linux-kernel - Re: [PATCH] linux/const.h: Explain how __is

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Date:   Wed, 2 Feb 2022 21:43:08 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Uecker <Martin.Uecker@....uni-goettingen.de>,
        Ingo Molnar <mingo@...nel.org>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] linux/const.h: Explain how __is_constexpr() works

On Mon, Jan 31, 2022 at 9:43 PM Kees Cook <keescook@...omium.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