Use zero octets for rtp packet padding

RFC3550 Section 4 mention
"Octets designated as padding have the value zero."

Bug: None
Change-Id: Ife4c6226143c79ad7d152bc6099ba1d81f5492dd
Reviewed-on: https://webrtc-review.googlesource.com/c/103983
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25109}

logging/rtc_event_log/rtc_event_log_unittest_helper.cc

@@ -299,7 +299,7 @@ void EventGenerator::RandomizeRtpPacket(
   for (size_t i = 0; i < payload_size; i++) {
     payload[i] = prng_.Rand<uint8_t>();
   }
-  RTC_CHECK(rtp_packet->SetPadding(padding_size, &prng_));
+  RTC_CHECK(rtp_packet->SetPadding(padding_size));
 }
 
 std::unique_ptr<RtcEventRtpPacketIncoming> EventGenerator::NewRtpPacketIncoming(

modules/rtp_rtcp/source/receive_statistics_unittest.cc

@@ -48,10 +48,7 @@ RtpPacketReceived CreateRtpPacket(uint32_t ssrc,
     packet.SetCsrcs(csrcs);
   }
   packet.SetPayloadSize(payload_size);
-  if (padding_size > 0) {
-    Random random(17);
-    packet.SetPadding(padding_size, &random);
-  }
+  packet.SetPadding(padding_size);
   return packet;
 }
 

modules/rtp_rtcp/source/rtp_packet.cc

@@ -19,7 +19,6 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/numerics/safe_conversions.h"
-#include "rtc_base/random.h"
 
 namespace webrtc {
 namespace {
@@ -359,22 +358,20 @@ uint8_t* RtpPacket::SetPayloadSize(size_t size_bytes) {
   return WriteAt(payload_offset_);
 }
 
-bool RtpPacket::SetPadding(uint8_t size_bytes, Random* random) {
-  RTC_DCHECK(random);
-  if (payload_offset_ + payload_size_ + size_bytes > capacity()) {
-    RTC_LOG(LS_WARNING) << "Cannot set padding size " << size_bytes << ", only "
+bool RtpPacket::SetPadding(size_t padding_bytes) {
+  if (payload_offset_ + payload_size_ + padding_bytes > capacity()) {
+    RTC_LOG(LS_WARNING) << "Cannot set padding size " << padding_bytes
+                        << ", only "
                         << (capacity() - payload_offset_ - payload_size_)
                         << " bytes left in buffer.";
     return false;
   }
-  padding_size_ = size_bytes;
+  padding_size_ = rtc::dchecked_cast<uint8_t>(padding_bytes);
   buffer_.SetSize(payload_offset_ + payload_size_ + padding_size_);
   if (padding_size_ > 0) {
     size_t padding_offset = payload_offset_ + payload_size_;
     size_t padding_end = padding_offset + padding_size_;
-    for (size_t offset = padding_offset; offset < padding_end - 1; ++offset) {
-      WriteAt(offset, random->Rand<uint8_t>());
-    }
+    memset(WriteAt(padding_offset), 0, padding_size_ - 1);
     WriteAt(padding_end - 1, padding_size_);
     WriteAt(0, data()[0] | 0x20);  // Set padding bit.
   } else {

modules/rtp_rtcp/source/rtp_packet.h

@@ -16,6 +16,7 @@
 #include "modules/rtp_rtcp/include/rtp_header_extension_map.h"
 #include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "rtc_base/copyonwritebuffer.h"
+#include "rtc_base/deprecation.h"
 
 namespace webrtc {
 class Random;
@@ -111,7 +112,12 @@ class RtpPacket {
   uint8_t* SetPayloadSize(size_t size_bytes);
   // Same as SetPayloadSize but doesn't guarantee to keep current payload.
   uint8_t* AllocatePayload(size_t size_bytes);
-  bool SetPadding(uint8_t size_bytes, Random* random);
+  RTC_DEPRECATED
+  bool SetPadding(uint8_t size_bytes, Random* random) {
+    return SetPadding(size_bytes);
+  }
+
+  bool SetPadding(size_t padding_size);
 
