diff --git a/tools/mainline_hook.sh b/tools/mainline_hook.sh
new file mode 100755
index 0000000..58afb49
--- /dev/null
+++ b/tools/mainline_hook.sh
@@ -0,0 +1,84 @@
+#!/bin/bash
+LOCAL_DIR="$( dirname "${BASH_SOURCE}" )"
+
+MAINLINE_FRAMEWORKS_AV_PATHS=(
+    media/extractors/
+    media/codec2/components/
+    media/libstagefright/codecs/amrnb
+    media/libstagefright/codecs/amrwb
+    media/libstagefright/codecs/amrwbenc
+    media/libstagefright/codecs/common
+    media/libstagefright/codecs/mp3dec
+    media/libstagefright/codecs/m4v_h263
+    media/libstagefright/flac/dec
+    media/libstagefright/mpeg2ts
+)
+
+MAINLINE_EXTERNAL_PROJECTS=(
+    external/aac
+    external/flac
+    external/libaac
+    external/libaom
+    external/libavc
+    external/libgav1
+    external/libgsm
+    external/libhevc
+    external/libmpeg2
+    external/libopus
+    external/libvpx
+    external/libxaac
+    external/sonivox
+    external/tremolo
+)
+
+DEV_BRANCH=qt-aml-media-dev
+RED=$(tput setaf 1)
+NORMAL=$(tput sgr0)
+WARNING_FULL="${RED}Please upload this change in ${DEV_BRANCH} unless it is restricted
+from mainline release until next dessert release. Low/moderate security bugs
+are restricted this way.${NORMAL}"
+WARNING_PARTIAL="${RED}It looks like your change has mainline and non-mainline changes;
+Consider separating them into two separate CLs -- one for mainline files,
+one for non-mainline files.${NORMAL}"
+PWD=`pwd`
+
+if git branch -vv | grep -q -P "^\*[^\[]+\[goog/qt-aml-media-dev"; then
+    # Change appears to be in mainline dev branch
+    exit 0
+fi
+
+for path in "${MAINLINE_EXTERNAL_PROJECTS[@]}"; do
+    if [[ $PWD =~ $path ]]; then
+        echo -e "${RED}The source of truth for '$path' is in ${DEV_BRANCH}.${NORMAL}"
+        echo -e ${WARNING_FULL}
+        exit 1
+    fi
+done
+
+if [[ ! $PWD =~ frameworks/av ]]; then
+    exit 0
+fi
+
+mainline_count=0
+total_count=0
+echo
+while read -r file ; do
+    (( total_count++ ))
+    for path in "${MAINLINE_FRAMEWORKS_AV_PATHS[@]}"; do
+        if [[ $file =~ ^$path ]]; then
+            echo -e "${RED}The source of truth for '$file' is in ${DEV_BRANCH}.${NORMAL}"
+            (( mainline_count++ ))
+            break
+        fi
+    done
+done < <(git show --name-only --pretty=format: $1 | grep -- "$2")
+
+if (( mainline_count != 0 )); then
+    if (( mainline_count == total_count )); then
+        echo -e ${WARNING_FULL}
+    else
+        echo -e ${WARNING_PARTIAL}
+    fi
+    exit 1
+fi
+exit 0