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

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Message-ID: <20180215173639.GA10146@bombadil.infradead.org>
Date:   Thu, 15 Feb 2018 09:36:39 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        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 07:40:50PM -0800, Matthew Wilcox wrote:
> > 	a_foo = kvzalloc_struct_buf(struct foo, struct bar, nr_bars);
> > 
> > or, of course.
> > 
> > 	a_foo = kvzalloc_struct_buf(typeof(*a_foo), typeof(a_foo->bar[0]),
> > 				    nr_bars);
> > 
> > or whatever.

This version works, although it's more typing in the callers:

-               attr_group = kvzalloc_struct(attr_group, attrs, i + 1,
+               attr_group = kvzalloc_struct(typeof(*attr_group), attrs, i + 1,
                                                                GFP_KERNEL);
...
-       dev_dax = kvzalloc_struct(dev_dax, res, count, GFP_KERNEL);
+       dev_dax = kvzalloc_struct(struct dev_dax, res, count, GFP_KERNEL);
...
-#define kvzalloc_struct(p, member, n, gfp)                             \
-       (typeof(p))kvzalloc_ab_c(n,                                     \
-               sizeof(*(p)->member) + __must_be_array((p)->member),    \
-               offsetof(typeof(*(p)), member), gfp)
+#define kvzalloc_struct(s, member, n, gfp) ({                          \
+       s *__p;                                                         \
+       (s *)kvzalloc_ab_c(n,                                           \
+               sizeof(*(__p)->member) + __must_be_array((__p)->member),\
+               offsetof(s, member), gfp);                              \
+})

Gives all the same checking as the current version, and doesn't involve
passing an uninitialised pointer to the macro.

It also looks pretty similar:

	p = kvzalloc(sizeof(*p), GFP_KERNEL);
	p = kvzalloc_struct(typeof(*p), array, count, GFP_KERNEL);
	p = kvzalloc_array(sizeof(*p), count);