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kernel: make sure the swap device has a reasonable capacity in setswapchan()

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

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

@@ -396,11 +396,13 @@

 	Dir d;

 	int n;

+	if(waserror()){

+		cclose(c);

+		nexterror();

+	}

 	if(swapimage.c != nil) {

-		if(swapalloc.free != conf.nswap){

-			cclose(c);

+		if(swapalloc.free != conf.nswap)

 			error(Einuse);

-		}

 		cclose(swapimage.c);

 		swapimage.c = nil;

 	}

@@ -411,11 +413,10 @@

 	 */

 	if(devtab[c->type]->dc != L'M'){

 		n = devtab[c->type]->stat(c, dirbuf, sizeof dirbuf);

-		if(n <= 0){

-			cclose(c);

+		if(n <= 0 || convM2D(dirbuf, n, &d, nil) == 0)

 			error("stat failed in setswapchan");

-		}

-		convM2D(dirbuf, n, &d, nil);

+		if(d.length < conf.nswppo*BY2PG)

+			error("swap device too small");

 		if(d.length < conf.nswap*BY2PG){

 			conf.nswap = d.length/BY2PG;

 			swapalloc.top = &swapalloc.swmap[conf.nswap];

@@ -424,4 +425,5 @@

 	}

 	c->flag &= ~CCACHE;

 	swapimage.c = c;

+	poperror();

 }