ramfs.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

	name = thdr.name;
	if(strcmp(name, ".")==0 || strcmp(name, "..")==0)
		return Ename;
	for(r=ram; r<&ram[nram]; r++)
		if(r->busy && parent==r->parent)
		if(strcmp((char*)name, r->name)==0)
			return Einuse;
	for(r=ram; r->busy; r++)
		if(r == &ram[Nram-1])
			return "no free ram resources";
	r->busy = 1;
	r->qid.path = ++path;
	r->qid.vers = 0;
	if(prm & DMDIR)
		r->qid.type |= QTDIR;
	r->parent = parent;
	free(r->name);
	r->name = estrdup(name);
	r->user = f->user;
	r->group = f->ram->group;
	r->muid = f->ram->muid;
	if(prm & DMDIR)
		prm = (prm&~0777) | (f->ram->perm&prm&0777);
	else
		prm = (prm&(~0777|0111)) | (f->ram->perm&prm&0666);
	r->perm = prm;
	r->ndata = 0;
	if(r-ram >= nram)
		nram = r - ram + 1;
	r->atime = time(0);
	r->mtime = r->atime;
	f->ram->mtime = r->atime;
	f->ram = r;
	rhdr.qid = r->qid;
	rhdr.iounit = messagesize-IOHDRSZ;
	f->open = 1;
	if(thdr.mode & ORCLOSE)
		f->rclose = 1;
	r->open++;
	return 0;
}

char*
rread(Fid *f)
{
	Ram *r;
	uchar *buf;
	long off;
	int n, m, cnt;

	if(f->ram->busy == 0)
		return Enotexist;
	n = 0;
	rhdr.count = 0;
	off = thdr.offset;
	buf = rdata;
	cnt = thdr.count;
	if(cnt > messagesize)	/* shouldn't happen, anyway */
		cnt = messagesize;
	if(f->ram->qid.type & QTDIR){
		for(r=ram+1; off > 0; r++){
			if(r->busy && r->parent==f->ram-ram)
				off -= ramstat(r, statbuf, sizeof statbuf);
			if(r == &ram[nram-1])
				return 0;
		}
		for(; r<&ram[nram] && n < cnt; r++){
			if(!r->busy || r->parent!=f->ram-ram)
				continue;
			m = ramstat(r, buf+n, cnt-n);
			if(m == 0)
				break;
			n += m;
		}
		rhdr.data = (char*)rdata;
		rhdr.count = n;
		return 0;
	}
	r = f->ram;
	if(off >= r->ndata)
		return 0;
	r->atime = time(0);
	n = cnt;
	if(off+n > r->ndata)
		n = r->ndata - off;
	rhdr.data = r->data+off;
	rhdr.count = n;
	return 0;
}

char*
rwrite(Fid *f)
{
	Ram *r;
	ulong off;
	int cnt;

	r = f->ram;
	if(r->busy == 0)
		return Enotexist;
	off = thdr.offset;
	if(r->perm & DMAPPEND)
		off = r->ndata;
	cnt = thdr.count;
	if(r->qid.type & QTDIR)
		return Eisdir;
	if(off+cnt >= Maxsize)		/* sanity check */
		return "write too big";
	if(off+cnt > r->ndata)
		r->data = erealloc(r->data, off+cnt);
	if(off > r->ndata)
		memset(r->data+r->ndata, 0, off-r->ndata);
	if(off+cnt > r->ndata)
		r->ndata = off+cnt;
	memmove(r->data+off, thdr.data, cnt);
	r->qid.vers++;
	r->mtime = time(0);
	rhdr.count = cnt;
	return 0;
}

static int
emptydir(Ram *dr)
{
	long didx = dr - ram;
	Ram *r;

	for(r=ram; r<&ram[nram]; r++)
		if(r->busy && didx==r->parent)
			return 0;
	return 1;
}

