Re: [PATCH v2 net 9/9] net: enetc: fix the off-by-one issue in enetc_map_tx_tso_buffs()

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Wed, Feb 19, 2025 at 01:42:47PM +0800, Wei Fang wrote:
> There is an off-by-one issue for the err_chained_bd path, it will free
> one more tx_swbd than expected. But there is no such issue for the
> err_map_data path.

It's clear that one of err_chained_bd or err_map_data is wrong, because
they operate with a different "count" but same "i". But how did you
determine which one is wrong? Is it based on static analysis? Because I
think the other one is wrong, more below.

> To fix this off-by-one issue and make the two error
> handling consistent, the loop condition of error handling is modified
> and the 'count++' operation is moved before enetc_map_tx_tso_data().
> 
> Fixes: fb8629e2cbfc ("net: enetc: add support for software TSO")
> Cc: stable@vger.kernel.org
> Signed-off-by: Wei Fang <wei.fang@nxp.com>
> ---
>  drivers/net/ethernet/freescale/enetc/enetc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
> index 9a24d1176479..fe3967268a19 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> @@ -832,6 +832,7 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
>  			txbd = ENETC_TXBD(*tx_ring, i);
>  			tx_swbd = &tx_ring->tx_swbd[i];
>  			prefetchw(txbd);
> +			count++;
>  
>  			/* Compute the checksum over this segment of data and
>  			 * add it to the csum already computed (over the L4
> @@ -848,7 +849,6 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
>  				goto err_map_data;
>  
>  			data_len -= size;
> -			count++;
>  			bd_data_num++;
>  			tso_build_data(skb, &tso, size);
>  
> @@ -874,13 +874,13 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
>  	dev_err(tx_ring->dev, "DMA map error");
>  
>  err_chained_bd:
> -	do {
> +	while (count--) {
>  		tx_swbd = &tx_ring->tx_swbd[i];
>  		enetc_free_tx_frame(tx_ring, tx_swbd);
>  		if (i == 0)
>  			i = tx_ring->bd_count;
>  		i--;
> -	} while (count--);
> +	}
>  
>  	return 0;
>  }

ah, there you go, here's the 3rd instance of TX DMA buffer unmapping :-/

Forget what I said in reply to patch 1/9 about having common code later.
After going through the whole set and now seeing this, I now think it's
better that you create the helper now, and consolidate the 2 instances
you touch anyway. Later you can make enetc_lso_hw_offload() reuse this
helper in net-next.

It should be something like this in the end (sorry, just 1 squashed diff):

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
index 6178157611db..a70e92dcbe2c 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.c
+++ b/drivers/net/ethernet/freescale/enetc/enetc.c
@@ -106,6 +106,24 @@ static void enetc_free_tx_frame(struct enetc_bdr *tx_ring,
 	}
 }
 
+/**
+ * enetc_unwind_tx_frame() - Unwind the DMA mappings of a multi-buffer TX frame
+ * @tx_ring: Pointer to the TX ring on which the buffer descriptors are located
+ * @count: Number of TX buffer descriptors which need to be unmapped
+ * @i: Index of the last successfully mapped TX buffer descriptor
+ */
+static void enetc_unwind_tx_frame(struct enetc_bdr *tx_ring, int count, int i)
+{
+	while (count--) {
+		struct enetc_tx_swbd *tx_swbd = &tx_ring->tx_swbd[i];
+
+		enetc_free_tx_frame(tx_ring, tx_swbd);
+		if (i == 0)
+			i = tx_ring->bd_count;
+		i--;
+	}
+}
+
 /* Let H/W know BD ring has been updated */
 static void enetc_update_tx_ring_tail(struct enetc_bdr *tx_ring)
 {
@@ -399,13 +417,7 @@ static int enetc_map_tx_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 dma_err:
 	dev_err(tx_ring->dev, "DMA map error");
 
-	while (count--) {
-		tx_swbd = &tx_ring->tx_swbd[i];
-		enetc_free_tx_frame(tx_ring, tx_swbd);
-		if (i == 0)
-			i = tx_ring->bd_count;
-		i--;
-	}
+	enetc_unwind_tx_frame(tx_ring, count, i);
 
 	return 0;
 }
@@ -752,7 +764,6 @@ static int enetc_lso_map_data(struct enetc_bdr *tx_ring, struct sk_buff *skb,
 
 static int enetc_lso_hw_offload(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 {
-	struct enetc_tx_swbd *tx_swbd;
 	struct enetc_lso_t lso = {0};
 	int err, i, count = 0;
 
@@ -776,13 +787,7 @@ static int enetc_lso_hw_offload(struct enetc_bdr *tx_ring, struct sk_buff *skb)
 	return count;
 
 dma_err:
-	do {
-		tx_swbd = &tx_ring->tx_swbd[i];
-		enetc_free_tx_frame(tx_ring, tx_swbd);
-		if (i == 0)
-			i = tx_ring->bd_count;
-		i--;
-	} while (--count);
+	enetc_unwind_tx_frame(tx_ring, count, i);
 
 	return 0;
 }
@@ -877,13 +882,7 @@ static int enetc_map_tx_tso_buffs(struct enetc_bdr *tx_ring, struct sk_buff *skb
 	dev_err(tx_ring->dev, "DMA map error");
 
 err_chained_bd:
-	while (count--) {
-		tx_swbd = &tx_ring->tx_swbd[i];
-		enetc_free_tx_frame(tx_ring, tx_swbd);
-		if (i == 0)
-			i = tx_ring->bd_count;
-		i--;
-	}
+	enetc_unwind_tx_frame(tx_ring, count, i);
 
 	return 0;
 }

With the definitions laid out explicitly in a kernel-doc, doesn't the
rest of the patch look a bit wrong? Why would you increment "count"
before enetc_map_tx_tso_data() succeeds? Why isn't the problem that "i"
needs to be rolled back on enetc_map_tx_tso_data() failure instead?