netdev - [PATCH net-next] net: atlantic: Avoid warning about potential string truncation

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Message-Id: <20240821-atlantic-str-v1-1-fa2cfe38ca00@kernel.org>
Date: Wed, 21 Aug 2024 16:58:57 +0100
From: Simon Horman <horms@...nel.org>
To: Igor Russkikh <irusskikh@...vell.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: [PATCH net-next] net: atlantic: Avoid warning about potential
 string truncation

W=1 builds with GCC 14.2.0 warn that:

.../aq_ethtool.c:278:59: warning: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 6 [-Wformat-truncation=]
  278 |                                 snprintf(tc_string, 8, "TC%d ", tc);
      |                                                           ^~
.../aq_ethtool.c:278:56: note: directive argument in the range [-2147483641, 254]
  278 |                                 snprintf(tc_string, 8, "TC%d ", tc);
      |                                                        ^~~~~~~
.../aq_ethtool.c:278:33: note: ‘snprintf’ output between 5 and 15 bytes into a destination of size 8
  278 |                                 snprintf(tc_string, 8, "TC%d ", tc);
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

tc is always in the range 0 - cfg->tcs. And as cfg->tcs is a u8,
the range is 0 - 255. Further, on inspecting the code, it seems
that cfg->tcs will never be more than AQ_CFG_TCS_MAX (8), so
the range is actually 0 - 8.

So, it seems that the condition that GCC flags will not occur.
But, nonetheless, it would be nice if it didn't emit the warning.

It seems that this can be achieved by changing the format specifier
from %d to %u, in which case I believe GCC recognises an upper bound
on the range of tc of 0 - 255. After some experimentation I think
this is due to the combination of the use of %u and the type of
cfg->tcs (u8).

Empirically, updating the type of the tc variable to unsigned int
has the same effect.

As both of these changes seem to make sense in relation to what the code
is actually doing - iterating over unsigned values - do both.

Compile tested only.

Signed-off-by: Simon Horman <horms@...nel.org>
---
 drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c b/drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c
index 38f22918c699..440ff4616fec 100644
--- a/drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c
+++ b/drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c
@@ -266,7 +266,7 @@ static void aq_ethtool_get_strings(struct net_device *ndev,
 		const int rx_stat_cnt = ARRAY_SIZE(aq_ethtool_queue_rx_stat_names);
 		const int tx_stat_cnt = ARRAY_SIZE(aq_ethtool_queue_tx_stat_names);
 		char tc_string[8];
-		int tc;
+		unsigned int tc;

 		memset(tc_string, 0, sizeof(tc_string));
 		memcpy(p, aq_ethtool_stat_names,
@@ -275,7 +275,7 @@ static void aq_ethtool_get_strings(struct net_device *ndev,

 		for (tc = 0; tc < cfg->tcs; tc++) {
 			if (cfg->is_qos)
-				snprintf(tc_string, 8, "TC%d ", tc);
+				snprintf(tc_string, 8, "TC%u ", tc);

 			for (i = 0; i < cfg->vecs; i++) {
 				for (si = 0; si < rx_stat_cnt; si++) {