jehanne/sys/src/kern/port/sysfile.c

/* Copyright (C) Charles Forsyth
 * See /doc/license/NOTICE.Plan9-9k.txt for details about the licensing.
 */
/* Portions of this file are Copyright (C) 2015-2018 Giacomo Tesio <giacomo@tesio.it>
 * See /doc/license/gpl-2.0.txt for details about the licensing.
 */
/* Portions of this file are Copyright (C) 2015 Ronald G. Minnich <rminnich@gmail.com>
 * See /doc/license/gpl-2.0.txt for details about the licensing.
 */
#include	"u.h"
#include	"../port/lib.h"
#include	"mem.h"
#include	"dat.h"
#include	"fns.h"
#include	"../port/error.h"

/*
 * The sys*() routines needn't poperror() as they return directly to syscall().
 */

void
unlockfgrp(Fgrp *f)
{
	int ex;

	ex = f->exceed;
	f->exceed = 0;
	unlock(&f->l);
	if(ex)
		pprint("warning: process exceeds %d file descriptors\n", ex);
}

static int
growfd(Fgrp *f, int fd)	/* fd is always >= 0 */
{
	Chan **newfd, **oldfd;

	if(fd < f->nfd)
		return 0;
	if(fd >= f->nfd+DELTAFD)
		return -1;	/* out of range */
	/*
	 * Unbounded allocation is unwise
	 */
	if(f->nfd >= 5000){
Exhausted:
		jehanne_print("no free file descriptors\n");
		return -1;
	}
	newfd = jehanne_malloc((f->nfd+DELTAFD)*sizeof(Chan*));
	if(newfd == nil)
		goto Exhausted;
	oldfd = f->fd;
	jehanne_memmove(newfd, oldfd, f->nfd*sizeof(Chan*));
	f->fd = newfd;
	jehanne_free(oldfd);
	f->nfd += DELTAFD;
	if(fd > f->maxfd){
		if(fd/100 > f->maxfd/100)
			f->exceed = (fd/100)*100;
		f->maxfd = fd;
	}
	return 1;
}

/*
 *  this assumes that the fgrp is locked
 */
int
findfreefd(Fgrp *f, int start)
{
	int fd;

	for(fd=start; fd<f->nfd; fd++)
		if(f->fd[fd] == nil)
			break;
	if(fd >= f->nfd && growfd(f, fd) < 0)
		return -1;
	return fd;
}

int
newfd(Chan *c)
{
	int fd;
	Fgrp *f;

	f = up->fgrp;
	lock(&f->l);
	fd = findfreefd(f, 0);
	if(fd < 0){
		unlockfgrp(f);
		return -1;
	}
	if(fd > f->maxfd)
		f->maxfd = fd;
	f->fd[fd] = c;
	unlockfgrp(f);
	return fd;
}

Chan*
fdtochan(int fd, unsigned long mode, int chkmnt, int iref)
{
	Chan *c;
	Fgrp *f;

	c = nil;
	f = up->fgrp;

	lock(&f->l);
	if(fd<0 || f->nfd<=fd || (c = f->fd[fd])==0) {
		unlock(&f->l);
		error(Ebadfd);
	}
	if(iref)
		incref(&c->r);
	unlock(&f->l);

	if(chkmnt && (c->flag&CMSG)) {
		if(iref)
			cclose(c);
		error(Ebadusefd);
	}

	if(mode==~0 || c->mode==ORDWR)
		return c;

