resolved: do not allocate packets with minimum size

dns_packet_new() is sometimes called with mtu == 0, and in that case we should
allocate more than the absolute minimum (which is the dns packet header size),
otherwise we have to resize immediately again after appending the first data to
the packet.

This partially reverts the previous commit.

diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c
index 821b66e266ae5ba8f259de55cc531475d3630dac..d1f0f760a4829b0babca734807e7c5f8f49d0c22 100644 (file)
--- a/src/resolve/resolved-dns-packet.c
+++ b/src/resolve/resolved-dns-packet.c
@@ -28,6 +28,9 @@
 
 #define EDNS0_OPT_DO (1<<15)
 
+#define DNS_PACKET_SIZE_START 512
+assert_cc(DNS_PACKET_SIZE_START > UDP_PACKET_HEADER_SIZE)
+
 typedef struct DnsPacketRewinder {
         DnsPacket *packet;
         size_t saved_rindex;
@@ -47,7 +50,14 @@ int dns_packet_new(DnsPacket **ret, DnsProtocol protocol, size_t mtu) {
 
         assert(ret);
 
-        a = MAX(mtu, DNS_PACKET_HEADER_SIZE);
+        /* When dns_packet_new() is called with mtu == 0, allocate more than the
+         * absolute minimum (which is the dns packet header size), to avoid
+         * resizing immediately again after appending the first data to the packet.
+         */
+        if (mtu < UDP_PACKET_HEADER_SIZE)
+                a = DNS_PACKET_SIZE_START;
+        else
+                a = MAX(mtu, DNS_PACKET_HEADER_SIZE);
 
         /* round up to next page size */
         a = PAGE_ALIGN(ALIGN(sizeof(DnsPacket)) + a) - ALIGN(sizeof(DnsPacket));