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

[David Laight]

From: Kees Cook
> Sent: 31 January 2022 20:44
> 
> The __is_constexpr() macro is dark magic. Shed some light on it with
> a comment to explain how and why it works.
> 
...
> 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.
> + * - (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)))

This has been making my head hurt all day.
The above isn't really a true description - ?: doesn't work that way.
Try the following for size:

- The conditional operator (?:) requires that both expressions have the
  the same type (after numeric promotions).
  The type of the result is a compile time constant and doesn't depend on any
  variables.
- If the expressions have distinct non-NULL pointer types then they are both
  cast to (void *) and the result has type 'void *'.
- A NULL pointer can be made from any integer constant expression that
  evaluates to 0, not just a literal 0.
- So the type of (0 ? (void *)(x) : (int *)8) is 'int *' if (x) is zero
  (because of the NULL) and (void *) otherwise because the pointer types
  don't match.

You can test this by evaluating:
	sizeof *(0 ? (float *)4 : (int *)4)
This is 1 because of the implicit (void *) cast.

I'd also delete the l from the 0l - it isn't needed.
(Or at least use L)

	David

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