From 626e9ef495474c95e3143ddae1a498d391c2a008 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
Date: Tue, 27 Jun 2017 14:20:00 -0400
Subject: [PATCH] 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.
---
 src/resolve/resolved-dns-packet.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c
index 7262a50eee..c1ee755d9f 100644
--- 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));