linux-hardening - Re: [PATCH v3][next] skbuff: Proactively round up to kmalloc bucket size

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Date:   Fri, 21 Oct 2022 09:10:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>,
        Menglong Dong <imagedong@...cent.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3][next] skbuff: Proactively round up to kmalloc bucket
 size

On Thu, Oct 20, 2022 at 10:42:47AM +0200, Paolo Abeni wrote:
> >  	size = SKB_DATA_ALIGN(size);
> >  	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > -	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
> > -	if (unlikely(!data))
> > -		goto nodata;
> 
> I'm sorry for not noticing the above in the previous iteration, but I
> think this revision will produce worse code than the V1, as
> kmalloc_reserve() now pollutes an additional register.
> 
> Why did you prefer adding an additional parameter to kmalloc_reserve()?
> I think computing the alloc_size in the caller is even more readable.
> 
> Additionally, as a matter of personal preference, I would not introduce
> an additional variable for alloc_size, just:
> 
> 	// ...
> 	size = kmalloc_size_roundup(size);
> 	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
> 
> The rationale is smaller diff, and consistent style with the existing
> code where 'size' is already adjusted multiple times icrementally.

Sure, I can do that. I will respin it. :)

-- 
Kees Cook