linux-kernel - Re: [PATCH 4/5] slab: Set freed variables to NULL by default

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Message-ID: <Z-WqEUWOvwzBt5ni@casper.infradead.org>
Date: Thu, 27 Mar 2025 19:42:09 +0000
From: Matthew Wilcox <willy@...radead.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>, linux-mm@...ck.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Marco Elver <elver@...gle.com>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 4/5] slab: Set freed variables to NULL by default

On Fri, Mar 21, 2025 at 01:41:00PM -0700, Kees Cook wrote:
>  /**
> - * kfree - free previously allocated memory
> + * __kfree - free previously allocated memory

No.  After applying this patch, we now have documentation for __kfree --
a function we want precisely 0 people to call directly, and no
documentation for kfree(), which is the function we want everybody to
use.

I don't know what the right solution to kernel documentation is here,
but this is definitely not it.  Something I've done in the past is to
note that kernel-doc ignores preprocessor, so we could do something
like:

#if 0
/**
 * kfree - blah blah
 * @blah: blah
 *
 * blah blah blah
 */
void kfree(const void *object) { }
#endif
void __kfree(const void *object)
{
...
}

We'd get warnings if people asked the kbuild system to show them all
EXPORT_SYMBOL functions which don't have kdoc, but nobody's that
invested in having complete documentation.  I don't know if this is
a good approach to solving the problem.