net: eth: Add a helper to pad a short Ethernet frame

Add a helper to pad a short Ethernet frame to the minimum required
length, which can be used by backends' code.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

include/net/eth.h

@@ -31,6 +31,7 @@
 
 #define ETH_ALEN 6
 #define ETH_HLEN 14
+#define ETH_ZLEN 60     /* Min. octets in frame without FCS */
 
 struct eth_header {
     uint8_t  h_dest[ETH_ALEN];   /* destination eth addr */
@@ -422,4 +423,20 @@ bool
 eth_parse_ipv6_hdr(const struct iovec *pkt, int pkt_frags,
                    size_t ip6hdr_off, eth_ip6_hdr_info *info);
 
+/**
+ * eth_pad_short_frame - pad a short frame to the minimum Ethernet frame length
+ *
+ * If the Ethernet frame size is shorter than 60 bytes, it will be padded to
+ * 60 bytes at the address @padded_pkt.
+ *
+ * @padded_pkt: buffer address to hold the padded frame
+ * @padded_buflen: pointer holding length of @padded_pkt. If the frame is
+ *                 padded, the length will be updated to the padded one.
+ * @pkt: address to hold the original Ethernet frame
+ * @pkt_size: size of the original Ethernet frame
+ * @return true if the frame is padded, otherwise false
+ */
+bool eth_pad_short_frame(uint8_t *padded_pkt, size_t *padded_buflen,
+                         const void *pkt, size_t pkt_size);
+
 #endif

net/eth.c

@@ -548,3 +548,20 @@ bool eth_parse_ipv6_hdr(const struct iovec *pkt, int pkt_frags,
     info->l4proto = ext_hdr.ip6r_nxt;
     return true;
 }
+
+bool eth_pad_short_frame(uint8_t *padded_pkt, size_t *padded_buflen,
+                         const void *pkt, size_t pkt_size)
+{
+    assert(padded_buflen && *padded_buflen >= ETH_ZLEN);
+
+    if (pkt_size >= ETH_ZLEN) {
+        return false;
+    }
+
+    /* pad to minimum Ethernet frame length */
+    memcpy(padded_pkt, pkt, pkt_size);
+    memset(&padded_pkt[pkt_size], 0, ETH_ZLEN - pkt_size);
+    *padded_buflen = ETH_ZLEN;
+
+    return true;
+}