ethtool: Allow zero-length register dumps again

Some drivers (ab)use the ethtool_ops::get_regs operation to expose
only a hardware revision ID.  Commit
a77f5db3 ('ethtool: Allocate register
dump buffer with vmalloc()') had the side-effect of breaking these, as
vmalloc() returns a null pointer for size=0 whereas kmalloc() did not.

For backward-compatibility, allow zero-length dumps again.
Reported-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Cc: stable@kernel.org [2.6.37+]
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/ethtool.c

@@ -923,7 +923,7 @@ static int ethtool_get_regs(struct net_device *dev, char __user *useraddr)
 		regs.len = reglen;
 
 	regbuf = vzalloc(reglen);
-	if (!regbuf)
+	if (reglen && !regbuf)
 		return -ENOMEM;
 
 	ops->get_regs(dev, &regs, regbuf);
@@ -932,7 +932,7 @@ static int ethtool_get_regs(struct net_device *dev, char __user *useraddr)
 	if (copy_to_user(useraddr, &regs, sizeof(regs)))
 		goto out;
 	useraddr += offsetof(struct ethtool_regs, data);
-	if (copy_to_user(useraddr, regbuf, regs.len))
+	if (regbuf && copy_to_user(useraddr, regbuf, regs.len))
 		goto out;
 	ret = 0;