid != attr.va_gid;
#ifdef MAC
	will_transition = mac_vnode_execve_will_transition(oldcred, imgp->vp,
	    interpvplabel, imgp);
	credential_changing |= will_transition;
#endif

	if (credential_changing &&
#ifdef CAPABILITY_MODE
	    ((oldcred->cr_flags & CRED_FLAG_CAPMODE) == 0) &&
#endif
	    (imgp->vp->v_mount->mnt_flag & MNT_NOSUID) == 0 &&
	    (p->p_flag & P_TRACED) == 0) {
		imgp->credential_setid = true;
		VOP_UNLOCK(imgp->vp, 0);
		imgp->newcred = crdup(oldcred);
		if (attr.va_mode & S_ISUID) {
			euip = uifind(attr.va_uid);
			change_euid(imgp->newcred, euip);
		}
		vn_lock(imgp->vp, LK_EXCLUSIVE | LK_RETRY);
		if (attr.va_mode & S_ISGID)
			change_egid(imgp->newcred, attr.va_gid);
		/*
		 * Implement correct POSIX saved-id behavior.
		 *
		 * XXXMAC: Note that the current logic will save the
		 * uid and gid if a MAC domain transition occurs, even
		 * though maybe it shouldn't.
		 */
		change_svuid(imgp->newcred, imgp->newcred->cr_uid);
		change_svgid(imgp->newcred, imgp->newcred->cr_gid);
	} else {
		/*
		 * Implement correct POSIX saved-id behavior.
		 *
		 * XXX: It's not clear that the existing behavior is
		 * POSIX-compliant.  A number of sources indicate that the
		 * saved uid/gid should only be updated if the new ruid is
		 * not equal to the old ruid, or the new euid is not equal
		 * to the old euid and the new euid is not equal to the old
		 * ruid.  The FreeBSD code always updates the saved uid/gid.
		 * Also, this code uses the new (replaced) euid and egid as
		 * the source, which may or may not be the right ones to use.
		 */
		if (oldcred->cr_svuid != oldcred->cr_uid ||
		    oldcred->cr_svgid != oldcred->cr_gid) {
			VOP_UNLOCK(imgp->vp, 0);
			imgp->newcred = crdup(oldcred);
			vn_lock(imgp->vp, LK_EXCLUSIVE | LK_RETRY);
			change_svuid(imgp->newcred, imgp->newcred->cr_uid);
			change_svgid(imgp->newcred, imgp->newcred->cr_gid);
		}
	}
	/* The new credentials are installed into the process later. */

	/*
	 * Do the best to calculate the full path to the image file.
	 */
	if (args->fname != NULL && args->fname[0] == '/')
		imgp->execpath = args->fname;
	else {
		VOP_UNLOCK(imgp->vp, 0);
		if (vn_fullpath(td, imgp->vp, &imgp->execpath,
		    &imgp->freepath) != 0)
			imgp->execpath = args->fname;
		vn_lock(imgp->vp, LK_EXCLUSIVE | LK_RETRY);
	}

	/*
	 *	If the current process has a special image activator it
	 *	wants to try first, call it.   For example, emulating shell
	 *	scripts differently.
	 */
	error = -1;
	if ((img_first = imgp->proc->p_sysent->sv_imgact_try) != NULL)
		error = img_first(imgp);

	/*
	 *	Loop through the list of image activators, calling each one.
	 *	An activator returns -1 if there is no match, 0 on success,
	 *	and an error otherwise.
	 */
	for (i = 0; error == -1 && execsw[i]; ++i) {
		if (execsw[i]->ex_imgact == NULL ||
		    execsw[i]->ex_imgact == img_first) {
			continue;
		}
		error = (*execsw[i]->ex_imgact)(imgp);
	}

	if (error) {
		if (error == -1) {
			if (textset == 0)
				VOP_UNSET_TEXT(imgp->vp);
			error = ENOEXEC;
		}
		goto exec_fail_dealloc;
	}

