netdev - Re: net: phy: continue searching for C45 MMDs even if first returned ffff:ffff

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Message-ID: <20200720163121.sihkkncthvwnfqd7@skbuf>
Date:   Mon, 20 Jul 2020 19:31:21 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: net: phy: continue searching for C45 MMDs even if first returned
 ffff:ffff

On Mon, Jul 20, 2020 at 05:21:13PM +0100, Colin Ian King wrote:
> Hi,
> 
> Static analysis by Coverity has found a potential issue with the
> following commit in /drivers/net/phy/phy_device.c:
> 
> commit bba238ed037c60242332dd1e4c5778af9eba4d9b
> Author: Vladimir Oltean <vladimir.oltean@....com>
> Date:   Sun Jul 12 19:48:15 2020 +0300
> 
>     net: phy: continue searching for C45 MMDs even if first returned
> ffff:ffff
> 
> The analysis is as follows:
> 
> 735         * for 802.3 c45 complied PHYs, so don't probe it at first.
> 736         */
> 
> dead_error_condition: The condition (devs_in_pkg & 0x1fffffffU) ==
> 0x1fffffffU cannot be true.
> 
> 737        for (i = 1; i < MDIO_MMD_NUM && devs_in_pkg == 0 &&
> 
> const: At condition (devs_in_pkg & 0x1fffffffU) == 0x1fffffffU, the
> value of devs_in_pkg must be equal to 0.
> 
> 738             (devs_in_pkg & 0x1fffffff) == 0x1fffffff; i++) {
> 
> Logically dead code (DEADCODE)dead_error_line: Execution cannot reach
> this statement: if (i == 30 || i == 31) {
> 
> To summarize, if devs_in_pkg is zero, then (devs_in_pkg & 0x1fffffffU)
> == 0x1fffffffU can never be true, so the loop is never iterated over.
> 
> Colin

You are absolutely correct. The check should have been || and not &&.
I have a patch in my tree where I am fixing that. I was giving it some
more thorough testing to understand why it was working, though, and how
I could've missed it. One hypothesis I can't rule out is that I tested
it using || but submitted it using && somehow (although I don't remember
doing that).

Thanks,
-Vladimir