Re: [PATCHv2 net] sctp: check and update stream->out_curr when allocating stream_out

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Nov 29, 2018 at 02:42:56PM +0800, Xin Long wrote:
> Now when using stream reconfig to add out streams, stream->out
> will get re-allocated, and all old streams' information will
> be copied to the new ones and the old ones will be freed.
> 
> So without stream->out_curr updated, next time when trying to
> send from stream->out_curr stream, a panic would be caused.
> 
> This patch is to check and update stream->out_curr when
> allocating stream_out.
> 
> v1->v2:
>   - define fa_index() to get elem index from stream->out_curr.
> 
> Fixes: 5bbbbe32a431 ("sctp: introduce stream scheduler foundations")
> Reported-by: Ying Xu <yinxu@redhat.com>
> Reported-by: syzbot+e33a3a138267ca119c7d@syzkaller.appspotmail.com
> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> ---
>  net/sctp/stream.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/net/sctp/stream.c b/net/sctp/stream.c
> index 3892e76..30e7809 100644
> --- a/net/sctp/stream.c
> +++ b/net/sctp/stream.c
> @@ -84,6 +84,19 @@ static void fa_zero(struct flex_array *fa, size_t index, size_t count)
>  	}
>  }
>  
> +static size_t fa_index(struct flex_array *fa, void *elem, size_t count)
> +{
> +	size_t index = 0;
> +
> +	while (count--) {
> +		if (elem == flex_array_get(fa, index))
> +			break;
> +		index++;
> +	}
> +
> +	return index;
> +}
> +
>  /* Migrates chunks from stream queues to new stream queues if needed,
>   * but not across associations. Also, removes those chunks to streams
>   * higher than the new max.
> @@ -147,6 +160,13 @@ static int sctp_stream_alloc_out(struct sctp_stream *stream, __u16 outcnt,
>  
>  	if (stream->out) {
>  		fa_copy(out, stream->out, 0, min(outcnt, stream->outcnt));
> +		if (stream->out_curr) {
> +			size_t index = fa_index(stream->out, stream->out_curr,
> +						stream->outcnt);
> +
> +			BUG_ON(index == stream->outcnt);
> +			stream->out_curr = flex_array_get(out, index);
> +		}
>  		fa_free(stream->out);
>  	}
>  
> -- 
> 2.1.0
> 
> 

I'm having a hard time understanding why, as I noted earlier, you don't just
write a function in the flex_array code that can resize the number of elements
in your array.  If you do that, you can avoid both all the copying, and the need
to lookup the in-use pointer again

Neil