linux-kernel - Re: [PATCH v2 2/8] mm: Add kvmalloc_ab_c and kvzalloc

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Date:   Wed, 14 Feb 2018 13:29:37 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>, linux-mm@...ck.org,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2 2/8] mm: Add kvmalloc_ab_c and kvzalloc_struct

On Wed, Feb 14, 2018 at 01:24:09PM -0800, Joe Perches wrote:
> On Wed, 2018-02-14 at 13:12 -0800, Matthew Wilcox wrote:
> > On Wed, Feb 14, 2018 at 12:45:52PM -0800, Joe Perches wrote:
> > > Perhaps kv[zm]alloc_buf_and_array is better naming.
> > 
> > I think that's actively misleading.  The programmer isn't allocating a
> > buf, they're allocating a struct.  kvzalloc_hdr_arr was the earlier name,
> > and that made some sense; they're allocating an array with a header.
> > But nobody thinks about it like that; they're allocating a structure
> > with a variably sized array at the end of it.
> > 
> > If C macros had decent introspection, I'd like it to be:
> > 
> > 	sev = kvzalloc_struct(elems, GFP_KERNEL);
> > 
> > and have the macro examine the structure pointed to by 'sev', check
> > the last element was an array, calculate the size of the array element,
> > and call kvzalloc_ab_c.  But we don't live in that world, so I have to
> > get the programmer to tell me the structure and the name of the last
> > element in it.
> 
> Look at your patch 4
> 
> -       dev_dax = kzalloc(sizeof(*dev_dax) + sizeof(*res) * count, GFP_KERNEL);
> +       dev_dax = kvzalloc_struct(dev_dax, res, count, GFP_KERNEL);
> 
> Here what is being allocated is exactly a struct
> and an array.

No, it's a struct *containing* an array.  Look at patches 5 & 8 where I
have to convert the structs to contain the array which was silently
being allocated immediately after them.

> And this doesn't compile either.

Does for me.  What error are you seeing?