	/* In Jehanne OTRUNC is not a "kernel reserved flag" (see libc.h):
	 * it's up to the server/device handing the request to ensure
	 * that OREAD|OTRUNC or any other combination produce an error...
	 *
	if((mode&OTRUNC) && c->mode==OREAD) {
		if(iref)
			cclose(c);
		error(Ebadusefd);
	}
	 */

	if((mode&c->mode) != mode) {
		if(iref)
			cclose(c);
		error(Ebadusefd);
	}

	return c;
}

unsigned long
openmode(unsigned long omode)
{
	if((omode&OEXEC) && (omode&ORDWR))
		error(Ebadarg);

	if((omode&OKMODE) == OEXEC)
		return (omode|OREAD)&~OEXEC;
	return omode;
}

int
sysfd2path(int fd, char* buf, int nbuf)
{
	Chan *c;

	buf = validaddr(buf, nbuf, 1);

	c = fdtochan(fd, -1, 0, 1);
	jehanne_snprint(buf, nbuf, "%s", chanpath(c));
	cclose(c);

	return 0;
}

int
sysdup(int ofd, int nfd)
{
	Chan *nc, *oc;
	Fgrp *f;

	oc = fdtochan(ofd, -1, 0, 1);

	if(nfd != -1){
		f = up->fgrp;
		lock(&f->l);
		if(nfd < 0 || growfd(f, nfd) < 0) {
			unlockfgrp(f);
			cclose(oc);
			error(Ebadfd);
		}
		if(nfd > f->maxfd)
			f->maxfd = nfd;

		nc = f->fd[nfd];
		f->fd[nfd] = oc;
		unlockfgrp(f);
		if(nc != nil)
			cclose(nc);
	}else{
		if(waserror()) {
			cclose(oc);
			nexterror();
		}
		nfd = newfd(oc);
		if(nfd < 0)
			error(Enofd);
		poperror();
	}

	return nfd;
}

int
sysopen(char *aname, uint32_t omode)
{
	int fd;
	Chan *c;

	openmode(omode);	/* error check only */

	c = nil;
	if(waserror()){
		if(c != nil)
			cclose(c);
		nexterror();
	}
	aname = validaddr(aname, 1, 0);
	if((omode&OKMODE) == OSTAT)
		c = namec(aname, Aaccess, 0, 0);
	else
		c = namec(aname, Aopen, omode, 0);
	fd = newfd(c);
	if(fd < 0)
		error(Enofd);
	poperror();

	return fd;
}

void
fdclose(int fd, int flag)
{
	int i;
	Chan *c;
	Fgrp *f;

	f = up->fgrp;
	lock(&f->l);
	c = f->fd[fd];
	if(c == nil){
		/* can happen for users with shared fd tables */
		unlock(&f->l);
		return;
	}
	if(flag){
		if(c == nil || !(c->flag&flag)){
			unlock(&f->l);
			return;
		}
	}
	f->fd[fd] = nil;
	if(fd == f->maxfd)
		for(i = fd; --i >= 0 && f->fd[i] == 0; )
			f->maxfd = i;

	unlock(&f->l);
	cclose(c);
}

int
sysclose(int fd)
{
	fdtochan(fd, -1, 0, 0);
	fdclose(fd, 0);

	return 0;
}

int32_t
unionread(Chan *c, void *va, int32_t n)
{
	int i;
	int32_t nr;
	Mhead *mh;
	Mount *mount;

	qlock(&c->umqlock);
	mh = c->umh;
	rlock(&mh->lock);
	mount = mh->mount;
	/* bring mount in sync with c->uri and c->umc */
	for(i = 0; mount != nil && i < c->uri; i++)
		mount = mount->next;

	nr = 0;
	while(mount != nil){
		/* Error causes component of union to be skipped */
		if(mount->to && !waserror()){
			if(c->umc == nil){
				c->umc = cclone(mount->to);
				c->umc = c->umc->dev->open(c->umc, OREAD);
			}

			nr = c->umc->dev->read(c->umc, va, n, c->umc->offset);
			c->umc->offset += nr;
			poperror();
		}
		if(nr > 0)
			break;

