Re: [PATCH] pcapng: fix write more packets than IOV_MAX limit

[Stephen Hemminger]

On Mon, 25 Jul 2022 17:28:11 +0200
Mário Kuka <k...@cesnet.cz> wrote:

> The rte_pcapng_write_packets() function fails when we try to write more
> packets than the IOV_MAX limit. The error is caused by the writev()
> system call, which is limited by the IOV_MAX limit. The iovcnt argument
> is valid if it is greater than 0 and less than or equal to IOV_MAX as
> defined in <limits.h>.
> 
> To avoid this problem, we can split the iovec buffer into smaller
> chunks with a maximum size of IOV_MAX and write them sequentially by
> calling the writev() repeatedly.
> 
> Fixes: 8d23ce8f5ee9 ("pcapng: add new library for writing pcapng files")
> Cc: step...@networkplumber.org
> 
> Signed-off-by: Mário Kuka <k...@cesnet.cz>
> ---

Something like this (compile tested only).

diff --git a/lib/pcapng/rte_pcapng.c b/lib/pcapng/rte_pcapng.c
index 06ad712bd1eb..e41cf909e120 100644
--- a/lib/pcapng/rte_pcapng.c
+++ b/lib/pcapng/rte_pcapng.c
@@ -551,33 +551,16 @@ rte_pcapng_copy(uint16_t port_id, uint32_t queue,
        return NULL;
 }
 
-/* Count how many segments are in this array of mbufs */
-static unsigned int
-mbuf_burst_segs(struct rte_mbuf *pkts[], unsigned int n)
-{
-       unsigned int i, iovcnt;
-
-       for (iovcnt = 0, i = 0; i < n; i++) {
-               const struct rte_mbuf *m = pkts[i];
-
-               __rte_mbuf_sanity_check(m, 1);
-
-               iovcnt += m->nb_segs;
-       }
-       return iovcnt;
-}
-
 /* Write pre-formatted packets to file. */
 ssize_t
 rte_pcapng_write_packets(rte_pcapng_t *self,
                         struct rte_mbuf *pkts[], uint16_t nb_pkts)
 {
-       int iovcnt = mbuf_burst_segs(pkts, nb_pkts);
-       struct iovec iov[iovcnt];
-       unsigned int i, cnt;
-       ssize_t ret;
+       struct iovec iov[IOV_MAX];
+       unsigned int i, cnt = 0;
+       ssize_t ret, total = 0;
 
-       for (i = cnt = 0; i < nb_pkts; i++) {
+       for (i = 0; i < nb_pkts; i++) {
                struct rte_mbuf *m = pkts[i];
                struct pcapng_enhance_packet_block *epb;
 
@@ -589,6 +572,20 @@ rte_pcapng_write_packets(rte_pcapng_t *self,
                        return -1;
                }
 
+               /*
+                * Handle case of highly fragmented and large burst size
+                * Note: this assumes that max segments per mbuf < IOV_MAX
+                */
+               if (unlikely(cnt + m->nb_segs >= IOV_MAX)) {
+                       ret = writev(self->outfd, iov, cnt);
+                       if (unlikely(ret < 0)) {
+                               rte_errno = errno;
+                               return -1;
+                       }
+                       total += ret;
+                       cnt = 0;
+               }
+
                /*
                 * The DPDK port is recorded during pcapng_copy.
                 * Map that to PCAPNG interface in file.
@@ -601,10 +598,12 @@ rte_pcapng_write_packets(rte_pcapng_t *self,
                } while ((m = m->next));
        }
 
-       ret = writev(self->outfd, iov, iovcnt);
-       if (unlikely(ret < 0))
+       ret = writev(self->outfd, iov, cnt);
+       if (unlikely(ret < 0)) {
                rte_errno = errno;
-       return ret;
+               return -1;
+       }
+       return total + ret;
 }
 
 /* Create new pcapng writer handle */