xref: /freebsd/sys/kern/kern_exec.c (revision b1fc0ec1a7a49dede256c4d357878fa2ba19cf93)

/*
 * 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.
 *
 * $FreeBSD$
 */

#include <sys/param.h>
#include <sys/systm.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/proc.h>
#include <sys/pioctl.h>
#include <sys/malloc.h>
#include <sys/namei.h>
#include <sys/sysent.h>
#include <sys/shm.h>
#include <sys/sysctl.h>
#include <sys/vnode.h>

#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>

#include <machine/reg.h>

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

static register_t *exec_copyout_strings __P((struct image_params *));

/* XXX This should be vm_size_t. */
static u_long ps_strings = PS_STRINGS;
SYSCTL_ULONG(_kern, KERN_PS_STRINGS, ps_strings, CTLFLAG_RD, &ps_strings, 0, "");

/* XXX This should be vm_size_t. */
static u_long usrstack = USRSTACK;
SYSCTL_ULONG(_kern, KERN_USRSTACK, usrstack, CTLFLAG_RD, &usrstack, 0, "");

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

int ps_argsopen = 1;
SYSCTL_INT(_kern, OID_AUTO, ps_argsopen, CTLFLAG_RW, &ps_argsopen, 0, "");

/*
 * 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

/*
 * execve() system call.
 */
int
execve(p, uap)
	struct proc *p;
	register struct execve_args *uap;
{
	struct nameidata nd, *ndp;
	struct ucred *newcred, *oldcred;
	register_t *stack_base;
	int error, len, i;
	struct image_params image_params, *imgp;
	struct vattr attr;
	int (*img_first) __P((struct image_params *));

	imgp = &image_params;

	/*
	 * Initialize part of the common data
	 */
	imgp->proc = p;
	imgp->uap = uap;
	imgp->attr = &attr;
	imgp->argc = imgp->envc = 0;
	imgp->argv0 = NULL;
	imgp->entry_addr = 0;
	imgp->vmspace_destroyed = 0;
	imgp->interpreted = 0;
	imgp->interpreter_name[0] = '\0';
	imgp->auxargs = NULL;
	imgp->vp = NULL;
	imgp->firstpage = NULL;
	imgp->ps_strings = 0;
	imgp->auxarg_size = 0;

	/*
	 * Allocate temporary demand zeroed space for argument and
	 *	environment strings
	 */
	imgp->stringbase = (char *)kmem_alloc_wait(exec_map, ARG_MAX + PAGE_SIZE);
	if (imgp->stringbase == NULL) {
		error = ENOMEM;
		goto exec_fail;
	}
	imgp->stringp = imgp->stringbase;
	imgp->stringspace = ARG_MAX;
	imgp->image_header = imgp->stringbase + ARG_MAX;

	/*
	 * Translate the file name. namei() returns a vnode pointer
	 *	in ni_vp amoung other things.
	 */
	ndp = &nd;
	NDINIT(ndp, LOOKUP, LOCKLEAF | FOLLOW | SAVENAME,
	    UIO_USERSPACE, uap->fname, p);

interpret:

	error = namei(ndp);
	if (error) {
		kmem_free_wakeup(exec_map, (vm_offset_t)imgp->stringbase,
			ARG_MAX + PAGE_SIZE);
		goto exec_fail;
	}

	imgp->vp = ndp->ni_vp;
	imgp->fname = uap->fname;

	/*
	 * Check file permissions (also 'opens' file)
	 */
	error = exec_check_permissions(imgp);
	if (error) {
		VOP_UNLOCK(imgp->vp, 0, p);
		goto exec_fail_dealloc;
	}

	error = exec_map_first_page(imgp);
	VOP_UNLOCK(imgp->vp, 0, p);
	if (error)
		goto exec_fail_dealloc;

	/*
	 *	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)
			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);
		/* free name buffer and old vnode */
		NDFREE(ndp, NDF_ONLY_PNBUF);
		vrele(ndp->ni_vp);
		/* set new name to that of the interpreter */
		NDINIT(ndp, LOOKUP, LOCKLEAF | FOLLOW | SAVENAME,
		    UIO_SYSSPACE, imgp->interpreter_name, p);
		goto interpret;
	}

	/*
	 * Copy out strings (args and env) and initialize stack base
	 */
	stack_base = exec_copyout_strings(imgp);
	p->p_vmspace->vm_minsaddr = (char *)stack_base;

	/*
	 * 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)
		(*p->p_sysent->sv_fixup)(&stack_base, imgp);
	else
		suword(--stack_base, imgp->argc);

	/*
	 * For security and other reasons, the file descriptor table cannot
	 * be shared after an exec.
	 */
	if (p->p_fd->fd_refcnt > 1) {
		struct filedesc *tmp;

		tmp = fdcopy(p);
		fdfree(p);
		p->p_fd = tmp;
	}

	/* Stop profiling */
	stopprofclock(p);

