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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 20 Mar 2019 13:50:59 -0700
Message-ID: <CAKwvOd=6916txvLmtEC3BcZgjLTNdtU-ou=YM+_dsd8CC-T_zw@mail.gmail.com>
Subject: Re: -Wsometimes-uninitialized Clang warning in net/tipc/node.c
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Jon Maloy <jon.maloy@ericsson.com>,
        Ying Xue <ying.xue@windriver.com>,
        "David S. Miller" <davem@davemloft.net>,
        tipc-discussion@lists.sourceforge.net, netdev@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux@googlegroups.com
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On Wed, Mar 20, 2019 at 12:07 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Thu, Mar 07, 2019 at 05:17:23PM -0700, Nathan Chancellor wrote:
> > Hi all,
> >
> > We are trying to get Clang's -Wsometimes-uninitialized turned on for the
> > kernel as it can catch some bugs that GCC can't. This warning came up:
> >
> > net/tipc/node.c:831:6: warning: variable 'maddr' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> >         if (!tipc_link_is_establishing(l)) {
> >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > net/tipc/node.c:847:46: note: uninitialized use occurs here
> >         tipc_bearer_xmit(n->net, bearer_id, &xmitq, maddr);
> >                                                     ^~~~~
> > net/tipc/node.c:831:2: note: remove the 'if' if its condition is always true
> >         if (!tipc_link_is_establishing(l)) {
> >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > net/tipc/node.c:821:31: note: initialize the variable 'maddr' to silence this warning
> >         struct tipc_media_addr *maddr;
> >                                      ^
> >                                       = NULL
> > 1 warning generated.
> >
> > This definitely appears to be a legitimate warning but I'm not sure of
> > the proper solution (should maddr be initialized to NULL or should it be
> > set to something different in the else branch). Your input would be
> > greatly appreciated.
> >
> > Cheers,
> > Nathan
>
> Gentle ping (if there was a response to this, I didn't receive it). I
> know I sent it in the middle of a merge window so I get if it slipped
> through the cracks.
>
> Thanks,
> Nathan

The use in tipc_bearer_xmit() isn't even guaranteed to set the in/out
parameter, so even if the if is taken doesn't guarantee that maddr is
always initialized before calling tipc_bearer_xmit().

At the minimum, we should initialize maddr to NULL.  I think we'd
prefer to risk the possibility of a null pointer dereference to the
possibility of working with uninitialized memory.  To be clear, both
are bad, but one is easier to spot/debug later than the other.

Thanks for bringing this up for discussion, sorry I missed it before.
-- 
Thanks,
~Nick Desaulniers