From 9eb1ff3ac00fd3dce597fb2392309d424d6c2f38 Mon Sep 17 00:00:00 2001
From: Mirko Bonadei <mbonadei@webrtc.org>
Date: Thu, 17 Nov 2022 09:57:02 +0000
Subject: [PATCH] Revert "video_layer_allocation: clean up unused code"

This reverts commit 05b58ad77e79efc5b4750f40b5092f945f0fff4d.

Reason for revert: UB because the shift exponent (-2) is negative
(UB happens at this line https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc;l=279;drc=05b58ad77e79efc5b4750f40b5092f945f0fff4d).

Original change's description:
> video_layer_allocation: clean up unused code
>
> remove unused support for more than four spatial layer descriptions
> of temporal layers
>
> BUG=webrtc:12000
>
> Change-Id: I087bcd020897898636bdf9c838abafa8c73c53f3
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/281320
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Commit-Queue: Philipp Hancke <phancke@microsoft.com>
> Reviewed-by: Per Kjellander <perkj@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#38646}

Bug: webrtc:12000, webrtc:14678
Change-Id: Ib94a0dead98aeb84af9b91c0ca6ad0893e8f2874
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/283840
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Owners-Override: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#38660}
---
 .../rtp_video_layers_allocation_extension.cc  | 24 +++++++++++++------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc b/modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc
index d44cb56205..5172ed4ce7 100644
--- a/modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc
+++ b/modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc
@@ -184,12 +184,16 @@ bool RtpVideoLayersAllocationExtension::Write(
   }
   ++write_at;
 
-  {  // Number of temporal layers per spatial layer (at most kMaxSpatialIds).
-    static_assert(VideoLayersAllocation::kMaxSpatialIds == 4);
+  {  // Number of temporal layers.
     int bit_offset = 8;
     *write_at = 0;
     for (const auto& layer : allocation.active_spatial_layers) {
-      bit_offset -= 2;
+      if (bit_offset == 0) {
+        bit_offset = 6;
+        *++write_at = 0;
+      } else {
+        bit_offset -= 2;
+      }
       *write_at |=
           ((layer.target_bitrate_per_temporal_layer.size() - 1) << bit_offset);
     }
@@ -265,9 +269,8 @@ bool RtpVideoLayersAllocationExtension::Parse(
     return false;
   }
 
-  // Read number of temporal layers per spatial layer (at most kMaxSpatialIds),
-  // create `allocation->active_spatial_layers` while iterating though it.
-  static_assert(VideoLayersAllocation::kMaxSpatialIds == 4);
+  // Read number of temporal layers,
+  // Create `allocation->active_spatial_layers` while iterating though it.
   int bit_offset = 8;
   for (int stream_idx = 0; stream_idx < num_rtp_streams; ++stream_idx) {
     for (int sid = 0; sid < VideoLayersAllocation::kMaxSpatialIds; ++sid) {
@@ -275,7 +278,14 @@ bool RtpVideoLayersAllocationExtension::Parse(
         continue;
       }
 
-      bit_offset -= 2;
+      if (bit_offset == 0) {
+        bit_offset = 6;
+        if (++read_at == end) {
+          return false;
+        }
+      } else {
+        bit_offset -= 2;
+      }
       int num_temporal_layers = 1 + ((*read_at >> bit_offset) & 0b11);
       allocation->active_spatial_layers.emplace_back();
       auto& layer = allocation->active_spatial_layers.back();