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

[Kees Cook <keescook@chromium.org>]

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@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
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@med.uni-goettingen.de>
+ *
+ * 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)))
-- 
2.30.2