linux-kernel - Re: [net-dsa-mv88e6xxx] question about potential use of uninitialized variable

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Date:   Thu, 11 May 2017 21:48:14 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vivien Didelot <vivien.didelot@...oirfairelinux.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [net-dsa-mv88e6xxx] question about potential use of
 uninitialized variable

Hi Andrew,

Quoting Andrew Lunn <andrew@...n.ch>:

> On Thu, May 11, 2017 at 04:35:37PM -0500, Gustavo A. R. Silva wrote:
>>
>> Hello everybody,
>>
>> While looking into Coverity ID 1398130 I ran into the following
>> piece of code at drivers/net/dsa/mv88e6xxx/chip.c:849:
>>
>>  849static uint64_t _mv88e6xxx_get_ethtool_stat(struct mv88e6xxx_chip *chip,
>>  850                                            struct mv88e6xxx_hw_stat *s,
>>  851                                            int port, u16 bank1_select,
>>  852                                            u16 histogram)
>>  853{
>>  854        u32 low;
>>  855        u32 high = 0;
>>  856        u16 reg = 0;
>>  857        int err;
>>  858        u64 value;
>>  859
>>  860        switch (s->type) {
>>  861        case STATS_TYPE_PORT:
>>  862                err = mv88e6xxx_port_read(chip, port, s->reg, &reg);
>>  863                if (err)
>>  864                        return UINT64_MAX;
>>  865
>>  866                low = reg;
>>  867                if (s->sizeof_stat == 4) {
>>  868                        err = mv88e6xxx_port_read(chip, port,
>> s->reg + 1, &reg);
>>  869                        if (err)
>>  870                                return UINT64_MAX;
>>  871                        high = reg;
>>  872                }
>>  873                break;
>>  874        case STATS_TYPE_BANK1:
>>  875                reg = bank1_select;
>>  876                /* fall through */
>>  877        case STATS_TYPE_BANK0:
>>  878                reg |= s->reg | histogram;
>>  879                mv88e6xxx_g1_stats_read(chip, reg, &low);
>>  880                if (s->sizeof_stat == 8)
>>  881                        mv88e6xxx_g1_stats_read(chip, reg + 1, &high);
>>  882        }
>>  883        value = (((u64)high) << 16) | low;
>>  884        return value;
>>  885}
>>
>> My question here is if there is any chance for the execution path to
>> directly jump from line 860 to line 883, hence ending up using the
>> uninitialized variable _low_?
>
> Hi Gustavo
>
> It would require that s->type not have one of the listed case values.
> Currently all members of mv88e6xxx_hw_stats due use expected values.
> However, it would not hurt to add a
>
> 	 default:
> 		return UINT64_MAX;
>
> Do you want to submit a patch?
>

Sure, I'll send it shortly.

Thanks for clarifying!
--
Gustavo A. R. Silva