linux-kernel - Re: Suggestion for fixing the variable length array used in the kernel.

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Date:	Sun, 10 Mar 2013 14:38:42 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Christopher Li <sparse@...isli.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux-Sparse <linux-sparse@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Suggestion for fixing the variable length array used in the
 kernel.

On Sat, Mar 09, 2013 at 03:00:54PM -0800, Christopher Li wrote:
> On Sat, Mar 9, 2013 at 2:34 PM, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> > The problems is if we go over the 8k stack.  So big arrays are bad.
> > Also if the dynamically sized array is inside a loop then normally
> > GCC frees it after each iteration, but on some arches it didn't free
> > it until after the last iteration.
> 
> So it seems that you agree those variable array usage should be
> better change to use kmalloc or some thing.
> 
> > Btw, I've Smatch has cross function analysis, and I'd like to use
> > it here to figure out if the max size for dynamically sized arrays.
> > I ran into a problem:
> >
> > The code looks like this:
> >         char buf[a];
> > The size expression should be an EXPR_SYMBOL, but smatch gets:
> >         char buf[*a];
> 
> Sparse currently does not deal with the dynamic array size right now.
> It only want to get constant value from the array size.
> 
> The part that evaluate the array size is actually correct. Remember
> the EXPR_SYMBOL
> actually contain the *address* of symbol "a". So the proper
> sizeof(buf) is actually
> the content of "*a". That part is fine.

It's evaluating it correctly, but Smatch normally expects
expressions which haven't been evaluated yet.

I can probably hack my own Sparse tree for what I need.  It's not a
big deal.

regards,
dan carpenter
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