From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <53a650f30ccb9218b76af22fb32a1bc1@plan9.bell-labs.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] how to avoid a memset() optimization
From: "Russ Cox" <rsc@plan9.bell-labs.com>
In-Reply-To: <Pine.LNX.4.30.0301061034040.19189-100000@athena>
MIME-Version: 1.0
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Date: Mon,  6 Jan 2003 10:49:14 -0500
Topicbox-Message-UUID: 3bc929bc-eacb-11e9-9e20-41e7f4b1d025

> you just need a marker to see how far you got. (I'm not even sure
> what the context of this message is, but every time i see the goto
> solution to this I have to say something). ;)

the goto seems much clearer to me.
also easier to edit.  what happens when
you have to lock A½?  you have to rewrite
all your case statements!

g% grep goto /sys/games/lib/fortunes |sed 1q
If you want to go somewhere, goto is the best way to get there. K Thompson
g% 

also you can't always write the code in your if-style,
but you can always use a goto.  for example, you can't
rewrite this without goto (or at least, if you do, it ends
up a lot less clear or a lot more duplicated):

static int
p9skclient(Conv *c)
{
	char *user;
	char cchal[CHALLEN];
	uchar secret[8];
	char buf[MAXAUTH];
	int speakfor, ret;
	Attr *a;
	Authenticator au;
	Key *k;
	Ticket t;
	Ticketreq tr;

	ret = -1;
	a = nil;
	k = nil;

	/* p9sk1: send client challenge */
	if(c->proto == &p9sk1){
		c->state = "write challenge";
		memrandom(cchal, CHALLEN);
		if(convwrite(c, cchal, CHALLEN) < 0)
			goto out;
	}

	/* read ticket request */
	c->state = "read tickreq";
	if(convread(c, buf, TICKREQLEN) < 0)
		goto out;
	convM2TR(buf, &tr);

	/* p9sk2: use server challenge as client challenge */
	if(c->proto == &p9sk2)
		memmove(cchal, tr.chal, CHALLEN);

	/*
	 * find a key.
	 *
	 * if the user is the factotum owner, any key will do.
	 * if not, then if we have a speakfor key,
	 * we will only vouch for the user's local identity.
	 *
	 * this logic is duplicated in p9any.c
	 */
	user = strfindattr(c->attr, "user");
	a = delattr(copyattr(c->attr), "role");
	a = addattr(a, "proto=p9sk1");

	if(strcmp(c->sysuser, owner) == 0){
		speakfor = 0;
		a = addattr(a, "proto=p9sk1 user? dom=%q", tr.authdom);
	}else if(user==nil || strcmp(c->sysuser, user)==0){
		speakfor = 1;
		a = delattr(a, "user");
		a = addattr(a, "proto=p9sk1 user? dom=%q role=speakfor", tr.authdom);
	}else{
		werrstr("will not authenticate for %q as %q", c->sysuser, user);
		goto out;
	}

	for(;;){
		c->state = "find key";
		k = keyfetch(c, "%A", a);
		if(k == nil)
			goto out;
		
		/* relay ticket request to auth server, get tickets */
		strcpy(tr.hostid, strfindattr(k->attr, "user"));
		if(speakfor)
			strcpy(tr.uid, c->sysuser);
		else
			strcpy(tr.uid, tr.hostid);

		c->state = "get tickets";
		if(gettickets(&tr, buf, k) < 0)
			goto out;

		convM2T(buf, &t, k->priv);
		if(t.num == AuthTc)
			break;

		/* we don't agree with the auth server about the key; try again */
		c->state = "replace key";
		if((k = keyreplace(c, k, "key mismatch with auth server")) == nil){
			werrstr("key mismatch with auth server");
			goto out;
		}
	}

	/* send second ticket and authenticator to server */
	c->state = "write ticket+auth";
	memmove(buf, buf+TICKETLEN, TICKETLEN);
	au.num = AuthAc;
	memmove(au.chal, tr.chal, CHALLEN);
	au.id = 0;
	convA2M(&au, buf+TICKETLEN, t.key);
	if(convwrite(c, buf, TICKETLEN+AUTHENTLEN) < 0)
		goto out;

	/* read authenticator from server */
	c->state = "read auth";
	if(convread(c, buf, AUTHENTLEN) < 0)
		goto out;
	convM2A(buf, &au, t.key);
	if(au.num != AuthAs || memcmp(au.chal, cchal, CHALLEN) != 0 || au.id != 0){
		werrstr("server lies through his teeth");
		goto out;
	}

	/* success */
	c->attr = addcap(c->attr, c->sysuser, &t);
	des56to64((uchar*)t.key, secret);
	c->attr = addattr(c->attr, "secret=%.8H", secret);
	ret = 0;

out:
	freeattr(a);
	keyclose(k);
	return ret;
}