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pc: sanity check bda value in lowraminit() to avoid overflow with efi

--- a/sys/src/9/pc/memory.c

+++ b/sys/src/9/pc/memory.c

@@ -376,7 +376,7 @@

 static void

 lowraminit(void)

 {

-	ulong n, pa, x;

+	ulong pa, x;

 	uchar *bda;

 	/*

@@ -387,9 +387,11 @@

 	 */

 	x = PADDR(CPU0END);

 	bda = (uchar*)KADDR(0x400);

-	n = ((bda[0x14]<<8)|bda[0x13])*KB-x;

-	mapfree(&rmapram, x, n);

-	memset(KADDR(x), 0, n);			/* keep us honest */

+	pa = ((bda[0x14]<<8)|bda[0x13])*KB;

+	if(x < pa){

+		mapfree(&rmapram, x, pa-x);

+		memset(KADDR(x), 0, pa-x);		/* keep us honest */

+	}

 	x = PADDR(PGROUND((ulong)end));

 	pa = MemMin;