[PATCH] ethtool: fix ethtool_get_regs() to work with zero length registers

[Kalle Valo <kvalo@xxxxxxxxxxxxxxxx>]

cfg80211 exports zero length register size as it currently only uses
struct ethtool_regs.version to export struct wiphy.hw_version. But the
problem is that ethtool_get_regs() assumes that the driver (cfg80211 in this
case) always has non-zero length for registers. With cfg80211
it would always fail and return -ENOMEM to user space.

Fix this by checking the register length from the driver and exporting
struct ethtool_regs to user space if the length is zero.

With this patch it's possible to get the hardware id from wireless drivers.
Tested with wl12xx and ath6kl.

Tested-by: Gery Kahn <geryk@xxxxxx>
Signed-off-by: Kalle Valo <kvalo@xxxxxxxxxxxxxxxx>
---
 net/core/ethtool.c |   20 +++++++++++++-------
 1 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index fd14116..6f073f4 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1213,7 +1213,7 @@ static int ethtool_get_regs(struct net_device *dev, char __user *useraddr)
 {
 	struct ethtool_regs regs;
 	const struct ethtool_ops *ops = dev->ethtool_ops;
-	void *regbuf;
+	void *regbuf = NULL;
 	int reglen, ret;
 
 	if (!ops->get_regs || !ops->get_regs_len)
@@ -1226,18 +1226,24 @@ static int ethtool_get_regs(struct net_device *dev, char __user *useraddr)
 	if (regs.len > reglen)
 		regs.len = reglen;
 
-	regbuf = vzalloc(reglen);
-	if (!regbuf)
-		return -ENOMEM;
+	if (reglen > 0) {
+		regbuf = vzalloc(reglen);
+		if (!regbuf)
+			return -ENOMEM;
+	}
 
 	ops->get_regs(dev, &regs, regbuf);
 
 	ret = -EFAULT;
 	if (copy_to_user(useraddr, &regs, sizeof(regs)))
 		goto out;
-	useraddr += offsetof(struct ethtool_regs, data);
-	if (copy_to_user(useraddr, regbuf, regs.len))
-		goto out;
+
+	if (regs.len > 0) {
+		useraddr += offsetof(struct ethtool_regs, data);
+		if (copy_to_user(useraddr, regbuf, regs.len))
+			goto out;
+	}
+
 	ret = 0;
 
  out:

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