Re: [PATCH] linux/const.h: Explain how __is_constexpr() works

[Jani Nikula]

On Mon, 31 Jan 2022, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> The __is_constexpr() macro is dark magic. Shed some light on it with
> a comment to explain how and why it works.
>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Martin Uecker <Martin.Uecker@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
> Cc: Rikard Falkeborn <rikard.falkeborn@xxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> Jon, since this is pure comment, do you want to take it through the docs tree?
> ---
>  include/linux/const.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/include/linux/const.h b/include/linux/const.h
> index 435ddd72d2c4..7122d6a1f8ce 100644
> --- a/include/linux/const.h
> +++ b/include/linux/const.h
> @@ -7,6 +7,30 @@
>   * This returns a constant expression while determining if an argument is
>   * a constant expression, most importantly without evaluating the argument.
>   * Glory to Martin Uecker <Martin.Uecker@xxxxxxxxxxxxxxxxxxxxx>
> + *
> + * Details:
> + * - sizeof() is an integer constant expression, and does not evaluate the
> + *   value of its operand; it only examines the type of its operand.
> + * - The results of comparing two integer constant expressions is also
> + *   an integer constant expression.
> + * - The use of literal "8" is to avoid warnings about unaligned pointers;
> + *   these could otherwise just be "1"s.

I thought the first literal 8 was just for looks, and it mattered only
for the last literal 8.

It's been a while when I looked all of this up, but this pretty much
matches what I remember. LGTM.

BR,
Jani.


> + * - (long)(x) is used to avoid warnings about 64-bit types on 32-bit
> + *   architectures.
> + * - The C standard defines an "integer constant expression" as different
> + *   from a "null pointer constant" (an integer constant 0 pointer).
> + * - The conditional operator ("... ? ... : ...") returns the type of the
> + *   operand that isn't a null pointer constant. This behavior is the
> + *   central mechanism of the macro.
> + * - 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 *)".
> + * - sizeof(int) == 4 and sizeof(void) == 1.
> + * - The ultimate comparison to "sizeof(int)" chooses between either:
> + *     sizeof(*((int *) (8)) == sizeof(int)   (x was a constant expression)
> + *     sizeof(*((void *)(8)) == sizeof(void)  (x was not a constant expression)
>   */
>  #define __is_constexpr(x) \
>  	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

-- 
Jani Nikula, Intel Open Source Graphics Center