Subject: Re: [PATCH net V3 2/2] ptr_ring: fail on large queue size (>64K)

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Feb 2018 06:52:39 +0200

On Thu, Feb 08, 2018 at 11:59:25AM +0800, Jason Wang wrote:
> We need limit the maximum size of queue, otherwise it may cause
> several side effects e.g slab will warn when the size exceeds
> KMALLOC_MAX_SIZE. Using KMALLOC_MAX_SIZE still looks too so this patch
> tries to limit it to 64K. This value could be revisited if we found a
> real case that needs more.
> 
> Reported-by: [email-masked]
> Fixes: 2e0ab8ca83c12 ("ptr_ring: array based FIFO for pointers")
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  include/linux/ptr_ring.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 2af71a7..5858d48 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -44,6 +44,8 @@ struct ptr_ring {
>  	void **queue;
>  };
>  

Seems like a weird location for a define. Either put defines on
top of the file, or near where they are used. I prefer the
second option.

> +#define PTR_RING_MAX_ALLOC 65536
> +

I guess it's an arbitrary number. Seems like a sufficiently large one,
but pls add a comment so readers don't wonder. And please explain what
it does:

/* Callers can create ptr_ring structures with userspace-supplied
 * parameters. This sets a limit on the size to make that usecase
 * safe. If you ever change this, make sure to audit all callers.
 */

Also I think we should generally use either hex 0x10000 or (1 << 16).

>  /* Note: callers invoking this in a loop must use a compiler barrier,
>   * for example cpu_relax().
>   *
> @@ -466,6 +468,8 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  
>  static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
>  {
> +	if (size > PTR_RING_MAX_ALLOC)
> +		return NULL;
>  	return kvmalloc_array(size, sizeof(void *), gfp | __GFP_ZERO);
>  }
>  
> -- 
> 2.7.4