linux-hardening - Re: [PATCH v6 1/5] slab: Introduce kmalloc

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Message-ID: <aZbCT3-q3O4M5osr@stanley.mountain>
Date: Thu, 19 Feb 2026 10:57:03 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Kees Cook <kees@...nel.org>
Cc: 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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Matthew Wilcox <willy@...radead.org>,
	John Hubbard <jhubbard@...dia.com>, Joe Perches <joe@...ches.com>,
	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 v6 1/5] slab: Introduce kmalloc_obj() and family

On Fri, Jan 09, 2026 at 09:31:03AM -0800, Kees Cook wrote:
> On Thu, Jan 08, 2026 at 03:01:00PM +0100, Vlastimil Babka wrote:
> > On 12/4/25 00:30, Kees Cook wrote:
> > > [...]
> > > +/**
> > > + * __alloc_objs - Allocate objects of a given type using
> > > + * @KMALLOC: which size-based kmalloc wrapper to allocate with.
> > > + * @GFP: GFP flags for the allocation.
> > > + * @TYPE: type to allocate space for.
> > > + * @COUNT: how many @TYPE objects to allocate.
> > > + *
> > > + * Returns: Newly allocated pointer to (first) @TYPE of @COUNT-many
> > > + * allocated @TYPE objects, or NULL on failure.
> > > + */
> > > +#define __alloc_objs(KMALLOC, GFP, TYPE, COUNT)				\
> > > +({									\
> > > +	const size_t __obj_size = size_mul(sizeof(TYPE), COUNT);	\
> > 
> > I assume with the hardcoded 1 for COUNT, this size_mul() will be eliminated
> > by the compiler and not add unnecessary runtime overhead? Otherwise we
> > should have two core #define variants.
> 
> You're correct: the compiler completely collapses it with 0 runtime
> overhead; a variant is not needed.
> 
> > I also noted that the existing kmalloc_array() and kvmalloc_array() do
> > check_mul_overflow() and return NULL silently on overflow. This AFAIU will
> > make SIZE_MAX passed to the underlying kmalloc/kvmalloc and thus will cause
> > a warning. That's IMHO a good thing.
> 
> Right -- I prefer seeing the SIZE_MAX yelling from the allocator. Should
> we change how k*malloc_array() behaves?

Huh, yeah, it's weird to not get a stack trace if kmalloc() fails.  On
the other hand, if we're allocating SIZE_MAX, that is probably a user
controlled size.  Since we don't want people spamming dmesg, it should
probably be WARN_ONCE().

regards,
dan carpenter