[PATCH net v3] net: dsa: motorcomm: Prevent rx_bytes/tx_bytes underflow

[David Yang <mmyangfl@gmail.com>]

There is no direct rx_bytes/tx_bytes counter excluding the FCS; both
have to be derived as good_bytes - ETH_FCS_LEN * frames. Three hazards
can pair a frame count with a mismatched byte count and make the
subtraction underflow, reporting rx_bytes or tx_bytes as about 2^64:

- a register read fails midway and the half-refreshed buffer is
  published as is, pairing fresh frame counters with stale byte
  counters;
- get_stats64() on 64-bit kernels reads the published counters without
  any sequencing across fields (u64_stats_sync is a no-op there), so a
  writer update can slip between the frame and byte reads;
- the MIB bank cannot be read as a whole; counters keep counting while
  the read loop walks the table.

Handle this in the writer: do not publish a half-read buffer, keep the
last complete sample on read failure, do the arithmetic on the complete
sample under priv->reg_lock, and publish the derived byte counters only
when check_sub_overflow() succeeds, so that get_stats64() copies plain
published values.

Found by LLM.

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Signed-off-by: David Yang <mmyangfl@gmail.com>
---
v3:
 - prevent underflow anyway
v2: https://lore.kernel.org/r/20261103134705.867613-1-mmyangfl@gmail.com
 - more issues found by sashiko
v1: https://lore.kernel.org/r/20261102101334.3525485-1-mmyangfl@gmail.com
 drivers/net/dsa/motorcomm/mib.c | 25 +++++++++++++++++--------
 drivers/net/dsa/motorcomm/mib.h |  2 ++
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/drivers/net/dsa/motorcomm/mib.c b/drivers/net/dsa/motorcomm/mib.c
index 6b170fc6e187..09b3e6b98bdd 100644
--- a/drivers/net/dsa/motorcomm/mib.c
+++ b/drivers/net/dsa/motorcomm/mib.c
@@ -95,6 +95,8 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 	unsigned long flags;
 	u64 rx_frames;
 	u64 tx_frames;
+	u64 rx_bytes;
+	u64 tx_bytes;
 	int res = 0;
 
 	mib = &pm->stats;
@@ -127,6 +129,12 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 		buf[i] = val;
 	}
 
+	if (res) {
+		dev_err(dev, "Failed to %s port %d: %i\n", "read stats for",
+			port, res);
+		return res;
+	}
+
 	flags = u64_stats_update_begin_irqsave(&pm->syncp);
 	for (size_t i = 0; i < ARRAY_SIZE(yt921x_mib_descs); i++)
 		u64_stats_set(&((u64_stats_t *)mib)[i], buf[i]);
@@ -146,12 +154,15 @@ static int yt921x_mib_read(struct yt921x_priv *priv, int port)
 		    u64_stats_read(&mib->tx_jumbo);
 	u64_stats_set(&pm->rx_frames, rx_frames);
 	u64_stats_set(&pm->tx_frames, tx_frames);
+	if (!check_sub_overflow(u64_stats_read(&mib->rx_good_bytes),
+				ETH_FCS_LEN * rx_frames, &rx_bytes))
+		u64_stats_set(&pm->rx_bytes, rx_bytes);
+	if (!check_sub_overflow(u64_stats_read(&mib->tx_good_bytes),
+				ETH_FCS_LEN * tx_frames, &tx_bytes))
+		u64_stats_set(&pm->tx_bytes, tx_bytes);
 	u64_stats_update_end_irqrestore(&pm->syncp, flags);
 
-	if (res)
-		dev_err(dev, "Failed to %s port %d: %i\n", "read stats for",
-			port, res);
-	return res;
+	return 0;
 }
 
 void yt921x_mib_poll(struct work_struct *work)
@@ -391,10 +402,8 @@ yt921x_dsa_get_stats64(struct dsa_switch *ds, int port,
 
 		stats->rx_packets = u64_stats_read(&pm->rx_frames);
 		stats->tx_packets = u64_stats_read(&pm->tx_frames);
-		stats->rx_bytes = u64_stats_read(&mib->rx_good_bytes) -
-				  ETH_FCS_LEN * stats->rx_packets;
-		stats->tx_bytes = u64_stats_read(&mib->tx_good_bytes) -
-				  ETH_FCS_LEN * stats->tx_packets;
+		stats->rx_bytes = u64_stats_read(&pm->rx_bytes);
+		stats->tx_bytes = u64_stats_read(&pm->tx_bytes);
 		stats->rx_dropped = u64_stats_read(&mib->rx_dropped);
 		/* stats->tx_dropped */
 		stats->multicast = u64_stats_read(&mib->rx_multicast);
diff --git a/drivers/net/dsa/motorcomm/mib.h b/drivers/net/dsa/motorcomm/mib.h
index 9c24a52d46cc..2e2b832cb245 100644
--- a/drivers/net/dsa/motorcomm/mib.h
+++ b/drivers/net/dsa/motorcomm/mib.h
@@ -136,6 +136,8 @@ struct yt921x_mib {
 	struct yt921x_mib_stats stats;
 	u64_stats_t rx_frames;
 	u64_stats_t tx_frames;
+	u64_stats_t rx_bytes;
+	u64_stats_t tx_bytes;
 	/* protected by priv->reg_lock */
 	u64 data[YT921X_MIB_STAT_NUM];
 };
-- 
2.53.0