netdev - Re: Potential invalid ~ operator in net/mac80211/cfg.c

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Date:   Fri, 5 Feb 2021 18:20:11 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Potential invalid ~ operator in net/mac80211/cfg.c

On 05/02/2021 18:19, Colin Ian King wrote:
> On 05/02/2021 18:05, Johannes Berg wrote:
>> Hi Colin,
>>
>>> while working through a backlog of older static analysis reports from
>>> Coverity
>>
>> So ... yeah. Every time I look at Coverity (not frequently, I must
>> admit) I see the same thing, and get confused.
>>
>>> I found an interesting use of the ~ operator that looks
>>> incorrect to me in function ieee80211_set_bitrate_mask():
>>>
>>>                 for (j = 0; j < IEEE80211_HT_MCS_MASK_LEN; j++) {
>>>                         if (~sdata->rc_rateidx_mcs_mask[i][j]) {
>>>                                 sdata->rc_has_mcs_mask[i] = true;
>>>                                 break;
>>>                         }
>>>                 }
>>>
>>>                 for (j = 0; j < NL80211_VHT_NSS_MAX; j++) {
>>>                         if (~sdata->rc_rateidx_vht_mcs_mask[i][j]) {
>>>                                 sdata->rc_has_vht_mcs_mask[i] = true;
>>>                                 break;
>>>                         }
>>>                 }
>>>
>>> For the ~ operator in both if stanzas, Coverity reports:
>>>
>>> Logical vs. bitwise operator (CONSTANT_EXPRESSION_RESULT)
>>> logical_vs_bitwise:
>>>
>>> ~sdata->rc_rateidx_mcs_mask[i][j] is always 1/true regardless of the
>>> values of its operand. This occurs as the logical operand of if.
>>>     Did you intend to use ! rather than ~?
>>>
>>> I've checked the results of this and it does seem that ~ is incorrect
>>> and always returns true for the if expression. So it probably should be
>>> !, but I'm not sure if I'm missing something deeper here and wondering
>>> why this has always worked.
>>
>> But is it really always true?
>>
>> I _think_ it was intended to check that it's not 0xffffffff or
>> something?
>>
>> https://lore.kernel.org/linux-wireless/516C0C7F.3000204@openwrt.org/
>>
>> But maybe that isn't actually quite right due to integer promotion?
>> OTOH, that's a u8, so it should do the ~ in u8 space, and then compare
>> to 0 also?
> 
> rc_rateidx_vht_mcs_mask is a u64, so I think the expression could be
> expressed as:

oops, fat fingered that, it is a u16 not a u64

> 
> if ((uint16_t)~sdata->rc_rateidx_mcs_mask[i][j]) ..
> 
> this is only true if all the 16 bits in the mask are 0xffff
> 
>>
>> johannes
>>
>