		/* Advance to next element */
		c->uri++;
		if(c->umc){
			cclose(c->umc);
			c->umc = nil;
		}
		mount = mount->next;
	}
	runlock(&mh->lock);
	qunlock(&c->umqlock);
	return nr;
}

static void
unionrewind(Chan *c)
{
	qlock(&c->umqlock);
	c->uri = 0;
	if(c->umc){
		cclose(c->umc);
		c->umc = nil;
	}
	qunlock(&c->umqlock);
}

static usize
dirfixed(uint8_t *p, uint8_t *e, Dir *d)
{
	int len;
	Dev *dev;

	len = GBIT16(p)+BIT16SZ;
	if(p + len > e)
		return 0;

	p += BIT16SZ;	/* ignore size */
	dev = devtabget(GBIT16(p), 1);			//XDYNX
	if(dev != nil){
		d->type = dev->dc;
		//devtabdecr(dev);
	}
	else
		d->type = -1;
	p += BIT16SZ;
	d->dev = GBIT32(p);
	p += BIT32SZ;
	d->qid.type = GBIT8(p);
	p += BIT8SZ;
	d->qid.vers = GBIT32(p);
	p += BIT32SZ;
	d->qid.path = GBIT64(p);
	p += BIT64SZ;
	d->mode = GBIT32(p);
	p += BIT32SZ;
	d->atime = GBIT32(p);
	p += BIT32SZ;
	d->mtime = GBIT32(p);
	p += BIT32SZ;
	d->length = GBIT64(p);

	return len;
}

static char*
dirname(uint8_t *p, usize *n)
{
	p += BIT16SZ+BIT16SZ+BIT32SZ+BIT8SZ+BIT32SZ+BIT64SZ
		+ BIT32SZ+BIT32SZ+BIT32SZ+BIT64SZ;
	*n = GBIT16(p);

	return (char*)p+BIT16SZ;
}

static usize
dirsetname(char *name, usize len, uint8_t *p, usize n, usize maxn)
{
	char *oname;
	usize nn, olen;

	if(n == BIT16SZ)
		return BIT16SZ;

	oname = dirname(p, &olen);

	nn = n+len-olen;
	PBIT16(p, nn-BIT16SZ);
	if(nn > maxn)
		return BIT16SZ;

	if(len != olen)
		jehanne_memmove(oname+len, oname+olen, p+n-(uint8_t*)(oname+olen));
	PBIT16((uint8_t*)(oname-2), len);
	jehanne_memmove(oname, name, len);

	return nn;
}

/*
 * Mountfix might have caused the fixed results of the directory read
 * to overflow the buffer.  Catch the overflow in c->dirrock.
 */
static void
mountrock(Chan *c, uint8_t *p, uint8_t **pe)
{
	uint8_t *e, *r;
	int len, n;

	e = *pe;

	/* find last directory entry */
	for(;;){
		len = BIT16SZ+GBIT16(p);
		if(p+len >= e)
			break;
		p += len;
	}

	/* save it away */
	qlock(&c->rockqlock);
	if(c->nrock+len > c->mrock){
		n = ROUNDUP(c->nrock+len, 1024);
		r = smalloc(n);
		jehanne_memmove(r, c->dirrock, c->nrock);
		jehanne_free(c->dirrock);
		c->dirrock = r;
		c->mrock = n;
	}
	jehanne_memmove(c->dirrock+c->nrock, p, len);
	c->nrock += len;
	qunlock(&c->rockqlock);

	/* drop it */
	*pe = p;
}

/*
 * Satisfy a directory read with the results saved in c->dirrock.
 */
static int
mountrockread(Chan *c, uint8_t *op, int32_t n, int32_t *nn)
{
	int32_t dirlen;
	uint8_t *rp, *erp, *ep, *p;

	/* common case */
	if(c->nrock == 0)
		return 0;

