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

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Date:   Fri, 5 Feb 2021 17:38:47 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Johannes Berg <johannes.berg@...el.com>
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: Potential invalid ~ operator in net/mac80211/cfg.c

Hi there,

while working through a backlog of older static analysis reports from
Coverity 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.

Colin