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.
+ * - (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