	/*
	 * Special interpreter operation, cleanup and loop up to try to
	 * activate the interpreter.
	 */
	if (imgp->interpreted) {
		exec_unmap_first_page(imgp);
		/*
		 * VV_TEXT needs to be unset for scripts.  There is a short
		 * period before we determine that something is a script where
		 * VV_TEXT will be set. The vnode lock is held over this
		 * entire period so nothing should illegitimately be blocked.
		 */
		VOP_UNSET_TEXT(imgp->vp);
		/* free name buffer and old vnode */
		if (args->fname != NULL)
			NDFREE(&nd, NDF_ONLY_PNBUF);
#ifdef MAC
		mac_execve_interpreter_enter(newtextvp, &interpvplabel);
#endif
		if (imgp->opened) {
			VOP_CLOSE(newtextvp, FREAD, td->td_ucred, td);
			imgp->opened = 0;
		}
		vput(newtextvp);
		vm_object_deallocate(imgp->object);
		imgp->object = NULL;
		imgp->credential_setid = false;
		if (imgp->newcred != NULL) {
			crfree(imgp->newcred);
			imgp->newcred = NULL;
		}
		imgp->execpath = NULL;
		free(imgp->freepath, M_TEMP);
		imgp->freepath = NULL;
		/* set new name to that of the interpreter */
		NDINIT(&nd, LOOKUP, LOCKLEAF | FOLLOW | SAVENAME,
		    UIO_SYSSPACE, imgp->interpreter_name, td);
		args->fname = imgp->interpreter_name;
		goto interpret;
	}

	/*
	 * NB: We unlock the vnode here because it is believed that none
	 * of the sv_copyout_strings/sv_fixup operations require the vnode.
	 */
	VOP_UNLOCK(imgp->vp, 0);

	if (disallow_high_osrel &&
	    P_OSREL_MAJOR(p->p_osrel) > P_OSREL_MAJOR(__FreeBSD_version)) {
		error = ENOEXEC;
		uprintf("Osrel %d for image %s too high\n", p->p_osrel,
		    imgp->execpath != NULL ? imgp->execpath : "<unresolved>");
		vn_lock(imgp->vp, LK_SHARED | LK_RETRY);
		goto exec_fail_dealloc;
	}

	/* ABI enforces the use of Capsicum. Switch into capabilities mode. */
	if (SV_PROC_FLAG(p, SV_CAPSICUM))
		sys_cap_enter(td, NULL);

	/*
	 * Copy out strings (args and env) and initialize stack base
	 */
	if (p->p_sysent->sv_copyout_strings)
		stack_base = (*p->p_sysent->sv_copyout_strings)(imgp);
	else
		stack_base = exec_copyout_strings(imgp);

	/*
	 * If custom stack fixup routine present for this process
	 * let it do the stack setup.
	 * Else stuff argument count as first item on stack
	 */
	if (p->p_sysent->sv_fixup != NULL)
		(*p->p_sysent->sv_fixup)(&stack_base, imgp);
	else
		suword(--stack_base, imgp->args->argc);

	if (args->fdp != NULL) {
		/* Install a brand new file descriptor table. */
		fdinstall_remapped(td, args->fdp);
		args->fdp = NULL;
	} else {
		/*
		 * Keep on using the existing file descriptor table. For
		 * security and other reasons, the file descriptor table
		 * cannot be shared after an exec.
		 */
		fdunshare(td);
		/* close files on exec */
		fdcloseexec(td);
	}

	/*
	 * Malloc things before we need locks.
	 */
	i = imgp->args->begin_envv - imgp->args->begin_argv;
	/* Cache arguments if they fit inside our allowance */
	if (ps_arg_cache_limit >= i + sizeof(struct pargs)) {
		newargs = pargs_alloc(i);
		bcopy(imgp->args->begin_argv, newargs->ar_args, i);
	}

	/*
	 * For security and other reasons, signal handlers cannot
	 * be shared after an exec. The new process gets a copy of the old
	 * handlers. In execsigs(), the new process will have its signals
	 * reset.
	 */
	if (sigacts_shared(p->p_sigacts)) {
		oldsigacts = p->p_sigacts;
		newsigacts = sigacts_alloc();
		sigacts_copy(newsigacts, oldsigacts);
	}

	vn_lock(imgp->vp, LK_SHARED | LK_RETRY);

	PROC_LOCK(p);
	if (oldsigacts)
		p->p_sigacts = newsigacts;
	/* Stop profiling */
	stopprofclock(p);

	/* reset caught signals */
	execsigs(p);

	/* name this process - nameiexec(p, ndp) */
	bzero(p->p_comm, sizeof(p->p_comm));
	if (args->fname)
		bcopy(nd.ni_cnd.cn_nameptr, p->p_comm,
		    min(nd.ni_cnd.cn_namelen, MAXCOMLEN));
	else if (vn_commname(newtextvp, p->p_comm, sizeof(p->p_comm)) != 0)
		bcopy(fexecv_proc_title, p->p_comm, sizeof(fexecv_proc_title));
	bcopy(p->p_comm, td->td_name, sizeof(td->td_name));
#ifdef KTR
	sched_clear_tdname(td);
#endif

