[PATCH] net: stmmac: Fix possible division by zero

Fix for divide by zero error reported by Coverity.

Addresses-Coverity: 1494557 ("Division or modulo by zero")

Signed-off-by: Ameer Hamza <redacted>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index da8306f60730..f44400323407 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -863,7 +863,7 @@ int stmmac_init_tstamp_counter(struct stmmac_priv *priv, u32 systime_flags)
 	stmmac_config_sub_second_increment(priv, priv->ptpaddr,
 					   priv->plat->clk_ptp_rate,
 					   xmac, &sec_inc);
-	temp = div_u64(1000000000ULL, sec_inc);
+	temp = div_u64(1000000000ULL, (sec_inc > 0) ? sec_inc : 1);
 
 	/* Store sub second increment for later use */
 	priv->sub_second_inc = sec_inc;

-- 
2.25.1

On Fri,  3 Dec 2021 03:37:29 +0500 Ameer Hamza wrote:

Fix for divide by zero error reported by Coverity.

Addresses-Coverity: 1494557 ("Division or modulo by zero")

Signed-off-by: Ameer Hamza <redacted>

Please include justification that this issue can actually happen, Fixes
tag pointing to the commit which introduced the problem, and CC the
author and reviewers of that commit.

In stmmac_init_tstamp_counter() routine, there is a possiblity of division
by zero. If priv->plat->clk_ptp_rate becomes greater than 1 GHz,
config_sub_second_increment() subroutine may calculate sec_inc as zero
depending upon the PTP_TCR_TSCFUPDT register value, which will cause
divide by zero exception.

Fixes: a6da2bbb0005e ("net: stmmac: retain PTP clock time during SIOCSHWTSTAMP ioctls")
Addresses-Coverity: 1494557 ("Division or modulo by zero")
Signed-off-by: Ameer Hamza <redacted>

---
Changes in v2:
Added fix tag, bug justification, and commit author.
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index da8306f60730..f44400323407 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -863,7 +863,7 @@ int stmmac_init_tstamp_counter(struct stmmac_priv *priv, u32 systime_flags)
 	stmmac_config_sub_second_increment(priv, priv->ptpaddr,
 					   priv->plat->clk_ptp_rate,
 					   xmac, &sec_inc);
-	temp = div_u64(1000000000ULL, sec_inc);
+	temp = div_u64(1000000000ULL, (sec_inc > 0) ? sec_inc : 1);
 
 	/* Store sub second increment for later use */
 	priv->sub_second_inc = sec_inc;

-- 
2.25.1

On Mon, Dec 06, 2021 at 07:23:37PM +0500, Ameer Hamza wrote:

It can be written cleanly with max(a,b):

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 4e05c1d92935..e2e232bff511 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -855,7 +855,7 @@ int stmmac_init_tstamp_counter(struct stmmac_priv *priv, u32 systime_flags)
        stmmac_config_sub_second_increment(priv, priv->ptpaddr,
                                           priv->plat->clk_ptp_rate,
                                           xmac, &sec_inc);
-       temp = div_u64(1000000000ULL, sec_inc);
+       temp = div_u64(1000000000ULL, max(sec_inc, 1));
 
        /* Store sub second increment for later use */
        priv->sub_second_inc = sec_inc;

 
 	/* Store sub second increment for later use */
 	priv->sub_second_inc = sec_inc;
-- 
2.25.1