	/* copy out what we can */
	qlock(&c->rockqlock);
	rp = c->dirrock;
	erp = rp+c->nrock;
	p = op;
	ep = p+n;
	while(rp+BIT16SZ <= erp){
		dirlen = BIT16SZ+GBIT16(rp);
		if(p+dirlen > ep)
			break;
		jehanne_memmove(p, rp, dirlen);
		p += dirlen;
		rp += dirlen;
	}

	if(p == op){
		qunlock(&c->rockqlock);
		return 0;
	}

	/* shift the rest */
	if(rp != erp)
		jehanne_memmove(c->dirrock, rp, erp-rp);
	c->nrock = erp - rp;

	*nn = p - op;
	qunlock(&c->rockqlock);
	return 1;
}

static void
mountrewind(Chan *c)
{
	c->nrock = 0;
}

/*
 * Rewrite the results of a directory read to reflect current
 * name space bindings and mounts.  Specifically, replace
 * directory entries for bind and mount points with the results
 * of statting what is mounted there.  Except leave the old names.
 */
static int32_t
mountfix(Chan *c, uint8_t *op, int32_t n, int32_t maxn)
{
	char *name;
	int nbuf;
	Chan *nc;
	Mhead *mh;
	Mount *mount;
	usize dirlen, nname, r, rest;
	int32_t l;
	uint8_t *buf, *e, *p;
	Dir d;

	p = op;
	buf = nil;
	nbuf = 0;
	for(e=&p[n]; p+BIT16SZ<e; p+=dirlen){
		dirlen = dirfixed(p, e, &d);
		if(dirlen == 0)
			break;
		nc = nil;
		mh = nil;
		if(findmount(&nc, &mh, d.type, d.dev, d.qid)){
			/*
			 * If it's a union directory and the original is
			 * in the union, don't rewrite anything.
			 */
			for(mount=mh->mount; mount; mount=mount->next)
				if(eqchanddq(mount->to, d.type, d.dev, d.qid, 1))
					goto Norewrite;

			name = dirname(p, &nname);
			/*
			 * Do the stat but fix the name.  If it fails,
			 * leave old entry.
			 * BUG: If it fails because there isn't room for
			 * the entry, what can we do?  Nothing, really.
			 * Might as well skip it.
			 */
			if(buf == nil){
				buf = smalloc(4096);
				nbuf = 4096;
			}
			if(waserror())
				goto Norewrite;
			l = nc->dev->stat(nc, buf, nbuf);
			r = dirsetname(name, nname, buf, l, nbuf);
			if(r == BIT16SZ)
				error("dirsetname");
			poperror();

			/*
			 * Shift data in buffer to accomodate new entry,
			 * possibly overflowing into rock.
			 */
			rest = e - (p+dirlen);
			if(r > dirlen){
				while(p+r+rest > op+maxn){
					mountrock(c, p, &e);
					if(e == p){
						dirlen = 0;
						goto Norewrite;
					}
					rest = e - (p+dirlen);
				}
			}
			if(r != dirlen){
				jehanne_memmove(p+r, p+dirlen, rest);
				dirlen = r;
				e = p+dirlen+rest;
			}

			/*
			 * Rewrite directory entry.
			 */
			jehanne_memmove(p, buf, r);

		    Norewrite:
			cclose(nc);
			putmhead(mh);
		}
	}
	if(buf)
		jehanne_free(buf);

	if(p != e)
		error("oops in mountfix");

	return e-op;
}

long
syspread(int fd, void *p, long n, int64_t off)
{
	int32_t nn;
	long nnn;
	int sequential;
	Chan *c;

	if(n >= 0)
		p = validaddr(p, n, 1);
	else if(p != nil) {
		/* in Jehanne, a negative length can be meaningful to
		 * the target device/server, but with a negative length
		 * to read the buffer must be nil
		 */
InvalidAddress:
		pprint("trap: invalid address %#p/%lld in pread pc=%#P\n", p, n, userpc(nil));
		postnote(up, 1, "sys: bad address in pread", NDebug);
		error(Ebadarg);
	}

	c = fdtochan(fd, OREAD, 1, 1);

	up->blockingfd = fd;
	if(waserror()){
		up->blockingfd = -1;
		cclose(c);
		nexterror();
	}