	/*
	 * mark as execed, wakeup the process that vforked (if any) and tell
	 * it that it now has its own resources back
	 */
	p->p_flag |= P_EXEC;
	if ((p->p_flag2 & P2_NOTRACE_EXEC) == 0)
		p->p_flag2 &= ~P2_NOTRACE;
	if (p->p_flag & P_PPWAIT) {
		p->p_flag &= ~(P_PPWAIT | P_PPTRACE);
		cv_broadcast(&p->p_pwait);
	}

	/*
	 * Implement image setuid/setgid installation.
	 */
	if (imgp->credential_setid) {
		/*
		 * Turn off syscall tracing for set-id programs, except for
		 * root.  Record any set-id flags first to make sure that
		 * we do not regain any tracing during a possible block.
		 */
		setsugid(p);

#ifdef KTRACE
		if (p->p_tracecred != NULL &&
		    priv_check_cred(p->p_tracecred, PRIV_DEBUG_DIFFCRED, 0))
			ktrprocexec(p, &tracecred, &tracevp);
#endif
		/*
		 * Close any file descriptors 0..2 that reference procfs,
		 * then make sure file descriptors 0..2 are in use.
		 *
		 * Both fdsetugidsafety() and fdcheckstd() may call functions
		 * taking sleepable locks, so temporarily drop our locks.
		 */
		PROC_UNLOCK(p);
		VOP_UNLOCK(imgp->vp, 0);
		fdsetugidsafety(td);
		error = fdcheckstd(td);
		vn_lock(imgp->vp, LK_SHARED | LK_RETRY);
		if (error != 0)
			goto exec_fail_dealloc;
		PROC_LOCK(p);
#ifdef MAC
		if (will_transition) {
			mac_vnode_execve_transition(oldcred, imgp->newcred,
			    imgp->vp, interpvplabel, imgp);
		}
#endif
	} else {
		if (oldcred->cr_uid == oldcred->cr_ruid &&
		    oldcred->cr_gid == oldcred->cr_rgid)
			p->p_flag &= ~P_SUGID;
	}
	/*
	 * Set the new credentials.
	 */
	if (imgp->newcred != NULL) {
		proc_set_cred(p, imgp->newcred);
		crfree(oldcred);
		oldcred = NULL;
	}

	/*
	 * Store the vp for use in procfs.  This vnode was referenced by namei
	 * or fgetvp_exec.
	 */
	oldtextvp = p->p_textvp;
	p->p_textvp = newtextvp;

#ifdef KDTRACE_HOOKS
	/*
	 * Tell the DTrace fasttrap provider about the exec if it
	 * has declared an interest.
	 */
	if (dtrace_fasttrap_exec)
		dtrace_fasttrap_exec(p);
#endif

	/*
	 * Notify others that we exec'd, and clear the P_INEXEC flag
	 * as we're now a bona fide freshly-execed process.
	 */
	KNOTE_LOCKED(p->p_klist, NOTE_EXEC);
	p->p_flag &= ~P_INEXEC;

	/* clear "fork but no exec" flag, as we _are_ execing */
	p->p_acflag &= ~AFORK;

	/*
	 * Free any previous argument cache and replace it with
	 * the new argument cache, if any.
	 */
	oldargs = p->p_args;
	p->p_args = newargs;
	newargs = NULL;

#ifdef	HWPMC_HOOKS
	/*
	 * Check if system-wide sampling is in effect or if the
	 * current process is using PMCs.  If so, do exec() time
	 * processing.  This processing needs to happen AFTER the
	 * P_INEXEC flag is cleared.
	 *
	 * The proc lock needs to be released before taking the PMC
	 * SX.
	 */
	if (PMC_SYSTEM_SAMPLING_ACTIVE() || PMC_PROC_IS_USING_PMCS(p)) {
		PROC_UNLOCK(p);
		VOP_UNLOCK(imgp->vp, 0);
		pe.pm_credentialschanged = credential_changing;
		pe.pm_entryaddr = imgp->entry_addr;

		PMC_CALL_HOOK_X(td, PMC_FN_PROCESS_EXEC, (void *) &pe);
		vn_lock(imgp->vp, LK_SHARED | LK_RETRY);
	} else
		PROC_UNLOCK(p);
#else  /* !HWPMC_HOOKS */
	PROC_UNLOCK(p);
#endif

	/* Set values passed into the program in registers. */
	if (p->p_sysent->sv_setregs)
		(*p->p_sysent->sv_setregs)(td, imgp, 
		    (u_long)(uintptr_t)stack_base);
	else
		exec_setregs(td, imgp, (u_long)(uintptr_t)stack_base);

	vfs_mark_atime(imgp->vp, td->td_ucred);