  private:
   struct ExtensionInfo {

modules/rtp_rtcp/source/rtp_packet_unittest.cc

@@ -19,6 +19,7 @@
 namespace webrtc {
 namespace {
 
+using ::testing::Each;
 using ::testing::ElementsAre;
 using ::testing::ElementsAreArray;
 using ::testing::IsEmpty;
@@ -367,11 +368,10 @@ TEST(RtpPacketTest, CreatePurePadding) {
   packet.SetSequenceNumber(kSeqNum);
   packet.SetTimestamp(kTimestamp);
   packet.SetSsrc(kSsrc);
-  Random random(0x123456789);
 
   EXPECT_LT(packet.size(), packet.capacity());
-  EXPECT_FALSE(packet.SetPadding(kPaddingSize + 1, &random));
-  EXPECT_TRUE(packet.SetPadding(kPaddingSize, &random));
+  EXPECT_FALSE(packet.SetPadding(kPaddingSize + 1));
+  EXPECT_TRUE(packet.SetPadding(kPaddingSize));
   EXPECT_EQ(packet.size(), packet.capacity());
 }
 
@@ -383,13 +383,48 @@ TEST(RtpPacketTest, CreateUnalignedPadding) {
   packet.SetTimestamp(kTimestamp);
   packet.SetSsrc(kSsrc);
   packet.SetPayloadSize(kPayloadSize);
-  Random r(0x123456789);
 
   EXPECT_LT(packet.size(), packet.capacity());
-  EXPECT_TRUE(packet.SetPadding(kMaxPaddingSize, &r));
+  EXPECT_TRUE(packet.SetPadding(kMaxPaddingSize));
   EXPECT_EQ(packet.size(), packet.capacity());
 }
 
+TEST(RtpPacketTest, WritesPaddingSizeToLastByte) {
+  const size_t kPaddingSize = 5;
+  RtpPacket packet;
+
+  EXPECT_TRUE(packet.SetPadding(kPaddingSize));
+  EXPECT_EQ(packet.data()[packet.size() - 1], kPaddingSize);
+}
+
+TEST(RtpPacketTest, UsesZerosForPadding) {
+  const size_t kPaddingSize = 5;
+  RtpPacket packet;
+
+  EXPECT_TRUE(packet.SetPadding(kPaddingSize));
+  EXPECT_THAT(rtc::MakeArrayView(packet.data() + 12, kPaddingSize - 1),
+              Each(0));
+}
+
+TEST(RtpPacketTest, CreateOneBytePadding) {
+  size_t kPayloadSize = 123;
+  RtpPacket packet(nullptr, 12 + kPayloadSize + 1);
+  packet.SetPayloadSize(kPayloadSize);
+
+  EXPECT_TRUE(packet.SetPadding(1));
+
+  EXPECT_EQ(packet.size(), 12 + kPayloadSize + 1);
+  EXPECT_EQ(packet.padding_size(), 1u);
+}
+
+TEST(RtpPacketTest, FailsToAddPaddingWithoutCapacity) {
+  size_t kPayloadSize = 123;
+  RtpPacket packet(nullptr, 12 + kPayloadSize);
+  packet.SetPayloadSize(kPayloadSize);
+
+  EXPECT_FALSE(packet.SetPadding(1));
+}
+
 TEST(RtpPacketTest, ParseMinimum) {
   RtpPacketReceived packet;
   EXPECT_TRUE(packet.Parse(kMinimumPacket, sizeof(kMinimumPacket)));

modules/rtp_rtcp/source/rtp_sender.cc

@@ -622,7 +622,7 @@ size_t RTPSender::SendPadData(size_t bytes,
           has_transport_seq_num || force_part_of_allocation_;
       options.included_in_feedback = has_transport_seq_num;
     }
-    padding_packet.SetPadding(padding_bytes_in_packet, &random_);
+    padding_packet.SetPadding(padding_bytes_in_packet);
     if (has_transport_seq_num) {
       AddPacketToTransportFeedback(options.packet_id, padding_packet,
                                    pacing_info);