netdev - Re: [PATCH] net: wireguard: avoid unused variable warning

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Date:   Tue, 5 May 2020 22:51:35 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] net: wireguard: avoid unused variable warning

On Tue, May 5, 2020 at 10:07 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> On Tue, May 5, 2020 at 8:13 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > clang points out a harmless use of uninitialized variables that
> > get passed into a local function but are ignored there:
> >
> > In file included from drivers/net/wireguard/ratelimiter.c:223:
> > drivers/net/wireguard/selftest/ratelimiter.c:173:34: error: variable 'skb6' is uninitialized when used here [-Werror,-Wuninitialized]
> >                 ret = timings_test(skb4, hdr4, skb6, hdr6, &test_count);
> >                                                ^~~~
> > drivers/net/wireguard/selftest/ratelimiter.c:123:29: note: initialize the variable 'skb6' to silence this warning
> >         struct sk_buff *skb4, *skb6;
> >                                    ^
> >                                     = NULL
> > drivers/net/wireguard/selftest/ratelimiter.c:173:40: error: variable 'hdr6' is uninitialized when used here [-Werror,-Wuninitialized]
> >                 ret = timings_test(skb4, hdr4, skb6, hdr6, &test_count);
> >                                                      ^~~~
> > drivers/net/wireguard/selftest/ratelimiter.c:125:22: note: initialize the variable 'hdr6' to silence this warning
> >         struct ipv6hdr *hdr6;
> >                             ^
>
> Seems like the code is a bit easier to read and is more uniform
> looking by just initializing those two variables to NULL, like the
> warning suggests. If you don't mind, I'll queue something up in my
> tree to this effect.

I think we really should fix clang to not make this suggestion, as that
normally leads to worse code ;-)

The problem with initializing variables to NULL (or 0) is that it hides
real bugs when the NULL initialization end up being used in a place
where a non-NULL value is required, so I try hard not to send patches
that add those.

It's your code though, so if you prefer to do it that way, just do that
and add me as "Reported-by:"

> By the way, I'm having a bit of a hard time reproducing the warning
> with either clang-10 or clang-9. Just for my own curiosity, would you
> mind sending the .config that results in this?

See https://pastebin.com/6zRSUYax

       Arnd