[PATCH v4 4/6] stddef: Add const_cast()

[Alejandro Colomar <alx@kernel.org>]

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This macro serves to cast const away without risking unwanted
conversions due to mistakes in the cast.

It's similar to C++'s const_cast(), but it is more restrictive: it can
only cast away 'const', and not arbitrary qualifiers.

David made it a bit more robust.

Link: https://lore.kernel.org/linux-hardening/20261111102535.434c2eee@pumpkin/
Co-developed-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 include/linux/stddef.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index e1851c50c89b..5201d71db387 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -194,4 +194,16 @@ enum {
 #define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS)				\
 	__TRAILING_OVERLAP(TYPE, NAME, FAM, /* no attrs */, MEMBERS)
 
+/**
+ * const_cast() - At most, drop const in the left-most derivation of a pointer
+ *
+ * @T: Type for the cast.
+ * @p: Pointer to be casted.
+ *
+ * If @p is of type const @T, then it is casted to @T.
+ * If @p is of type @T, then it is returned without any casts.
+ * This avoids mistakes that change anything other than 'const' in a cast.
+ */
+#define const_cast(T, p)  _Generic(p, const T: (T) (p), T: (p))
+
 #endif
-- 
2.55.0


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