char *
realremove(Ram *r)
{
	if(r->qid.type & QTDIR && !emptydir(r))
		return Enotempty;
	r->ndata = 0;
	if(r->data)
		free(r->data);
	r->data = 0;
	r->parent = 0;
	memset(&r->qid, 0, sizeof r->qid);
	free(r->name);
	r->name = nil;
	r->busy = 0;
	return nil;
}

char *
rclunk(Fid *f)
{
	char *e = nil;

	if(f->open)
		f->ram->open--;
	if(f->rclose)
		e = realremove(f->ram);
	f->busy = 0;
	f->open = 0;
	f->ram = 0;
	return e;
}

char *
rremove(Fid *f)
{
	Ram *r;

	if(f->open)
		f->ram->open--;
	f->busy = 0;
	f->open = 0;
	r = f->ram;
	f->ram = 0;
	if(r->qid.path == 0 || !perm(f, &ram[r->parent], Pwrite))
		return Eperm;
	ram[r->parent].mtime = time(0);
	return realremove(r);
}

char *
rstat(Fid *f)
{
	if(f->ram->busy == 0)
		return Enotexist;
	rhdr.nstat = ramstat(f->ram, statbuf, sizeof statbuf);
	rhdr.stat = statbuf;
	return 0;
}

char *
rwstat(Fid *f)
{
	Ram *r, *s;
	Dir dir;

	if(f->ram->busy == 0)
		return Enotexist;
	convM2D(thdr.stat, thdr.nstat, &dir, (char*)statbuf);
	r = f->ram;

	/*
	 * To change length, must have write permission on file.
	 */
	if(dir.length!=~0 && dir.length!=r->ndata){
	 	if(!perm(f, r, Pwrite))
			return Eperm;
	}

	/*
	 * To change name, must have write permission in parent
	 * and name must be unique.
	 */
	if(dir.name[0]!='\0' && strcmp(dir.name, r->name)!=0){
	 	if(!perm(f, &ram[r->parent], Pwrite))
			return Eperm;
		for(s=ram; s<&ram[nram]; s++)
			if(s->busy && s->parent==r->parent)
			if(strcmp(dir.name, s->name)==0)
				return Eexist;
	}

	/*
	 * To change mode, must be owner or group leader.
	 * Because of lack of users file, leader=>group itself.
	 */
	if(dir.mode!=~0 && r->perm!=dir.mode){
		if(strcmp(f->user, r->user) != 0)
		if(strcmp(f->user, r->group) != 0)
			return Enotowner;
	}

	/*
	 * To change group, must be owner and member of new group,
	 * or leader of current group and leader of new group.
	 * Second case cannot happen, but we check anyway.
	 */
	if(dir.gid[0]!='\0' && strcmp(r->group, dir.gid)!=0){
		if(strcmp(f->user, r->user) == 0)
	//	if(strcmp(f->user, dir.gid) == 0)
			goto ok;
		if(strcmp(f->user, r->group) == 0)
		if(strcmp(f->user, dir.gid) == 0)
			goto ok;
		return Enotowner;
		ok:;
	}

	/* all ok; do it */
	if(dir.mode != ~0){
		dir.mode &= ~DMDIR;	/* cannot change dir bit */
		dir.mode |= r->perm&DMDIR;
		r->perm = dir.mode;
	}
	if(dir.name[0] != '\0'){
		free(r->name);
		r->name = estrdup(dir.name);
	}
	if(dir.gid[0] != '\0')
		r->group = estrdup(dir.gid);
	if(dir.length!=~0 && dir.length!=r->ndata){
		r->data = erealloc(r->data, dir.length);
		if(r->ndata < dir.length)
			memset(r->data+r->ndata, 0, dir.length-r->ndata);
		r->ndata = dir.length;
	}
	ram[r->parent].mtime = time(0);
	return 0;
}

uint
ramstat(Ram *r, uchar *buf, uint nbuf)
{
	int n;
	Dir dir;

	dir.name = r->name;
	dir.qid = r->qid;
	dir.mode = r->perm;
	dir.length = r->ndata;
	dir.uid = r->user;
	dir.gid = r->group;
	dir.muid = r->muid;
	dir.atime = r->atime;
	dir.mtime = r->mtime;
	n = convD2M(&dir, buf, nbuf);
	if(n > 2)
		return n;
	return 0;
}

