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

From: Andrew Lunn <[email-masked]>
Date: Fri, 12 May 2017 04:33:33 +0200

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?

   Thanks
	Andrew