	SDT_PROBE1(proc, , , exec__success, args->fname);

exec_fail_dealloc:
	if (imgp->firstpage != NULL)
		exec_unmap_first_page(imgp);

	if (imgp->vp != NULL) {
		if (args->fname)
			NDFREE(&nd, NDF_ONLY_PNBUF);
		if (imgp->opened)
			VOP_CLOSE(imgp->vp, FREAD, td->td_ucred, td);
		if (error != 0)
			vput(imgp->vp);
		else
			VOP_UNLOCK(imgp->vp, 0);
	}

	if (imgp->object != NULL)
		vm_object_deallocate(imgp->object);

	free(imgp->freepath, M_TEMP);

	if (error == 0) {
		PROC_LOCK(p);
		if (p->p_ptevents & PTRACE_EXEC)
			td->td_dbgflags |= TDB_EXEC;
		PROC_UNLOCK(p);

		/*
		 * Stop the process here if its stop event mask has
		 * the S_EXEC bit set.
		 */
		STOPEVENT(p, S_EXEC, 0);
	} else {
exec_fail:
		/* we're done here, clear P_INEXEC */
		PROC_LOCK(p);
		p->p_flag &= ~P_INEXEC;
		PROC_UNLOCK(p);

		SDT_PROBE1(proc, , , exec__failure, error);
	}

	if (imgp->newcred != NULL && oldcred != NULL)
		crfree(imgp->newcred);

#ifdef MAC
	mac_execve_exit(imgp);
	mac_execve_interpreter_exit(interpvplabel);
#endif
	exec_free_args(args);

	/*
	 * Handle deferred decrement of ref counts.
	 */
	if (oldtextvp != NULL)
		vrele(oldtextvp);
#ifdef KTRACE
	if (tracevp != NULL)
		vrele(tracevp);
	if (tracecred != NULL)
		crfree(tracecred);
#endif
	pargs_drop(oldargs);
	pargs_drop(newargs);
	if (oldsigacts != NULL)
		sigacts_free(oldsigacts);
	if (euip != NULL)
		uifree(euip);

	if (error && imgp->vmspace_destroyed) {
		/* sorry, no more process anymore. exit gracefully */
		exit1(td, 0, SIGABRT);
		/* NOT REACHED */
	}

#ifdef KTRACE
	if (error == 0)
		ktrprocctor(p);
#endif

	return (error);
}

int
exec_map_first_page(imgp)
	struct image_params *imgp;
{
	int rv, i, after, initial_pagein;
	vm_page_t ma[VM_INITIAL_PAGEIN];
	vm_object_t object;

	if (imgp->firstpage != NULL)
		exec_unmap_first_page(imgp);

	object = imgp->vp->v_object;
	if (object == NULL)
		return (EACCES);
	VM_OBJECT_WLOCK(object);
#if VM_NRESERVLEVEL > 0
	vm_object_color(object, 0);
#endif
	ma[0] = vm_page_grab(object, 0, VM_ALLOC_NORMAL);
	if (ma[0]->valid != VM_PAGE_BITS_ALL) {
		if (!vm_pager_has_page(object, 0, NULL, &after)) {
			vm_page_lock(ma[0]);
			vm_page_free(ma[0]);
			vm_page_unlock(ma[0]);
			vm_page_xunbusy(ma[0]);
			VM_OBJECT_WUNLOCK(object);
			return (EIO);
		}
		initial_pagein = min(after, VM_INITIAL_PAGEIN);
		KASSERT(initial_pagein <= object->size,
		    ("%s: initial_pagein %d object->size %ju",
		    __func__, initial_pagein, (uintmax_t )object->size));
		for (i = 1; i < initial_pagein; i++) {
			if ((ma[i] = vm_page_next(ma[i - 1])) != NULL) {
				if (ma[i]->valid)
					break;
				if (vm_page_tryxbusy(ma[i]))
					break;
			} else {
				ma[i] = vm_page_alloc(object, i,
				    VM_ALLOC_NORMAL | VM_ALLOC_IFNOTCACHED);
				if (ma[i] == NULL)
					break;
			}
		}
		initial_pagein = i;
		rv = vm_pager_get_pages(object, ma, initial_pagein, NULL, NULL);
		if (rv != VM_PAGER_OK) {
			for (i = 0; i < initial_pagein; i++) {
				vm_page_lock(ma[i]);
				vm_page_free(ma[i]);
				vm_page_unlock(ma[i]);
				vm_page_xunbusy(ma[i]);
			}
			VM_OBJECT_WUNLOCK(object);
			return (EIO);
		}
		for (i = 1; i < initial_pagein; i++)
			vm_page_readahead_finish(ma[i]);
	}
	vm_page_xunbusy(ma[0]);
	vm_page_lock(ma[0]);
	vm_page_hold(ma[0]);
	vm_page_activate(ma[0]);
	vm_page_unlock(ma[0]);
	VM_OBJECT_WUNLOCK(object);