	/* close files on exec */
	fdcloseexec(p);

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

	/* name this process - nameiexec(p, ndp) */
	len = min(ndp->ni_cnd.cn_namelen,MAXCOMLEN);
	bcopy(ndp->ni_cnd.cn_nameptr, p->p_comm, len);
	p->p_comm[len] = 0;

	/*
	 * mark as execed, wakeup the process that vforked (if any) and tell
	 * it that it now has its own resources back
	 */
	PROC_LOCK(p);
	p->p_flag |= P_EXEC;
	if (p->p_pptr && (p->p_flag & P_PPWAIT)) {
		p->p_flag &= ~P_PPWAIT;
		wakeup((caddr_t)p->p_pptr);
	}

	/*
	 * XXX: Note, the whole execve() is incredibly racey right now
	 * with regards to debugging and privilege/credential management.
	 * In particular, it's possible to race during exec() to attach
	 * debugging to a process that will gain privilege.
	 *
	 * This MUST be fixed prior to any release.
	 */

	/*
	 * Implement image setuid/setgid.
	 *
	 * Don't honor setuid/setgid if the filesystem prohibits it or if
	 * the process is being traced.
	 */
	oldcred = p->p_ucred;
	newcred = NULL;
	if ((((attr.va_mode & VSUID) && oldcred->cr_uid != attr.va_uid) ||
	     ((attr.va_mode & VSGID) && oldcred->cr_gid != attr.va_gid)) &&
	    (imgp->vp->v_mount->mnt_flag & MNT_NOSUID) == 0 &&
	    (p->p_flag & P_TRACED) == 0) {
		PROC_UNLOCK(p);
		/*
		 * Turn off syscall tracing for set-id programs, except for
		 * root.
		 */
		if (p->p_tracep && suser_xxx(oldcred, NULL, PRISON_ROOT)) {
			p->p_traceflag = 0;
			vrele(p->p_tracep);
			p->p_tracep = NULL;
		}
		/*
		 * Set the new credentials.
		 */
		newcred = crdup(oldcred);
		if (attr.va_mode & VSUID)
			change_euid(newcred, attr.va_uid);
		if (attr.va_mode & VSGID)
			change_egid(newcred, attr.va_gid);
		setsugid(p);
		setugidsafety(p);
	} else {
		if (oldcred->cr_uid == oldcred->cr_ruid &&
		    oldcred->cr_gid == oldcred->cr_rgid)
			p->p_flag &= ~P_SUGID;
		PROC_UNLOCK(p);
	}

	/*
	 * Implement correct POSIX saved-id behavior.
	 *
	 * XXX: It's not clear that the existing behavior is
	 * POSIX-compliant.  A number of sourses 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) {
		/*
		 * Avoid allocating a newcred if we don't have one yet and
		 * the saved uid/gid update would be a noop.
		 */
		if (newcred == NULL)
			newcred = crdup(oldcred);
		change_svuid(newcred, newcred->cr_uid);
		change_svgid(newcred, newcred->cr_gid);
	}
	if (newcred != NULL) {
		PROC_LOCK(p);
		p->p_ucred = newcred;
		PROC_UNLOCK(p);
		crfree(oldcred);
	}

	/*
	 * Store the vp for use in procfs
	 */
	if (p->p_textvp)		/* release old reference */
		vrele(p->p_textvp);
	VREF(ndp->ni_vp);
	p->p_textvp = ndp->ni_vp;

	/*
	 * notify others that we exec'd
	 */
	PROC_LOCK(p);
	KNOTE(&p->p_klist, NOTE_EXEC);

	/*
	 * If tracing the process, trap to debugger so breakpoints
	 * 	can be set before the program executes.
	 */
	_STOPEVENT(p, S_EXEC, 0);

	if (p->p_flag & P_TRACED)
		psignal(p, SIGTRAP);

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

	/* Set values passed into the program in registers. */
	setregs(p, imgp->entry_addr, (u_long)(uintptr_t)stack_base,
	    imgp->ps_strings);

	/* Free any previous argument cache */
	if (p->p_args && --p->p_args->ar_ref == 0)
		FREE(p->p_args, M_PARGS);
	p->p_args = NULL;

	/* Cache arguments if they fit inside our allowance */
	i = imgp->endargs - imgp->stringbase;
	if (ps_arg_cache_limit >= i + sizeof(struct pargs)) {
		PROC_UNLOCK(p);
		MALLOC(p->p_args, struct pargs *, sizeof(struct pargs) + i,
		    M_PARGS, M_WAITOK);
		KASSERT(p->p_args != NULL, ("malloc of p_args failed"));
		PROC_LOCK(p);
		p->p_args->ar_ref = 1;
		p->p_args->ar_length = i;
		bcopy(imgp->stringbase, p->p_args->ar_args, i);
	}
	PROC_UNLOCK(p);

exec_fail_dealloc:

