netdev - [PATCH] net: ethernet: dec: tulip: Fix length mask in receive length calculation

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Message-Id: <20200204230118.7877-1-mdf@kernel.org>
Date:   Tue,  4 Feb 2020 15:01:18 -0800
From:   Moritz Fischer <mdf@...nel.org>
To:     netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        mst@...hat.com, hkallweit1@...il.com, davem@...emloft.net,
        morats@...gle.com, Moritz Fischer <mdf@...nel.org>
Subject: [PATCH] net: ethernet: dec: tulip: Fix length mask in receive length calculation

The receive frame length calculation uses a wrong mask to calculate the
length of the received frames.

Per spec table 4-1 the length is contained in the FL (Frame Length)
field in bits 30:16.

This didn't show up as an issue so far since frames were limited to
1500 bytes which falls within the 11 bit window.

Signed-off-by: Moritz Fischer <mdf@...nel.org>
---
 drivers/net/ethernet/dec/tulip/de2104x.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/dec/tulip/de2104x.c b/drivers/net/ethernet/dec/tulip/de2104x.c
index d305d1b24b0a..42b798a3fad4 100644
--- a/drivers/net/ethernet/dec/tulip/de2104x.c
+++ b/drivers/net/ethernet/dec/tulip/de2104x.c
@@ -417,7 +417,10 @@ static void de_rx (struct de_private *de)
 		if (status & DescOwn)
 			break;
 
-		len = ((status >> 16) & 0x7ff) - 4;
+		/* the length is actually a 15 bit value here according
+		 * to Table 4-1 in the DE2104x spec so mask is 0x7fff
+		 */
+		len = ((status >> 16) & 0x7fff) - 4;
 		mapping = de->rx_skb[rx_tail].mapping;
 
 		if (unlikely(drop)) {
-- 
2.25.0