	imgp->firstpage = sf_buf_alloc(ma[0], 0);
	imgp->image_header = (char *)sf_buf_kva(imgp->firstpage);

	return (0);
}

void
exec_unmap_first_page(imgp)
	struct image_params *imgp;
{
	vm_page_t m;

	if (imgp->firstpage != NULL) {
		m = sf_buf_page(imgp->firstpage);
		sf_buf_free(imgp->firstpage);
		imgp->firstpage = NULL;
		vm_page_lock(m);
		vm_page_unhold(m);
		vm_page_unlock(m);
	}
}

/*
 * Destroy old address space, and allocate a new stack
 *	The new stack is only SGROWSIZ large because it is grown
 *	automatically in trap.c.
 */
int
exec_new_vmspace(imgp, sv)
	struct image_params *imgp;
	struct sysentvec *sv;
{
	int error;
	struct proc *p = imgp->proc;
	struct vmspace *vmspace = p->p_vmspace;
	vm_object_t obj;
	struct rlimit rlim_stack;
	vm_offset_t sv_minuser, stack_addr;
	vm_map_t map;
	u_long ssiz;

	imgp->vmspace_destroyed = 1;
	imgp->sysent = sv;

	/* May be called with Giant held */
	EVENTHANDLER_INVOKE(process_exec, p, imgp);

	/*
	 * Blow away entire process VM, if address space not shared,
	 * otherwise, create a new VM space so that other threads are
	 * not disrupted
	 */
	map = &vmspace->vm_map;
	if (map_at_zero)
		sv_minuser = sv->sv_minuser;
	else
		sv_minuser = MAX(sv->sv_minuser, PAGE_SIZE);
	if (vmspace->vm_refcnt == 1 && vm_map_min(map) == sv_minuser &&
	    vm_map_max(map) == sv->sv_maxuser) {
		shmexit(vmspace);
		pmap_remove_pages(vmspace_pmap(vmspace));
		vm_map_remove(map, vm_map_min(map), vm_map_max(map));
	} else {
		error = vmspace_exec(p, sv_minuser, sv->sv_maxuser);
		if (error)
			return (error);
		vmspace = p->p_vmspace;
		map = &vmspace->vm_map;
	}

	/* Map a shared page */
	obj = sv->sv_shared_page_obj;
	if (obj != NULL) {
		vm_object_reference(obj);
		error = vm_map_fixed(map, obj, 0,
		    sv->sv_shared_page_base, sv->sv_shared_page_len,
		    VM_PROT_READ | VM_PROT_EXECUTE,
		    VM_PROT_READ | VM_PROT_EXECUTE,
		    MAP_INHERIT_SHARE | MAP_ACC_NO_CHARGE);
		if (error) {
			vm_object_deallocate(obj);
			return (error);
		}
	}

	/* Allocate a new stack */
	if (imgp->stack_sz != 0) {
		ssiz = trunc_page(imgp->stack_sz);
		PROC_LOCK(p);
		lim_rlimit_proc(p, RLIMIT_STACK, &rlim_stack);
		PROC_UNLOCK(p);
		if (ssiz > rlim_stack.rlim_max)
			ssiz = rlim_stack.rlim_max;
		if (ssiz > rlim_stack.rlim_cur) {
			rlim_stack.rlim_cur = ssiz;
			kern_setrlimit(curthread, RLIMIT_STACK, &rlim_stack);
		}
	} else if (sv->sv_maxssiz != NULL) {
		ssiz = *sv->sv_maxssiz;
	} else {
		ssiz = maxssiz;
	}
	stack_addr = sv->sv_usrstack - ssiz;
	error = vm_map_stack(map, stack_addr, (vm_size_t)ssiz,
	    obj != NULL && imgp->stack_prot != 0 ? imgp->stack_prot :
		sv->sv_stackprot,
	    VM_PROT_ALL, MAP_STACK_GROWS_DOWN);
	if (error)
		return (error);

