linux-kernel - Re: [PATCH v5 2/4] slab: Introduce kmalloc

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Message-ID: <202511241940.8278C53C@keescook>
Date: Mon, 24 Nov 2025 19:47:32 -0800
From: Kees Cook <kees@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	"Gustavo A . R . Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Jann Horn <jannh@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Marco Elver <elver@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org,
	Randy Dunlap <rdunlap@...radead.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Harry Yoo <harry.yoo@...cle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Jonathan Corbet <corbet@....net>, Jakub Kicinski <kuba@...nel.org>,
	Yafang Shao <laoar.shao@...il.com>,
	Tony Ambardar <tony.ambardar@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Jan Hendrik Farr <kernel@...rr.cc>,
	Alexander Potapenko <glider@...gle.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-doc@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v5 2/4] slab: Introduce kmalloc_obj() and family

On Tue, Nov 25, 2025 at 01:09:42AM +0000, Matthew Wilcox wrote:
> so the Rust language people have clearly decided that this is too
> complicated for your average programmer to figure out, and you need
> explicit casts to make it work.

Yup, and this is correct: "intent" becomes explicit. Doing implicit
extension, narrowing, and truncation is just a mess. As part of the
Overflow Behavior Type work, Justin has also been looking at something
that would allow a typedef to not be allowed to be mixed with other
types (i.e. to do so requires explicit casts). For example:

__strong typedef unsigned short only_u16;
...
only_u16 foo;
int bar = ...;
...
foo = bar; // Build failure: mismatched types
foo = (u16)bar; // okay, explicit truncation

-Kees

-- 
Kees Cook