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

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Date:	Wed, 6 Mar 2013 20:46:35 -0800
From:	Christopher Li <sparse@...isli.org>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	Linux-Sparse <linux-sparse@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Suggestion for fixing the variable length array used in the kernel.

Hi,

I am looking at the current sparse warning on the kernel source.
One category of those warning are produce by the variable length array.
We all know that the kernel stack has a limit so we don't want to allocate
too much stack to the variable size array.

Is there a recommended way to fix those warnings? Is it worth while to
fix it at all? I am looking forward to some kind of guideline how to handle
this.


Some of them has estimated size limited, like the one fournd in decode_rs.c

	/* Err+Eras Locator poly and syndrome poly The maximum value
	 * of nroots is 8. So the necessary stack size will be about
	 * 220 bytes max.
	 */
	uint16_t lambda[nroots + 1], syn[nroots];
	uint16_t b[nroots + 1], t[nroots + 1], omega[nroots + 1];
	uint16_t root[nroots], reg[nroots + 1], loc[nroots];

Some of them did not said the size estimation but you kind of know
they are not likely to blow up the stack:
In xen_flush_tlb_others

	struct {
		struct mmuext_op op;
#ifdef CONFIG_SMP
		DECLARE_BITMAP(mask, num_processors);
#else
		DECLARE_BITMAP(mask, NR_CPUS);
#endif
	} *args;

And also some of them are harder to tell from the context if
there will be a size limit:

int snd_pcm_hw_refine(struct snd_pcm_substream *substream,
		      struct snd_pcm_hw_params *params)
{
	unsigned int k;
	struct snd_pcm_hardware *hw;
	struct snd_interval *i = NULL;
	struct snd_mask *m = NULL;
	struct snd_pcm_hw_constraints *constrs = &substream->runtime->hw_constraints;
	unsigned int rstamps[constrs->rules_num]; <---------------------------------


Chris
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