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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 23 Feb 2022 11:23:39 -0800
Message-ID: <CAHk-=wgLe-OSLTEHm=V7eRG6Fcr0dpAM1ZRV1a=R_g6pBOr8Bg@mail.gmail.com>
Subject: Re: [RFC PATCH 03/13] usb: remove the usage of the list iterator
 after the loop
To:     Jakob <jakobkoschel@gmail.com>, Arnd Bergmann <arnd@arndb.de>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Mike Rapoport <rppt@kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>
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On Wed, Feb 23, 2022 at 10:47 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Arnd - remind me, please.. Was there some other problem than just gcc-4.9?

Hmm. Interesting. I decided to just try it and see for the compiler I
have, and changing the gnu89 to gnu99 I get new warnings
(-Werror=shift-negative-value).

Very annoying.  Especially since negative values are in many contexts
actually *safer* than positive ones when used as a mask, because as
long as the top bit is set in 'int', if the end result is then
expanded to some wider type, the top bit stays set.

Example:

  unsigned long mask(unsigned long x)
  { return x & (~0 << 5); }

  unsigned long badmask(unsigned long x)
  { return x & (~0u << 5); }

One does it properly, the other is buggy.

Now, with an explicit "unsigned long" like this, some clueless
compiler person  might say "just use "~0ul", but that's completely
wrong - because quite often the type is *not* this visible, and the
signed version works *regardless* of type.

So this Werror=shift-negative-value warning seems to be actively
detrimental, and I'm not seeing the reason for it. Can somebody
explain the thinking for that stupid warning?

That said, we seem to only have two cases of it in the kernel, at
least by a x86-64 allmodconfig build. So we could examine the types
there, or we could just add '-Wno-shift-negative-value".

               Linus