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References: <20220131204357.1133674-1-keescook@chromium.org>
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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Wed, 2 Feb 2022 21:43:08 +0100
Message-ID: <CANiq72m+3UKC+PskuGYu_c_u1Ua-=bvkHi9Gess-59+pqXdSwA@mail.gmail.com>
Subject: Re: [PATCH] linux/const.h: Explain how __is_constexpr() works
To:     Kees Cook <keescook@chromium.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Martin Uecker <Martin.Uecker@med.uni-goettingen.de>,
        Ingo Molnar <mingo@kernel.org>,
        Rikard Falkeborn <rikard.falkeborn@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-hardening@vger.kernel.org
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On Mon, Jan 31, 2022 at 9:43 PM Kees Cook <keescook@chromium.org> wrote:
>
> + * - The conditional operator ("... ? ... : ...") returns the type of the
> + *   operand that isn't a null pointer constant. This behavior is the

Perhaps clarify that this happens only if it fits that case? ...

> + * - If (x) is an integer constant expression, then the "* 0l" resolves it
> + *   into a null pointer constant, which forces the conditional operator
> + *   to return the type of the last operand: "(int *)".
> + * - If (x) is not an integer constant expression, then the type of the
> + *   conditional operator is from the first operand: "(void *)".

... i.e. this one happens because it is specified as returning a
pointer to void (one could read it as returning the type of the first
operand).

What about something like:

  - The behavior (including its return type) of the conditional
operator ("... ? ... : ...") depends on the kind of expressions given
for the second and third operands. This is the central mechanism of
the macro.
  - If (x) is an integer constant expression, then the "* 0l" resolves
it into a null pointer constant. When one operand is a null pointer
constant and the other is a pointer, the conditional operator returns
the type of the pointer operand; that is, "int *".
  - If (x) is not an integer constant expression, then that operand is
a pointer to void (but not a null pointer constant). When one operand
is a pointer to void and the other a pointer to an object type, the
conditional operator returns a "void *" type.

Cheers,
Miguel