netdev - Re: kmem_cache_attr (was Re: [PATCH 04/36] usercopy: Prepare for usercopy whitelisting)

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Date:   Tue, 16 Jan 2018 12:07:49 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        David Windsor <dave@...lcore.net>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Jan Kara <jack@...e.cz>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Rik van Riel <riel@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: kmem_cache_attr (was Re: [PATCH 04/36] usercopy: Prepare for
 usercopy whitelisting)

On Tue, 16 Jan 2018, Matthew Wilcox wrote:

> > Sure this data is never changed. It can be const.
>
> It's changed at initialisation.  Look:
>
> kmem_cache_create(const char *name, size_t size, size_t align,
>                   slab_flags_t flags, void (*ctor)(void *))
>         s = create_cache(cache_name, size, size,
>                          calculate_alignment(flags, align, size),
>                          flags, ctor, NULL, NULL);
>
> The 'align' that ends up in s->align, is not the user-specified align.
> It's also dependent on runtime information (cache_line_size()), so it
> can't be calculated at compile time.

Then we would need another align field in struct kmem_cache that takes the
changes value?

> 'flags' also gets mangled:
>         flags &= CACHE_CREATE_MASK;

Well ok then that also belongs into kmem_cache and the original value
stays in kmem_cache_attr.

> unsigned int would be my preference.

Great.