	/*
	 * The offset is passed through on directories, normally.
	 * Sysseek complains, but pread is used by servers like exportfs,
	 * that shouldn't need to worry about this issue.
	 *
	 * Notice that c->devoffset is the offset that c's dev is seeing.
	 * The number of bytes read on this fd (c->offset) may be different
	 * due to rewritings in mountfix.
	 */
	if(off == ~0LL){	/* use and maintain channel's offset */
		off = c->offset;
		sequential = 1;
	} else {
		sequential = 0;
	}
	if(c->qid.type & QTDIR){
		if(p == nil){
			/* With union mount we can't use negative
			 * offsets on directories as it's impossible
			 * to predict which fs support them and
			 * how they interpret them.
			 */
			goto InvalidAddress;
		}
		/*
		 * Directory read:
		 * rewind to the beginning of the file if necessary;
		 * try to fill the buffer via mountrockread;
		 * set sequential to always maintain the Chan offset.
		 */
		if(off == 0LL){
			if(sequential){
				c->offset = 0;
				c->devoffset = 0;
			}
			mountrewind(c);
			unionrewind(c);
		}

		if(!mountrockread(c, p, n, &nn)){
			if(c->umh)
				nn = unionread(c, p, n);
			else{
				if(off != c->offset)
					error(Edirseek);
				nn = c->dev->read(c, p, n, c->devoffset);
			}
		}
		nnn = mountfix(c, p, nn, n);

		sequential = 1;
	} else {
		nnn = c->dev->read(c, p, n, off);
		nn = nnn;
	}

	if(sequential){
		lock(&c->l);
		c->devoffset += nn;
		c->offset += nnn;
		unlock(&c->l);
	}

	poperror();
	cclose(c);

	up->blockingfd = -1;

	return nnn;
}

long
syspwrite(int fd, void *p, long n, int64_t off)
{
	long r;
	int sequential;
	Chan *c;

	r = n;

	if(n >= 0)
		p = validaddr(p, n, 0);
	else if(p != nil) {
		/* in Jehanne, a negative length can be meaningful to
		 * the target device/server, but with a negative length
		 * to write the buffer must be nil
		 */
		pprint("trap: invalid address %#p/%ld in pwrite pc=%#P\n", p, n, userpc(nil));
		postnote(up, 1, "sys: bad address in pwrite", NDebug);
		error(Ebadarg);
	}
	n = 0;
	c = fdtochan(fd, OWRITE, 1, 1);

	if(off == ~0LL)
		sequential = 1;
	else
		sequential = 0;

	up->blockingfd = fd;
	if(waserror()) {
		up->blockingfd = -1;
		if(sequential){
			lock(&c->l);
			c->offset -= n;
			unlock(&c->l);
		}
		cclose(c);
		nexterror();
	}

	if(c->qid.type & QTDIR)
		error(Eisdir);

	n = r;

	if(sequential){	/* use and maintain channel's offset */
		lock(&c->l);
		off = c->offset;
		c->offset += n;
		unlock(&c->l);
	}

	r = c->dev->write(c, p, n, off);

	if(sequential && r < n){
		lock(&c->l);
		c->offset -= n - r;
		unlock(&c->l);
	}

	poperror();
	cclose(c);

	up->blockingfd = -1;

	return r;
}

static int64_t
sseek(int fd, int64_t offset, int whence)
{
	Chan *c;
	uint8_t buf[sizeof(Dir)+100];
	Dir dir;
	int n;

	c = fdtochan(fd, -1, 1, 1);
	if(waserror()){
		cclose(c);
		nexterror();
	}
	if(c->dev->dc == '|')
		error(Eisstream);

	switch(whence){
	case 0:
		if((c->qid.type & QTDIR) && offset != 0LL)
			error(Eisdir);
		c->offset = offset;
		break;

	case 1:
		if(c->qid.type & QTDIR)
			error(Eisdir);
		lock(&c->l);	/* lock for read/write update */
		offset += c->offset;
		c->offset = offset;
		unlock(&c->l);
		break;

	case 2:
		if(c->qid.type & QTDIR)
			error(Eisdir);
		n = c->dev->stat(c, buf, sizeof buf);
		if(jehanne_convM2D(buf, n, &dir, nil) == 0)
			error("internal error: stat error in seek");
		offset += dir.length;
		c->offset = offset;
		break;

	default:
		error(Ebadarg);
	}
	c->uri = 0;
	c->dri = 0;
	cclose(c);
	poperror();

	return offset;
}

long
sysseek(int fd, long offset, int whence)
{
	return sseek(fd, offset, whence);
}

void
validstat(uint8_t *s, usize n)
{
	usize m;
	char buf[64];

	if(statcheck(s, n) < 0)
		error(Ebadstat);
	/* verify that name entry is acceptable */
	s += STATFIXLEN - 4*BIT16SZ;	/* location of first string */
	/*
	 * s now points at count for first string.
	 * if it's too int32_t, let the server decide; this is
	 * only for his protection anyway. otherwise
	 * we'd have to allocate and waserror.
	 */
	m = GBIT16(s);
	s += BIT16SZ;
	if(m+1 > sizeof buf)
		return;
	jehanne_memmove(buf, s, m);
	buf[m] = '\0';
	/* name could be '/' */
	if(jehanne_strcmp(buf, "/") != 0)
		validname(buf, 0);
}

#if 0
static char*
pathlast(Path *p)
{
	char *s;

	if(p == nil)
		return nil;
	if(p->len == 0)
		return nil;
	s = jehanne_strrchr(p->s, '/');
	if(s)
		return s+1;
	return p->s;
}
#endif

int
sysfstat(int fd, uint8_t* p, int n)
{
	Chan *c;
	int r;

	p = validaddr(p, n, 1);
	c = fdtochan(fd, -1, 0, 1);
	if(waserror()) {
		cclose(c);
		nexterror();
	}
	r = c->dev->stat(c, p, n);
	poperror();
	cclose(c);

	return r;
}

/* white list of devices we allow mounting on.
 * At some point we can have build generate this if we ever
 * really start using it.
 */

static int dcok[] =  {
	'9'
};
static int checkdc(int dc)
{
	int i;
	/* we check for non-zero in case somebody ever puts a ,
	 * after the last element and we end up with 0 as the last thing ...
	 */
	for(i = 0; (i < nelem(dcok)) && dcok[i]; i++)
		if (dcok[i] == dc)
			return 1;
	return 0;
}

/* if dc is non-zero, it means we're doing a mount and dc is the mount device to use. */
static int
bindmount(int dc, int fd, int afd, char* arg0, char* arg1, int flag, char* spec)
{
	int i;
	Dev *dev;
	Chan *c0, *c1, *ac, *bc;

	if((flag&~MMASK) || (flag&MORDER)==(MBEFORE|MAFTER))
		error(Ebadarg);

	if(dc){
		validaddr(spec, 1, 0);
		spec = validnamedup(spec, 1);
		if(waserror()){
			jehanne_free(spec);
			nexterror();
		}
		if (! checkdc(dc))
			error(Ebadarg);

		if(up->pgrp->noattach)
			error(Enoattach);

		ac = nil;
		bc = fdtochan(fd, ORDWR, 0, 1);
		if(waserror()) {
			if(ac != nil)
				cclose(ac);
			cclose(bc);
			nexterror();
		}

		if(afd >= 0)
			ac = fdtochan(afd, ORDWR, 0, 1);

		dev = devtabget(dc, 0);
		c0 = dev->attach(bc, ac, spec, 0);