	/*
	 * free various allocated resources
	 */
	if (imgp->firstpage)
		exec_unmap_first_page(imgp);

	if (imgp->stringbase != NULL)
		kmem_free_wakeup(exec_map, (vm_offset_t)imgp->stringbase,
			ARG_MAX + PAGE_SIZE);

	if (imgp->vp) {
		NDFREE(ndp, NDF_ONLY_PNBUF);
		vrele(imgp->vp);
	}

	if (error == 0)
		return (0);

exec_fail:
	if (imgp->vmspace_destroyed) {
		/* sorry, no more process anymore. exit gracefully */
		exit1(p, W_EXITCODE(0, SIGABRT));
		/* NOT REACHED */
		return(0);
	} else {
		return(error);
	}
}

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


	if (imgp->firstpage) {
		exec_unmap_first_page(imgp);
	}

	VOP_GETVOBJECT(imgp->vp, &object);
	mtx_lock(&vm_mtx);

	ma[0] = vm_page_grab(object, 0, VM_ALLOC_NORMAL | VM_ALLOC_RETRY);

	if ((ma[0]->valid & VM_PAGE_BITS_ALL) != VM_PAGE_BITS_ALL) {
		initial_pagein = VM_INITIAL_PAGEIN;
		if (initial_pagein > object->size)
			initial_pagein = object->size;
		for (i = 1; i < initial_pagein; i++) {
			if ((ma[i] = vm_page_lookup(object, i)) != NULL) {
				if ((ma[i]->flags & PG_BUSY) || ma[i]->busy)
					break;
				if (ma[i]->valid)
					break;
				vm_page_busy(ma[i]);
			} else {
				ma[i] = vm_page_alloc(object, i, VM_ALLOC_NORMAL);
				if (ma[i] == NULL)
					break;
			}
		}
		initial_pagein = i;

		rv = vm_pager_get_pages(object, ma, initial_pagein, 0);
		ma[0] = vm_page_lookup(object, 0);

		if ((rv != VM_PAGER_OK) || (ma[0] == NULL) || (ma[0]->valid == 0)) {
			if (ma[0]) {
				vm_page_protect(ma[0], VM_PROT_NONE);
				vm_page_free(ma[0]);
			}
			mtx_unlock(&vm_mtx);
			return EIO;
		}
	}

	vm_page_wire(ma[0]);
	vm_page_wakeup(ma[0]);

	pmap_kenter((vm_offset_t) imgp->image_header, VM_PAGE_TO_PHYS(ma[0]));
	imgp->firstpage = ma[0];

	mtx_unlock(&vm_mtx);
	return 0;
}

void
exec_unmap_first_page(imgp)
	struct image_params *imgp;
{

	if (imgp->firstpage) {
		mtx_lock(&vm_mtx);
		pmap_kremove((vm_offset_t) imgp->image_header);
		vm_page_unwire(imgp->firstpage, 1);
		mtx_unlock(&vm_mtx);
		imgp->firstpage = NULL;
	}
}

/*
 * 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)
	struct image_params *imgp;
{
	int error;
	struct vmspace *vmspace = imgp->proc->p_vmspace;
	caddr_t	stack_addr = (caddr_t) (USRSTACK - MAXSSIZ);
	vm_map_t map = &vmspace->vm_map;

	mtx_assert(&vm_mtx, MA_OWNED);
	imgp->vmspace_destroyed = 1;

	/*
	 * Blow away entire process VM, if address space not shared,
	 * otherwise, create a new VM space so that other threads are
	 * not disrupted
	 */
	if (vmspace->vm_refcnt == 1) {
		if (vmspace->vm_shm)
			shmexit(imgp->proc);
		pmap_remove_pages(vmspace_pmap(vmspace), 0, VM_MAXUSER_ADDRESS);
		vm_map_remove(map, 0, VM_MAXUSER_ADDRESS);
	} else {
		vmspace_exec(imgp->proc);
		vmspace = imgp->proc->p_vmspace;
		map = &vmspace->vm_map;
	}

	/* Allocate a new stack */
	error = vm_map_stack (&vmspace->vm_map, (vm_offset_t)stack_addr,
			      (vm_size_t)MAXSSIZ, VM_PROT_ALL, VM_PROT_ALL, 0);
	if (error)
		return (error);

