linux-hardening - RE: struct_size() using sizeof() vs offsetof()

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Date:   Mon, 21 Aug 2023 08:38:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alejandro Colomar' <alx@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: RE: struct_size() using sizeof() vs offsetof()

From: Alejandro Colomar <alx@...nel.org>
> Sent: Thursday, August 17, 2023 7:38 PM
> 
> Hi Gustavo,
> 
> On 2023-08-17 18:05, Gustavo A. R. Silva wrote:
> >
> >> -               tp_c = kzalloc(sizeof(*tp_c), GFP_KERNEL);
> >> +               tp_c = kzalloc(struct_size(tp_c, hlist->ht, 1), GFP_KERNEL);
> >
> > I just sent a fix[1].
> >
> > Thanks for reporting this! :)

Perhaps struct_size() should include an assertion that:
	(offsetof(type, field[8]) > sizeof (type))
That will ensure that field is an array member and reasonably
near the end of the structure.

A more complex calculation (using _Alignof(type) and the offset/size
of field) could be used.
But I don't think you can actually detect it is field[] (or even the
last member).

	David

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