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

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Message-ID: <aSTKLsRNiEKtDqPI@casper.infradead.org>
Date: Mon, 24 Nov 2025 21:12:14 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Kees Cook <kees@...nel.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 Mon, Nov 24, 2025 at 12:38:57PM -0800, Kees Cook wrote:
> For code like:
> 
> 	u8 size;
> 	...
> 	size = struct_size(ptr, flex_member, count);
> 	ptr = kmalloc(size, gfp);
> 
> While struct_size() is designed to deal with overflows beyond SIZE_MAX,
> it can't do anything about truncation of its return value since it has
> no visibility into the lvalue type. So this code pattern happily
> truncates, allocates too little memory, and then usually does stuff like
> runs a for-loop based on "count" instead of "size" and walks right off
> the end of the heap allocation, clobbering whatever follows it.

Have we investigated a compiler warning like
-Wimplicit-arithmetic-truncation that would complain about this kind of
thing and could be shut up by an explicit cast:

	size = (u8)struct_size(ptr, flex_member, count);

or arithmetic that can be proven to not overflow:
	size = struct_size(ptr, flex_member, count) & 0xff;

Maybe such a warning already exists and it's just too noisy to even
start thinking about turning it on?