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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 1 Mar 2022 13:15:06 -0800
Message-ID: <CAHk-=wiuZGzc2UaNVPr6rZnK7buvaQWfadZMcDXavE=MeCXw3g@mail.gmail.com>
Subject: Re: [PATCH 1/6] Kbuild: compile kernel with gnu11 std
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     kernel test robot <lkp@intel.com>,
        Xiaomeng Tong <xiam0nd.tong@gmail.com>,
        kbuild-all@lists.01.org, Jakob Koschel <jakobkoschel@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Netdev <netdev@vger.kernel.org>
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On Tue, Mar 1, 2022 at 1:04 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Yeah, looks like that "<< 16" is likely just wrong.

.. and perhaps more importantly, I guess that means that -Wshift-overflow is

 (a) somehow new to -std=gnu11

 (b) possibly a lot more relevant and good than that
-Wshift-negative-value thing was

doing some grepping, it seems like we have never had that
'-Wshift-overflow' even in any extra warnings.

And trying it myself (keeping -std=gnu89), enabling it doesn't report
anything on a x86-64 allmodconfig build.

So I think this is likely a good new warning that -std=gnu11 brought
in by accident. No false positives that I can see, and one report for
a MIPS bug that looks real (but admittedly not a "sky-is-falling" one
;)

There's apparently a '-Wshift-overflow=2' mode too, but that warns
about things that change the sign bit, ie expressions like

        1<<31

warns.

And I would not be in the least surprised if we had a ton of those
kinds of things all over (but I didn't check).

So the plain -Wshift-overflow seems to work just fine, and while it's
surprising that it got enabled by gnu11, I think it's all good.

Famous last words.

                     Linus