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kernel: fix allocb for BLOCKALIGN*2 >= Hdrspc

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

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

@@ -19,7 +19,8 @@

 	uintptr addr;

 	size += Tlrspc;

-	if((b = mallocz(sizeof(Block)+size+Hdrspc, 0)) == nil)

+	size = ROUND(size, BLOCKALIGN);

+	if((b = mallocz(sizeof(Block)+BLOCKALIGN+Hdrspc+size, 0)) == nil)

 		return nil;

 	b->next = nil;

@@ -38,11 +39,8 @@

 	addr &= ~(BLOCKALIGN-1);

 	b->lim = (uchar*)addr;

-	/* leave sluff at beginning for added headers */

-	b->rp = b->lim - ROUND(size, BLOCKALIGN);

-	if(b->rp < b->base)

-		panic("_allocb");

-	b->wp = b->rp;

+	/* leave room at beginning for added headers */

+	b->wp = b->rp = b->lim - size;

 	return b;

 }