	/*
	 * vm_ssize and vm_maxsaddr are somewhat antiquated concepts, but they
	 * are still used to enforce the stack rlimit on the process stack.
	 */
	vmspace->vm_ssize = sgrowsiz >> PAGE_SHIFT;
	vmspace->vm_maxsaddr = (char *)stack_addr;

	return (0);
}

/*
 * Copy out argument and environment strings from the old process address
 * space into the temporary string buffer.
 */
int
exec_copyin_args(struct image_args *args, char *fname,
    enum uio_seg segflg, char **argv, char **envv)
{
	u_long argp, envp;
	int error;
	size_t length;

	bzero(args, sizeof(*args));
	if (argv == NULL)
		return (EFAULT);

	/*
	 * Allocate demand-paged memory for the file name, argument, and
	 * environment strings.
	 */
	error = exec_alloc_args(args);
	if (error != 0)
		return (error);

	/*
	 * Copy the file name.
	 */
	if (fname != NULL) {
		args->fname = args->buf;
		error = (segflg == UIO_SYSSPACE) ?
		    copystr(fname, args->fname, PATH_MAX, &length) :
		    copyinstr(fname, args->fname, PATH_MAX, &length);
		if (error != 0)
			goto err_exit;
	} else
		length = 0;

	args->begin_argv = args->buf + length;
	args->endp = args->begin_argv;
	args->stringspace = ARG_MAX;

	/*
	 * extract arguments first
	 */
	for (;;) {
		error = fueword(argv++, &argp);
		if (error == -1) {
			error = EFAULT;
			goto err_exit;
		}
		if (argp == 0)
			break;
		error = copyinstr((void *)(uintptr_t)argp, args->endp,
		    args->stringspace, &length);
		if (error != 0) {
			if (error == ENAMETOOLONG) 
				error = E2BIG;
			goto err_exit;
		}
		args->stringspace -= length;
		args->endp += length;
		args->argc++;
	}

	args->begin_envv = args->endp;

	/*
	 * extract environment strings
	 */
	if (envv) {
		for (;;) {
			error = fueword(envv++, &envp);
			if (error == -1) {
				error = EFAULT;
				goto err_exit;
			}
			if (envp == 0)
				break;
			error = copyinstr((void *)(uintptr_t)envp,
			    args->endp, args->stringspace, &length);
			if (error != 0) {
				if (error == ENAMETOOLONG)
					error = E2BIG;
				goto err_exit;
			}
			args->stringspace -= length;
			args->endp += length;
			args->envc++;
		}
	}

	return (0);

err_exit:
	exec_free_args(args);
	return (error);
}

int
exec_copyin_data_fds(struct thread *td, struct image_args *args,
    const void *data, size_t datalen, const int *fds, size_t fdslen)
{
	struct filedesc *ofdp;
	const char *p;
	int *kfds;
	int error;

	memset(args, '\0', sizeof(*args));
	ofdp = td->td_proc->p_fd;
	if (datalen >= ARG_MAX || fdslen > ofdp->fd_lastfile + 1)
		return (E2BIG);
	error = exec_alloc_args(args);
	if (error != 0)
		return (error);

	args->begin_argv = args->buf;
	args->stringspace = ARG_MAX;

	if (datalen > 0) {
		/*
		 * Argument buffer has been provided. Copy it into the
		 * kernel as a single string and add a terminating null
		 * byte.
		 */
		error = copyin(data, args->begin_argv, datalen);
		if (error != 0)
			goto err_exit;
		args->begin_argv[datalen] = '\0';
		args->endp = args->begin_argv + datalen + 1;
		args->stringspace -= datalen + 1;

		/*
		 * Traditional argument counting. Count the number of
		 * null bytes.
		 */
		for (p = args->begin_argv; p < args->endp; ++p)
			if (*p == '\0')
				++args->argc;
	} else {
		/* No argument buffer provided. */
		args->endp = args->begin_argv;
	}
	/* There are no environment variables. */
	args->begin_envv = args->endp;

	/* Create new file descriptor table. */
	kfds = malloc(fdslen * sizeof(int), M_TEMP, M_WAITOK);
	error = copyin(fds, kfds, fdslen * sizeof(int));
	if (error != 0) {
		free(kfds, M_TEMP);
		goto err_exit;
	}
	error = fdcopy_remapped(ofdp, kfds, fdslen, &args->fdp);
	free(kfds, M_TEMP);
	if (error != 0)
		goto err_exit;

	return (0);
err_exit:
	exec_free_args(args);
	return (error);
}

/*
 * Allocate temporary demand-paged, zero-filled memory for the file name,
 * argument, and environment strings.  Returns zero if the allocation succeeds
 * and ENOMEM otherwise.
 */
int
exec_alloc_args(struct image_args *args)
{

