netdev - Re: [PATCH net] net: stmmac: prevent division by 0 in stmmac_init_tstamp

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Message-ID: <d5ba9af3-6e61-4206-9eb0-7d3349807d23@omp.ru>
Date: Mon, 8 Sep 2025 21:43:48 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, Andrew Lunn
	<andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>, Alexandre
 Torgue <alexandre.torgue@...s.st.com>, <netdev@...r.kernel.org>,
	<linux-stm32@...md-mailman.stormreply.com>
CC: <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH net] net: stmmac: prevent division by 0 in
 stmmac_init_tstamp_counter()

On 9/8/25 9:26 PM, Sergey Shtylyov wrote:
[...]

>>> In stmmac_init_tstamp_counter(), the sec_inc variable is initialized to 0,
>>> and if stmmac_config_sub_second_increment() fails to set it to some non-0
>>
>> How that can happen?
> 
>    Let's see what the commit in my Fixes tag said about the problem it fixed:
> 
> ---
> When building with -Wsometimes-uninitialized, Clang warns:
> 
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:495:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:532:3: warning: variable 'ns' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:741:3: warning: variable 'sec_inc' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
> 
> Clang is concerned with the use of stmmac_do_void_callback (which
> stmmac_get_timestamp and stmmac_config_sub_second_increment wrap),
> as it may fail to initialize these values if the if condition was ever
> false (meaning the callbacks don't exist). It's not wrong because the
> callbacks (get_timestamp and config_sub_second_increment respectively)
> are the ones that initialize the variables. While it's unlikely that the
> callbacks are ever going to disappear and make that condition false, we
> can easily avoid this warning by zero initialize the variables.
> ---
> 
>    I think the original commit was just somewhat incomplete, as (adding 0-
> initializer into picture) it missed to add checking of sec_inc for 0 before
> invoking do_div()...

   Oops, it was div_u64()! :-)

MBR, Sergey