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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 30 Jun 2023 09:01:54 -0700
Message-ID: <CAHk-=whY5wuS0QRtEym4d8D2hrNE3pSL9LfoHpSOUKHb2sZtjA@mail.gmail.com>
Subject: Re: [GIT PULL] pid: use flex array
To:     David Laight <David.Laight@aculab.com>
Cc:     Christian Brauner <brauner@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, 30 Jun 2023 at 04:21, David Laight <David.Laight@aculab.com> wrote:
>
> The bit pattern used for a NULL pointer is implementation defined.

Oh, pretty much everything we do in the kernel is implementation defined.

And yes, we do indeed basically rely on the bit pattern for NULL being
all zeroes, and that example I gave with PCI_IOBASE is just one of
many.

And yes, compilers will sometimes complain about the things we do.

On s390, for example, the low memory range is special (and the kernel
mapping is special), so s390 uses all these pointers that are
*literally* small constants, and we've had to turn off some compiler
warnings because gcc would think that we're doing small offsets off
NULL.

But "implementation defined" is actually the good case.

The problem case is "undefined", when we sometimes want to do exactly
those kinds of things. We try to generally avoid it, but we sometimes
end up using compiler switches to say "turn off that part of your
logic" (strict overflow comes to mind).

             Linus