	args->buf = (char *)kmap_alloc_wait(exec_map, PATH_MAX + ARG_MAX);
	return (args->buf != NULL ? 0 : ENOMEM);
}

void
exec_free_args(struct image_args *args)
{

	if (args->buf != NULL) {
		kmap_free_wakeup(exec_map, (vm_offset_t)args->buf,
		    PATH_MAX + ARG_MAX);
		args->buf = NULL;
	}
	if (args->fname_buf != NULL) {
		free(args->fname_buf, M_TEMP);
		args->fname_buf = NULL;
	}
	if (args->fdp != NULL)
		fdescfree_remapped(args->fdp);
}

/*
 * Copy strings out to the new process address space, constructing new arg
 * and env vector tables. Return a pointer to the base so that it can be used
 * as the initial stack pointer.
 */
register_t *
exec_copyout_strings(imgp)
	struct image_params *imgp;
{
	int argc, envc;
	char **vectp;
	char *stringp;
	uintptr_t destp;
	register_t *stack_base;
	struct ps_strings *arginfo;
	struct proc *p;
	size_t execpath_len;
	int szsigcode, szps;
	char canary[sizeof(long) * 8];

	szps = sizeof(pagesizes[0]) * MAXPAGESIZES;
	/*
	 * Calculate string base and vector table pointers.
	 * Also deal with signal trampoline code for this exec type.
	 */
	if (imgp->execpath != NULL && imgp->auxargs != NULL)
		execpath_len = strlen(imgp->execpath) + 1;
	else
		execpath_len = 0;
	p = imgp->proc;
	szsigcode = 0;
	arginfo = (struct ps_strings *)p->p_sysent->sv_psstrings;
	if (p->p_sysent->sv_sigcode_base == 0) {
		if (p->p_sysent->sv_szsigcode != NULL)
			szsigcode = *(p->p_sysent->sv_szsigcode);
	}
	destp =	(uintptr_t)arginfo;

	/*
	 * install sigcode
	 */
	if (szsigcode != 0) {
		destp -= szsigcode;
		destp = rounddown2(destp, sizeof(void *));
		copyout(p->p_sysent->sv_sigcode, (void *)destp, szsigcode);
	}

	/*
	 * Copy the image path for the rtld.
	 */
	if (execpath_len != 0) {
		destp -= execpath_len;
		imgp->execpathp = destp;
		copyout(imgp->execpath, (void *)destp, execpath_len);
	}

	/*
	 * Prepare the canary for SSP.
	 */
	arc4rand(canary, sizeof(canary), 0);
	destp -= sizeof(canary);
	imgp->canary = destp;
	copyout(canary, (void *)destp, sizeof(canary));
	imgp->canarylen = sizeof(canary);

	/*
	 * Prepare the pagesizes array.
	 */
	destp -= szps;
	destp = rounddown2(destp, sizeof(void *));
	imgp->pagesizes = destp;
	copyout(pagesizes, (void *)destp, szps);
	imgp->pagesizeslen = szps;

	destp -= ARG_MAX - imgp->args->stringspace;
	destp = rounddown2(destp, sizeof(void *));

	/*
	 * If we have a valid auxargs ptr, prepare some room
	 * on the stack.
	 */
	if (imgp->auxargs) {
		/*
		 * 'AT_COUNT*2' is size for the ELF Auxargs data. This is for
		 * lower compatibility.
		 */
		imgp->auxarg_size = (imgp->auxarg_size) ? imgp->auxarg_size :
		    (AT_COUNT * 2);
		/*
		 * The '+ 2' is for the null pointers at the end of each of
		 * the arg and env vector sets,and imgp->auxarg_size is room
		 * for argument of Runtime loader.
		 */
		vectp = (char **)(destp - (imgp->args->argc +
		    imgp->args->envc + 2 + imgp->auxarg_size)
		    * sizeof(char *));
	} else {
		/*
		 * The '+ 2' is for the null pointers at the end of each of
		 * the arg and env vector sets
		 */
		vectp = (char **)(destp - (imgp->args->argc + imgp->args->envc
		    + 2) * sizeof(char *));
	}

	/*
	 * vectp also becomes our initial stack base
	 */
	stack_base = (register_t *)vectp;

	stringp = imgp->args->begin_argv;
	argc = imgp->args->argc;
	envc = imgp->args->envc;

	/*
	 * Copy out strings - arguments and environment.
	 */
	copyout(stringp, (void *)destp, ARG_MAX - imgp->args->stringspace);

