Re: [PATCH 2/4] port: fix ring writer buffer overflow

["Dumitrescu, Cristian" <cristian.dumitrescu@intel.com>]

> -----Original Message-----
> From: Robert Sanford [mailto:rsanford2@gmail.com]
> Sent: Monday, March 28, 2016 9:52 PM
> To: dev@dpdk.org; Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> Subject: [PATCH 2/4] port: fix ring writer buffer overflow
> 
> Ring writer tx_bulk functions may write past the end of tx_buf[].
> Solution is to double the size of tx_buf[].
> 
> Signed-off-by: Robert Sanford <rsanford@akamai.com>
> ---
>  lib/librte_port/rte_port_ring.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_port/rte_port_ring.c b/lib/librte_port/rte_port_ring.c
> index b847fea..765ecc5 100644
> --- a/lib/librte_port/rte_port_ring.c
> +++ b/lib/librte_port/rte_port_ring.c
> @@ -179,7 +179,7 @@ rte_port_ring_reader_stats_read(void *port,
>  struct rte_port_ring_writer {
>  	struct rte_port_out_stats stats;
> 
> -	struct rte_mbuf *tx_buf[RTE_PORT_IN_BURST_SIZE_MAX];
> +	struct rte_mbuf *tx_buf[2 * RTE_PORT_IN_BURST_SIZE_MAX];
>  	struct rte_ring *ring;
>  	uint32_t tx_burst_sz;
>  	uint32_t tx_buf_count;
> @@ -447,7 +447,7 @@ rte_port_ring_writer_stats_read(void *port,
>  struct rte_port_ring_writer_nodrop {
>  	struct rte_port_out_stats stats;
> 
> -	struct rte_mbuf *tx_buf[RTE_PORT_IN_BURST_SIZE_MAX];
> +	struct rte_mbuf *tx_buf[2 * RTE_PORT_IN_BURST_SIZE_MAX];
>  	struct rte_ring *ring;
>  	uint32_t tx_burst_sz;
>  	uint32_t tx_buf_count;
> --
> 1.7.1

Hi Robert,

How is the buffer overflow taking place?

After looking long and hard, I spotted that buffer overflow can potentially take place when the following conditions are met:
1. The input packet burst does not meet the conditions of (a) being contiguous (first n bits set in pkts_mask, all the other bits cleared) and (b) containing a full burst, i.e. at least tx_burst_sz packets (n >= tx_burst_size). This is the slow(er) code path taken when local variable expr != 0.
2. There are some packets already in the buffer.
3. The number of packets in the incoming burst (i.e. popcount(pkts_mask)) plus the number of packets already in the buffer exceeds the buffer size (RTE_PORT_IN_BURST_SIZE_MAX, i.e. 64).

Is this the buffer overflow scenario that you detected?

Thanks,
Cristian