#ifdef __ia64__
	{
		/*
		 * Allocate backing store. We really need something
		 * similar to vm_map_stack which can allow the backing
		 * store to grow upwards. This will do for now.
		 */
		vm_offset_t bsaddr;
		bsaddr = USRSTACK - 2*MAXSSIZ;
		error = vm_map_find(&vmspace->vm_map, 0, 0, &bsaddr,
				    4*PAGE_SIZE, 0,
				    VM_PROT_ALL, VM_PROT_ALL, 0);
		imgp->proc->p_md.md_bspstore = bsaddr;
	}
#endif

	/* vm_ssize and vm_maxsaddr are somewhat antiquated concepts in the
	 * VM_STACK case, but they are still used to monitor the size of the
	 * process stack so we can check the stack rlimit.
	 */
	vmspace->vm_ssize = SGROWSIZ >> PAGE_SHIFT;
	vmspace->vm_maxsaddr = (char *)USRSTACK - MAXSSIZ;

	return(0);
}

/*
 * Copy out argument and environment strings from the old process
 *	address space into the temporary string buffer.
 */
int
exec_extract_strings(imgp)
	struct image_params *imgp;
{
	char	**argv, **envv;
	char	*argp, *envp;
	int	error;
	size_t	length;

	/*
	 * extract arguments first
	 */

	argv = imgp->uap->argv;

	if (argv) {
		argp = (caddr_t) (intptr_t) fuword(argv);
		if (argp == (caddr_t) -1)
			return (EFAULT);
		if (argp)
			argv++;
		if (imgp->argv0)
			argp = imgp->argv0;
		if (argp) {
			do {
				if (argp == (caddr_t) -1)
					return (EFAULT);
				if ((error = copyinstr(argp, imgp->stringp,
				    imgp->stringspace, &length))) {
					if (error == ENAMETOOLONG)
						return(E2BIG);
					return (error);
				}
				imgp->stringspace -= length;
				imgp->stringp += length;
				imgp->argc++;
			} while ((argp = (caddr_t) (intptr_t) fuword(argv++)));
		}
	}

	imgp->endargs = imgp->stringp;

	/*
	 * extract environment strings
	 */

	envv = imgp->uap->envv;

	if (envv) {
		while ((envp = (caddr_t) (intptr_t) fuword(envv++))) {
			if (envp == (caddr_t) -1)
				return (EFAULT);
			if ((error = copyinstr(envp, imgp->stringp,
			    imgp->stringspace, &length))) {
				if (error == ENAMETOOLONG)
					return(E2BIG);
				return (error);
			}
			imgp->stringspace -= length;
			imgp->stringp += length;
			imgp->envc++;
		}
	}

	return (0);
}

/*
 * 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, *destp;
	register_t *stack_base;
	struct ps_strings *arginfo;
	int szsigcode;

	/*
	 * Calculate string base and vector table pointers.
	 * Also deal with signal trampoline code for this exec type.
	 */
	arginfo = (struct ps_strings *)PS_STRINGS;
	szsigcode = *(imgp->proc->p_sysent->sv_szsigcode);
	destp =	(caddr_t)arginfo - szsigcode - SPARE_USRSPACE -
		roundup((ARG_MAX - imgp->stringspace), sizeof(char *));

	/*
	 * install sigcode
	 */
	if (szsigcode)
		copyout(imgp->proc->p_sysent->sv_sigcode,
			((caddr_t)arginfo - szsigcode), szsigcode);

	/*
	 * 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->argc + imgp->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->argc + imgp->envc + 2) * sizeof(char *));

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

	stringp = imgp->stringbase;
	argc = imgp->argc;
	envc = imgp->envc;

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

	/*
	 * Fill in "ps_strings" struct for ps, w, etc.
	 */
	suword(&arginfo->ps_argvstr, (long)(intptr_t)vectp);
	suword(&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);
	suword(&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 proc *p = imgp->proc;
	struct vnode *vp = imgp->vp;
	struct vattr *attr = imgp->attr;
	int error;

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

	/*
	 * 1) Check if file execution is disabled for the filesystem that this
	 *	file resides on.
	 * 2) Insure that at least one execute bit is on - otherwise root
	 *	will always succeed, and we don't want to happen unless the
	 *	file really is executable.
	 * 3) Insure that the file is a regular file.
	 */
	if ((vp->v_mount->mnt_flag & MNT_NOEXEC) ||
	    ((attr->va_mode & 0111) == 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, p->p_ucred, p);
	if (error)
		return (error);

	/*
	 * Check number of open-for-writes on the file and deny execution
	 * if there are any.
	 */
	if (vp->v_writecount)
		return (ETXTBSY);

	/*
	 * Call filesystem specific open routine (which does nothing in the
	 * general case).
	 */
	error = VOP_OPEN(vp, FREAD, p->p_ucred, p);
	if (error)
		return (error);

	return (0);
}

/*
 * 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);
	if (newexecsw == NULL)
		return ENOMEM;
	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);
	if (newexecsw == NULL)
		return ENOMEM;
	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;
}