RE: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta

-----Original Message-----
From: Dan Carpenter <redacted>
Sent: Wednesday, November 3, 2021 6:21 PM
To: Pkshih <pkshih@realtek.com>
Cc: Colin King <redacted>; Kalle Valo <redacted>; David S . Miller
[off-list ref]; Jakub Kicinski [off-list ref]; linux-wireless@vger.kernel.org;
netdev@vger.kernel.org; kernel-janitors@vger.kernel.org; linux-kernel@vger.kernel.org
Subject: Re: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta

On Wed, Nov 03, 2021 at 12:36:17AM +0000, Pkshih wrote:

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diff --git a/drivers/net/wireless/realtek/rtw89/core.c

b/drivers/net/wireless/realtek/rtw89/core.c
index 06fb6e5b1b37..26f52a25f545 100644

--- a/drivers/net/wireless/realtek/rtw89/core.c
+++ b/drivers/net/wireless/realtek/rtw89/core.c

@@ -1534,9 +1534,14 @@ static bool rtw89_core_txq_agg_wait(struct rtw89_dev *rtwdev,
 {
 	struct rtw89_txq *rtwtxq = (struct rtw89_txq *)txq->drv_priv;
 	struct ieee80211_sta *sta = txq->sta;
-	struct rtw89_sta *rtwsta = (struct rtw89_sta *)sta->drv_priv;

'sta->drv_priv' is only a pointer, we don't really dereference the
data right here, so I think this is safe. More, compiler can optimize
this instruction that reorder it to the place just right before using.
So, it seems like a false alarm.

The warning is about "sta" not "sta->priv".  It's not a false positive.

I have heard discussions about compilers trying to work around these
bugs by re-ordering the code.  Is that an option in GCC?  It's not
something we should rely on, but I'm just curious if it exists in
released versions.

I say GCC does "reorder" the code, because the object codes of following
two codes are identical with default or -Os ccflags.

Huh...  That's cool.  GCC doesn't re-order it for me, but I'm on GCC 8
so maybe it will work when I get to a more modern version.

My GCC is 9.3.0. 
But, I don't try other versions.

--
Ping-Ke