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kernel: refactor devallowed() check in namec() for # devices

Now that we'r not changing amode from Aunmount
to Aopen anymore, we can move the devallowed()
check conditions for Aunmount after parsing # name
and get rid of the "devunmount" conditional.

The utflen(name) == 2 is equivalent to testing
if the attach spec and walk name is empty after
parsing the device character rune.

--- a/sys/src/9/port/chan.c

+++ b/sys/src/9/port/chan.c

@@ -1282,7 +1282,7 @@

 Chan*

 namec(char *aname, int amode, int omode, ulong perm)

 {

-	int len, n, t, nomount, devunmount;

+	int len, n, t, nomount;

 	Chan *c;

 	Chan *volatile cnew;

 	Path *volatile path;

@@ -1302,20 +1302,6 @@

 	name = aname;

 	/*

-	 * When sandboxing, unmounting a sharp from a union is a valid

-	 * operation even if the device is blocked.

-	 */

-	devunmount = 0;

-	if(amode == Aunmount){

-		/*

-		 * Doing any walks down the device could leak information

-		 * about the existence of files.

-		 */

-		if(name[0] == '#' && utflen(name) == 2)

-			devunmount = 1;

-	}

-

-	/*

 	 * Find the starting off point (the current slash, the root of

 	 * a device tree, or the current dot) as well as the name to

 	 * evaluate starting there.

@@ -1329,7 +1315,6 @@

 	case '#':

 		nomount = 1;

-		up->genbuf[0] = '\0';

 		n = 0;

 		while(*name != '\0' && (*name != '/' || n < 2)){

 			if(n >= sizeof(up->genbuf)-1)

@@ -1341,7 +1326,14 @@

 		t = devno(r, 1);

 		if(t == -1)

 			error(Ebadsharp);

-		if(!devunmount && !devallowed(up->pgrp, r))

+		/*

+		 * When sandboxing, unmounting a sharp from a union is a valid

+		 * operation even if the device is blocked.

+		 * Doing any walks down the device could leak information

+		 * about the existence of files.

+		 */

+		if((amode != Aunmount || up->genbuf[n] || *name)

+		&& !devallowed(up->pgrp, r))

 			error(Enoattach);

 		c = devtab[t]->attach(up->genbuf+n);

-- 

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