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

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Date:   Tue, 21 May 2019 15:53:32 -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>,
        rsmith@...gle.com
Subject: Re: [PATCH] staging: rtl8192u: Remove an unnecessary NULL check

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