		poperror();	/* ac bc */
		if(ac != nil)
			cclose(ac);
		cclose(bc);
	}else{
		spec = nil;
		c0 = namec(validaddr(arg0, 1, 0), Abind, 0, 0);
	}

	if(waserror()){
		cclose(c0);
		nexterror();
	}

	c1 = namec(validaddr(arg1, 1, 0), Amount, 0, 0);
	if(waserror()){
		cclose(c1);
		nexterror();
	}

	i = cmount(&c0, c1, flag, spec);

	poperror();
	cclose(c1);
	poperror();
	cclose(c0);
	if(dc){
		fdclose(fd, 0);
		poperror();
		jehanne_free(spec);
	}
	return i;
}

int
sysbind(char* name, char* old, int flag)
{
	return bindmount(0, -1, -1, name, old, flag, nil);
}

int
sysmount(int fd, int afd, char* old, uint32_t flag, char* aname, int dc)
{
	return bindmount(dc, fd, afd, nil, old, flag, aname);
}

int
sysunmount(char* name, char* old)
{
	Chan *cmount, *cmounted;

	cmount = namec(validaddr(old, 1, 0), Amount, 0, 0);
	cmounted = nil;
	if(name != nil) {
		if(waserror()) {
			cclose(cmount);
			nexterror();
		}

		/*
		 * This has to be namec(..., Aopen, ...) because
		 * if arg[0] is something like /srv/cs or /fd/0,
		 * opening it is the only way to get at the real
		 * Chan underneath.
		 */
		cmounted = namec(validaddr(name, 1, 0), Aopen, OREAD, 0);
		poperror();
	}

	if(waserror()) {
		cclose(cmount);
		if(cmounted != nil)
			cclose(cmounted);
		nexterror();
	}

	cunmount(cmount, cmounted);
	cclose(cmount);
	if(cmounted != nil)
		cclose(cmounted);
	poperror();

	return 0;
}

int
syscreate(char* aname, long omode, long perm)
{
	int fd;
	Chan *c;

	if(omode >= 0)
		openmode(omode);	/* error check only */
	c = nil;
	if(waserror()) {
		if(c != nil)
			cclose(c);
		nexterror();
	}
	c = namec(validaddr(aname, 1, 0), Acreate, omode, perm);
	fd = newfd(c);
	if(fd < 0)
		error(Enofd);
	poperror();

	return fd;
}

int
sysremove(char *aname)
{
	Chan *c;

	c = namec(validaddr(aname, 1, 0), Aremove, 0, 0);

	/*
	 * Removing mount points is disallowed to avoid surprises
	 * (which should be removed: the mount point or the mounted Chan?).
	 */
	if(c->ismtpt){
		cclose(c);
		error(Eismtpt);
	}
	if(waserror()){
		c->dev = nil;	/* see below */
		cclose(c);
		nexterror();
	}
	c->dev->remove(c);

	/*
	 * Remove clunks the fid, but we need to recover the Chan
	 * so fake it up.  rootclose() is known to be a nop.
Not sure this dicking around is right for Dev ref counts.
	 */
	c->dev = nil;
	poperror();
	cclose(c);

	return 0;
}

static int32_t
wstat(Chan* c, uint8_t* p, usize n)
{
	int32_t l;
	usize namelen;

	if(waserror()){
		cclose(c);
		nexterror();
	}

	/*
	 * Renaming mount points is disallowed to avoid surprises
	 * (which should be renamed? the mount point or the mounted Chan?).
	 */
	if(c->ismtpt){
		dirname(p, &namelen);
		if(namelen)
			nameerror(chanpath(c), Eismtpt);
	}
	l = c->dev->wstat(c, p, n);
	poperror();
	cclose(c);

	return l;
}

int
sysfwstat(int fd, uint8_t* p, int n)
{
	Chan *c;

	p = validaddr(p, n, 0);
	validstat(p, n);
	c = fdtochan(fd, -1, 1, 1);

	return wstat(c, p, n);
}