linux-kernel - Re: [PATCH] staging: r8188eu: fix NULL check for rcu pointer

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Message-ID: <Y9kxkhmS+yEs85s1@kadam>
Date:   Tue, 31 Jan 2023 18:19:46 +0300
From:   Dan Carpenter <error27@...il.com>
To:     Martin Kaiser <martin@...ser.cx>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Michael Straube <straube.linux@...il.com>,
        Pavel Skripkin <paskripkin@...il.com>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: r8188eu: fix NULL check for rcu pointer

[ Resending because my email setup is crap and silently eats my out
  going mail. ]

On Tue, Jan 31, 2023 at 10:00:57AM +0100, Martin Kaiser wrote:
> Fix the NULL check for padapter->pnetdev->rx_handler_data.
> 
> The current code calls rcu_dereference while it holds the rcu read lock
> and checks the pointer after releasing the lock. An rcu pointer may only be
> used between calls to rcu_read_lock and rcu_read_unlock.
> 
> Replace the check with rcu_access_pointer. My understanding is that this
> function returns the value of the pointer and needs no locking. We can
> then check the pointer but we must not dereference it.
> 
> Signed-off-by: Martin Kaiser <martin@...ser.cx>

This patch is fine but it's a clean up and not a fix.  The original code
doesn't dereference "br_port".  I guess the new code is faster and
especially if you have lockdep enabled, so maybe in that sense it is a
fix.

regards,
dan carpenter