	/*
	 * Fill in "ps_strings" struct for ps, w, etc.
	 */
	suword(&arginfo->ps_argvstr, (long)(intptr_t)vectp);
	suword32(&arginfo->ps_nargvstr, argc);

	/*
	 * Fill in argument portion of vector table.
	 */
	for (; argc > 0; --argc) {
		suword(vectp++, (long)(intptr_t)destp);
		while (*stringp++ != 0)
			destp++;
		destp++;
	}

	/* a null vector table pointer separates the argp's from the envp's */
	suword(vectp++, 0);

	suword(&arginfo->ps_envstr, (long)(intptr_t)vectp);
	suword32(&arginfo->ps_nenvstr, envc);

	/*
	 * Fill in environment portion of vector table.
	 */
	for (; envc > 0; --envc) {
		suword(vectp++, (long)(intptr_t)destp);
		while (*stringp++ != 0)
			destp++;
		destp++;
	}

	/* end of vector table is a null pointer */
	suword(vectp, 0);

	return (stack_base);
}

/*
 * Check permissions of file to execute.
 *	Called with imgp->vp locked.
 *	Return 0 for success or error code on failure.
 */
int
exec_check_permissions(imgp)
	struct image_params *imgp;
{
	struct vnode *vp = imgp->vp;
	struct vattr *attr = imgp->attr;
	struct thread *td;
	int error, writecount;

	td = curthread;

	/* Get file attributes */
	error = VOP_GETATTR(vp, attr, td->td_ucred);
	if (error)
		return (error);

#ifdef MAC
	error = mac_vnode_check_exec(td->td_ucred, imgp->vp, imgp);
	if (error)
		return (error);
#endif

	/*
	 * 1) Check if file execution is disabled for the filesystem that
	 *    this file resides on.
	 * 2) Ensure that at least one execute bit is on. Otherwise, a
	 *    privileged user will always succeed, and we don't want this
	 *    to happen unless the file really is executable.
	 * 3) Ensure that the file is a regular file.
	 */
	if ((vp->v_mount->mnt_flag & MNT_NOEXEC) ||
	    (attr->va_mode & (S_IXUSR | S_IXGRP | S_IXOTH)) == 0 ||
	    (attr->va_type != VREG))
		return (EACCES);

	/*
	 * Zero length files can't be exec'd
	 */
	if (attr->va_size == 0)
		return (ENOEXEC);

	/*
	 *  Check for execute permission to file based on current credentials.
	 */
	error = VOP_ACCESS(vp, VEXEC, td->td_ucred, td);
	if (error)
		return (error);

	/*
	 * Check number of open-for-writes on the file and deny execution
	 * if there are any.
	 */
	error = VOP_GET_WRITECOUNT(vp, &writecount);
	if (error != 0)
		return (error);
	if (writecount != 0)
		return (ETXTBSY);

	/*
	 * Call filesystem specific open routine (which does nothing in the
	 * general case).
	 */
	error = VOP_OPEN(vp, FREAD, td->td_ucred, td, NULL);
	if (error == 0)
		imgp->opened = 1;
	return (error);
}

/*
 * Exec handler registration
 */
int
exec_register(execsw_arg)
	const struct execsw *execsw_arg;
{
	const struct execsw **es, **xs, **newexecsw;
	int count = 2;	/* New slot and trailing NULL */

	if (execsw)
		for (es = execsw; *es; es++)
			count++;
	newexecsw = malloc(count * sizeof(*es), M_TEMP, M_WAITOK);
	xs = newexecsw;
	if (execsw)
		for (es = execsw; *es; es++)
			*xs++ = *es;
	*xs++ = execsw_arg;
	*xs = NULL;
	if (execsw)
		free(execsw, M_TEMP);
	execsw = newexecsw;
	return (0);
}

int
exec_unregister(execsw_arg)
	const struct execsw *execsw_arg;
{
	const struct execsw **es, **xs, **newexecsw;
	int count = 1;

	if (execsw == NULL)
		panic("unregister with no handlers left?\n");

	for (es = execsw; *es; es++) {
		if (*es == execsw_arg)
			break;
	}
	if (*es == NULL)
		return (ENOENT);
	for (es = execsw; *es; es++)
		if (*es != execsw_arg)
			count++;
	newexecsw = malloc(count * sizeof(*es), M_TEMP, M_WAITOK);
	xs = newexecsw;
	for (es = execsw; *es; es++)
		if (*es != execsw_arg)
			*xs++ = *es;
	*xs = NULL;
	if (execsw)
		free(execsw, M_TEMP);
	execsw = newexecsw;
	return (0);
}