Fid *
newfid(int fid)
{
	Fid *f, *ff;

	ff = 0;
	for(f = fids; f; f = f->next)
		if(f->fid == fid)
			return f;
		else if(!ff && !f->busy)
			ff = f;
	if(ff){
		ff->fid = fid;
		return ff;
	}
	f = emalloc(sizeof *f);
	f->ram = nil;
	f->fid = fid;
	f->next = fids;
	fids = f;
	return f;
}

void
io(void)
{
	char *err, buf[40];
	int n, pid, ctl;
	Fid *fid;

	pid = getpid();
	if(private){
		snprint(buf, sizeof buf, "/proc/%d/ctl", pid);
		ctl = open(buf, OWRITE);
		if(ctl < 0){
			fprint(2, "can't protect ramfs\n");
		}else{
			fprint(ctl, "noswap\n");
			fprint(ctl, "private\n");
			close(ctl);
		}
	}

	for(;;){
		/*
		 * reading from a pipe or a network device
		 * will give an error after a few eof reads.
		 * however, we cannot tell the difference
		 * between a zero-length read and an interrupt
		 * on the processes writing to us,
		 * so we wait for the error.
		 */
		n = read9pmsg(mfd[0], mdata, messagesize);
		if(n < 0){
			rerrstr(buf, sizeof buf);
			if(buf[0]=='\0' || strstr(buf, "hungup"))
				exits("");
			error("mount read");
		}
		if(n == 0)
			continue;
		if(convM2S(mdata, n, &thdr) == 0)
			continue;

		if(debug)
			fprint(2, "ramfs %d:<-%F\n", pid, &thdr);

		if(thdr.type<0 || thdr.type>=nelem(fcalls) || !fcalls[thdr.type])
			err = "bad fcall type";
		else if(((fid=newfid(thdr.fid))==nil || !fid->ram) && needfid[thdr.type])
			err = "fid not in use";
		else
			err = (*fcalls[thdr.type])(fid);
		if(err){
			rhdr.type = Rerror;
			rhdr.ename = err;
		}else{
			rhdr.type = thdr.type + 1;
			rhdr.fid = thdr.fid;
		}
		rhdr.tag = thdr.tag;
		if(debug)
			fprint(2, "ramfs %d:->%F\n", pid, &rhdr);/**/
		n = convS2M(&rhdr, mdata, messagesize);
		if(n == 0)
			error("convS2M error on write");
		if(write(mfd[1], mdata, n) != n)
			error("mount write");
	}
}

int
perm(Fid *f, Ram *r, int p)
{
	if((p*Pother) & r->perm)
		return 1;
	if(strcmp(f->user, r->group)==0 && ((p*Pgroup) & r->perm))
		return 1;
	if(strcmp(f->user, r->user)==0 && ((p*Powner) & r->perm))
		return 1;
	return 0;
}

void
error(char *s)
{
	fprint(2, "%s: %s: %r\n", argv0, s);
	exits(s);
}

void *
emalloc(ulong n)
{
	void *p;

	p = malloc(n);
	if(!p)
		error("out of memory");
	memset(p, 0, n);
	return p;
}

void *
erealloc(void *p, ulong n)
{
	p = realloc(p, n);
	if(!p)
		error("out of memory");
	return p;
}

char *
estrdup(char *q)
{
	char *p;
	int n;

	n = strlen(q)+1;
	p = malloc(n);
	if(!p)
		error("out of memory");
	memmove(p, q, n);
	return p;
}

void
usage(void)
{
	fprint(2, "usage: %s [-Dips] [-m mountpoint] [-S srvname]\n", argv0);
	exits("usage");
}