linux-kernel - Re: [PATCH] flex_array: Change behaviour on zero size allocations

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Date:	Tue, 01 Feb 2011 06:55:45 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Steffen Klassert <steffen.klassert@...unet.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH] flex_array: Change behaviour on zero size allocations

On Tue, 2011-02-01 at 12:03 +0100, Steffen Klassert wrote:
> rc = flex_array_prealloc(p->type_val_to_struct_array, 0,
>                          p->p_types.nprim - 1, GFP_KERNEL | __GFP_ZERO);
> if (rc)
>         goto out;
> 
> If p->p_types.nprim is zero, we allocare with total_nr_elements equal
> to zerro and then we try to prealloc with p->p_types.nprim - 1.
> flex_array_prealloc interprets this as an unsigned int and fails,
> because this is bigger than total_nr_elements, which is correct I
> think.
> 
> Thoughts?

The most we ever hold in a flex_array is ~2 million entries.  So we have
plenty of room to use a normal int if you want.

On the other hand, there's only one user of flex_array_prealloc(), and
making the "end" argument inclusive doesn't seem to be what that user
wants.  We might want to either make flex_array_prealloc() take start
and length, or instead make "end" be exclusive of the "end" index.

I thought that flex_array_prealloc would say, effectively: "all put()'s
would work up until 'end'".  But, looking at it now, that's probably not
how people will use it.  

-- Dave

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