[PATCH 5/5] types: Add standard __ob_trap and __ob_wrap scalar types

[Kees Cook <kees@kernel.org>]

While Linux's use of -fno-strict-overflow means that all arithmetic
operations have a defined behavior (2's-complement wrapping), there
isn't a way to unambiguously specify if a given variable was designed
or intended to wrap around by the author.

Introduce explicit trapping and wrapping types for all bit widths
including architecture word length (i.e. "long"), signed and unsigned,
for use going forward for unambiguous arithmetic, now available via
Clang 23+'s Overflow Behavior Types[1] (CONFIG_OVERFLOW_BEHAVIOR_TYPES=y).

Bike shedding time! How should these be named? We already have the short
bit width types, named as: {u,s}{8,16,32,64}. We need to construct new
type names that also indicate their overflow behavior: "trapping" or
"wrapping". And we need to capture the "architectural word" length type
too (i.e. what "unsigned long" or "size_t" captures).

Whole word addition:
- Pro: Unambiguous
- Con: Long. E.g. suffixed "u16_trap", or prefixed "wrap_u16"

Single letter addition, "t" for "trap" and "w" for "wrap":
- At the end: but "u8t" looks like the "t" is "type", like "uint8_t".
- At the front: but "wu8" looks like the "w" is "wide", like "wchar_t".

Current straw-man proposal is single letter suffix because it vaguely
felt like the least bad of all choices, and they should be short or
everyone will just continue to type "int". :)

Link: https://clang.llvm.org/docs/OverflowBehaviorTypes.html [1]
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Justin Stitt <justinstitt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Finn Thain <fthain@linux-m68k.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: "Thomas Weißschuh" <thomas.weissschuh@linutronix.de>
Cc: <llvm@lists.linux.dev>
---
 include/linux/types.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/types.h b/include/linux/types.h
index 7e71d260763c..786eb2c9775f 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -94,6 +94,30 @@ typedef unsigned int		uint;
 typedef unsigned long		ulong;
 typedef unsigned long long	ullong;
 
+/* Trapping types. */
+typedef u8 __ob_trap		u8t;
+typedef u16 __ob_trap		u16t;
+typedef u32 __ob_trap		u32t;
+typedef u64 __ob_trap		u64t;
+typedef unsigned long __ob_trap	ulongt;
+typedef s8 __ob_trap		s8t;
+typedef s16 __ob_trap		s16t;
+typedef s32 __ob_trap		s32t;
+typedef s64 __ob_trap		s64t;
+typedef signed long __ob_trap	slongt;
+
+/* Wrapping types. */
+typedef u8 __ob_wrap		u8w;
+typedef u16 __ob_wrap		u16w;
+typedef u32 __ob_wrap		u32w;
+typedef u64 __ob_wrap		u64w;
+typedef unsigned long __ob_wrap	ulongw;
+typedef s8 __ob_wrap		s8w;
+typedef s16 __ob_wrap		s16w;
+typedef s32 __ob_wrap		s32w;
+typedef s64 __ob_wrap		s64w;
+typedef signed long __ob_wrap	slongw;
+
 #ifndef __BIT_TYPES_DEFINED__
 #define __BIT_TYPES_DEFINED__
 
-- 
2.34.1