net/ibm/emac: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
removes the VLA used for the emac xaht registers size. Since the size
of registers can only ever be 4 or 8, as detected in emac_init_config(),
the max can be hardcoded and a runtime test added for robustness.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Christian Lamparter <chunkeey@gmail.com>
Cc: Ivan Mikhaylov <ivan@de.ibm.com>
Cc: netdev@vger.kernel.org
Co-developed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ibm/emac/core.c

@@ -423,7 +423,7 @@ static void emac_hash_mc(struct emac_instance *dev)
 {
 	const int regs = EMAC_XAHT_REGS(dev);
 	u32 *gaht_base = emac_gaht_base(dev);
-	u32 gaht_temp[regs];
+	u32 gaht_temp[EMAC_XAHT_MAX_REGS];
 	struct netdev_hw_addr *ha;
 	int i;
 
@@ -2964,6 +2964,10 @@ static int emac_init_config(struct emac_instance *dev)
 		dev->xaht_width_shift = EMAC4_XAHT_WIDTH_SHIFT;
 	}
 
+	/* This should never happen */
+	if (WARN_ON(EMAC_XAHT_REGS(dev) > EMAC_XAHT_MAX_REGS))
+		return -ENXIO;
+
 	DBG(dev, "features     : 0x%08x / 0x%08x\n", dev->features, EMAC_FTRS_POSSIBLE);
 	DBG(dev, "tx_fifo_size : %d (%d gige)\n", dev->tx_fifo_size, dev->tx_fifo_size_gige);
 	DBG(dev, "rx_fifo_size : %d (%d gige)\n", dev->rx_fifo_size, dev->rx_fifo_size_gige);

drivers/net/ethernet/ibm/emac/core.h

@@ -390,6 +390,9 @@ static inline int emac_has_feature(struct emac_instance *dev,
 #define	EMAC4SYNC_XAHT_SLOTS_SHIFT	8
 #define	EMAC4SYNC_XAHT_WIDTH_SHIFT	5
 
+/* The largest span between slots and widths above is 3 */
+#define	EMAC_XAHT_MAX_REGS		(1 << 3)
+
 #define	EMAC_XAHT_SLOTS(dev)         	(1 << (dev)->xaht_slots_shift)
 #define	EMAC_XAHT_WIDTH(dev)         	(1 << (dev)->xaht_width_shift)
 #define	EMAC_XAHT_REGS(dev)          	(1 << ((dev)->xaht_slots_shift - \