Re: [9fans] how to avoid a memset() optimization

[Sam <sah@softcardsystems.com>]

ooh, a challenge.  Nothing pleases me more than proving myself
wrong.  be back in a few. ;)

btw - i don't fear the goto.  I use it a lot in small loop
situations to avoid empty for loops and such.  I'll play
with this function and see what I find. ;)

Cheers,

Sam

On Mon, 6
Jan 2003, Russ Cox wrote:

> > 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;
> }
>