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

[Rasmus Villemoes]

On 31/01/2022 21.43, Kees Cook 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.

Just the second 8, the first could be a 0 or 12345 or whatever.

> + * - (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).

I don't see the point of this bullet. Yes, an ICE is a distinct concept
from a null pointer constant, obviously. One is defined in terms of the
other - and your parenthesis is not an accurate paraphrase of the
definition of a null pointer constant.

> + * - 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

yes, because then it becomes "An integer constant expression with the
value 0,".

, 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 *)".

Not entirely correct (and by "first" you probably meant second). It's
better to just quote chapter-and-verse.

C99, 6.5.15.6:

[...]
if one operand is a
null pointer constant, the result has the type of the other operand;
otherwise, one operand
is a pointer to void or a qualified version of void, in which case the
result type is a
pointer to an appropriately qualified version of void.

I.e., the second and third operands are treated symmetrically in the
standard.

> + * - 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)

Actually, since the first operand (the condition) is a non-zero number,
the _value_ of the whole expression is the value of the _second_
operand, but with a _type_ determined by the above rules. So the whole
ternary operator evalutes to either

  (void *)((void *)((long)(x) * 0l))

or

  (int *)((void *)((long)(x) * 0l))


I don't think adding slightly inaccurate comments would help a future
reader at all. Then it's better to just stick to Linus' "it's art, and
art should not be explained".

Rasmus