linux-kernel - Re: [PATCH] mm: ksize() should silently accept a NULL pointer

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Date:   Wed, 17 Jun 2020 09:26:43 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     William Kucharski <william.kucharski@...cle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: ksize() should silently accept a NULL pointer

On Wed, Jun 17, 2020 at 04:23:04PM +0000, Christopher Lameter wrote:
> On Tue, 16 Jun 2020, William Kucharski wrote:
> 
> > Other mm routines such as kfree() and kzfree() silently do the right
> > thing if passed a NULL pointer, so ksize() should do the same.
> 
> Ok so the size of an no object pointer is zero? Ignoring the freeing
> of a nonexisting object makes sense. But determining it size?

ksize() is misnamed.  It's not the size of the object, it's the number
of bytes allocated for that object.

A NULL pointer represents a freed object, or one that was never
allocated in the first place.  Clearly that's 0 bytes.  What other
answer would make sense?