linux-kernel - Re: [PATCH] Staging: rtlwifi: Remove NULL pointer dereference

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Message-ID: <1507920904.2087.2.camel@gmail.com>
Date:   Sat, 14 Oct 2017 00:25:04 +0530
From:   Shreeya Patel <shreeya.patel23498@...il.com>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Staging: rtlwifi: Remove NULL pointer dereference

On Thu, 2017-10-12 at 13:16 +1100, Tobin C. Harding wrote:
> On Wed, Oct 11, 2017 at 06:02:47PM +0530, Shreeya Patel wrote:
> > 
> > On Tue, 2017-10-10 at 11:06 +1100, Tobin C. Harding wrote:
> > > 
> > > On Tue, Oct 10, 2017 at 02:48:58AM +0530, Shreeya Patel wrote:
> > > > 
> > > > 
> > > > Remove NULL pointer dereference as it results in undefined
> > > > behaviour, and will usually lead to a runtime error.
> > > The diff does not show any pointer dereference so it is hard to
> > > understand what you are trying to do
> > > with this patch.
> > > 
> > > > 
> > > > 
> > > > Signed-off-by: Shreeya Patel <shreeya.patel23498@...il.com>
> > > > ---
> > > >  drivers/staging/rtlwifi/base.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/staging/rtlwifi/base.c
> > > > b/drivers/staging/rtlwifi/base.c
> > > > index b88b0e8..5bb8f98 100644
> > > > --- a/drivers/staging/rtlwifi/base.c
> > > > +++ b/drivers/staging/rtlwifi/base.c
> > > > @@ -781,7 +781,7 @@ static void _rtl_txrate_selectmode(struct
> > > > ieee80211_hw *hw,
> > > >  
> > > >  	struct rtl_priv *rtlpriv = rtl_priv(hw);
> > > >  	struct rtl_mac *mac = rtl_mac(rtl_priv(hw));
> > > > -	struct rtl_sta_info *sta_entry = NULL;
> > > > +	struct rtl_sta_info *sta_entry;
> > > Now the pointer just has garbage in it instead of the testable
> > > value
> > > of NULL. If you are concerned
> > > with the dereference perhaps you could add a NULL check, again
> > > it's
> > > hard to say without seeing the
> > > code.
> > Hello, 
> > 
> > Thanks for making me understand. 
> > 
> > Here is the code after declaration and initialization of
> > sta_entry. 
> > Will it be good to add a NULL check in this case? 
> > 
> > struct rtl_sta_info *sta_entry = NULL;
> > 	u8 ratr_index = SET_RATE_ID(RATR_INX_WIRELESS_MC);
> > 
> > 	if (sta) {
> > 		sta_entry = (struct rtl_sta_info *)sta->drv_priv;
> > 		ratr_index = sta_entry->ratr_index;
> > 	}
> Later in this function the macro SET_RATE_ID() is called, it relies
> on sta_entry being NULL if it
> was not explicitly set.
> 
> Here is the macro;
> 
> #define SET_RATE_ID(rate_id)					\
> 	((rtlpriv->cfg->spec_ver & RTL_SPEC_NEW_RATEID) ?	\
> 		rtl_mrate_idx_to_arfr_id(hw, rate_id,		\
> 			(sta_entry ? sta_entry->wireless_mode :	
> \
> 			 WIRELESS_MODE_G)) :			\
> 		rate_id)
> 
> > 
> > If we are making a pointer point to NULL then what if any other
> > variable is already pointing to NULL for some other purpose.
> > Instead, removing initialization will be good right?
> A pointer does not _point_ to NULL as such. A NULL pointer has a
> value of all zero bytes. Have you
> read (and completed all the exercises) in KnR
> 
> https://en.wikipedia.org/wiki/The_C_Programming_Language
> 
> It is, in my opinion, one of the best tech books ever written. If you
> have any holes in your C
> knowledge, this is the place to start.

Thank you so much.
I will make sure that I don't make the same mistake again.


> 
> Good luck,
> Tobin.