linux-kernel - Re: [PATCH] staging: rtl8192u: Remove an unnecessary NULL check

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Message-ID: <CAKwvOdmpHOMwVM+d_W3eeu3xC+nZqBTO_hx9Wf1z10yivxSe7A@mail.gmail.com>
Date:   Tue, 21 May 2019 15:57:46 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        John Whitmore <johnfwhitmore@...il.com>,
        devel@...verdev.osuosl.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Richard Smith <richardsmith@...gle.com>
Subject: Re: [PATCH] staging: rtl8192u: Remove an unnecessary NULL check

+ Richard for real this time.

On Tue, May 21, 2019 at 3:53 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, May 21, 2019 at 10:42 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Clang warns:
> >
> > drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:2663:47: warning:
> > address of array 'param->u.wpa_ie.data' will always evaluate to 'true'
> > [-Wpointer-bool-conversion]
> >             (param->u.wpa_ie.len && !param->u.wpa_ie.data))
> >                                     ~~~~~~~~~~~~~~~~~^~~~
> >
> > This was exposed by commit deabe03523a7 ("Staging: rtl8192u: ieee80211:
> > Use !x in place of NULL comparisons") because we disable the warning
> > that would have pointed out the comparison against NULL is also false:
> >
> > drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:2663:46: warning:
> > comparison of array 'param->u.wpa_ie.data' equal to a null pointer is
> > always false [-Wtautological-pointer-compare]
> >             (param->u.wpa_ie.len && param->u.wpa_ie.data == NULL))
> >                                     ~~~~~~~~~~~~~~~~^~~~    ~~~~
> >
> > Remove it so clang no longer warns.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/487
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c b/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
> > index f38f9d8b78bb..e0da0900a4f7 100644
> > --- a/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
> > +++ b/drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c
> > @@ -2659,8 +2659,7 @@ static int ieee80211_wpa_set_wpa_ie(struct ieee80211_device *ieee,
> >  {
> >         u8 *buf;
> >
> > -       if (param->u.wpa_ie.len > MAX_WPA_IE_LEN ||
> > -           (param->u.wpa_ie.len && !param->u.wpa_ie.data))
>
> Right so, the types in this expression:
>
> param: struct ieee_param*
> param->u: *anonymous union*
> param->u.wpa_ie: *anonymous struct*
> param->u.wpa_ie.len: u32
> param->u.wpa_ie.data: u8 [0]
> as defined in drivers/staging/rtl8192u/ieee80211/ieee80211.h#L295
> https://github.com/ClangBuiltLinux/linux/blob/9c7db5004280767566e91a33445bf93aa479ef02/drivers/staging/rtl8192u/ieee80211/ieee80211.h#L295-L322
>
> so this is a tricky case, because in general array members can never
> themselves be NULL, and usually I trust -Wpointer-bool-conversion, but
> this is a special case because of the flexible array member:
> https://en.wikipedia.org/wiki/Flexible_array_member. (It seems that
> having the 0 in the length explicitly was pre-c99 GNU extension:
> https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html). I wonder if
> -Wtautological-pointer-compare applies to Flexible Array Members or
> not (Richard, do you know)?  In general, you'd be setting
> param->u.wpa_ie to the return value of a dynamic memory allocation,
> not param->u.wpa_ie.data, so this check is fishy to me.
>
> > +       if (param->u.wpa_ie.len > MAX_WPA_IE_LEN)
> >                 return -EINVAL;
> >
> >         if (param->u.wpa_ie.len) {
> > --
> > 2.22.0.rc1
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers