Crash report: https://crash-stats.mozilla.org/report/index/2d8d1c35-417c-4d5c-b171-64df10230428

MOZ_CRASH Reason: Cannot load plugin as library -5977 126

Top 10 frames of crashing thread:

0  xul.dll  MOZ_Crash  mfbt/Assertions.h:261
0  xul.dll  mozilla::gmp::GMPLoader::Load  dom/media/gmp/GMPLoader.cpp:121
1  xul.dll  mozilla::gmp::GMPChild::RecvStartPlugin  dom/media/gmp/GMPChild.cpp:484
2  xul.dll  mozilla::gmp::PGMPChild::OnMessageReceived  ipc/ipdl/PGMPChild.cpp:787
3  xul.dll  mozilla::ipc::MessageChannel::DispatchSyncMessage  ipc/glue/MessageChannel.cpp:1767
3  xul.dll  mozilla::ipc::MessageChannel::DispatchMessage  ipc/glue/MessageChannel.cpp:1723
4  xul.dll  mozilla::ipc::MessageChannel::RunMessage  ipc/glue/MessageChannel.cpp:1525
5  xul.dll  mozilla::ipc::MessageChannel::MessageTask::Run  ipc/glue/MessageChannel.cpp:1623
6  xul.dll  MessageLoop::RunTask  ipc/chromium/src/base/message_loop.cc:492
6  xul.dll  MessageLoop::DeferOrRunPendingTask  ipc/chromium/src/base/message_loop.cc:501

Most of the crashes with 126 appear to have the Symantec DLLs inside of it, preventing the loading of the Widevine DLL. Given the GMP plugins are reasonably well sandboxed and limited API scope, we should just block these DLLs like we do for the utility process:
https://searchfox.org/mozilla-central/rev/3563da061ca2b32f7f77f5f68088dbf9b5332a9f/toolkit/xre/dllservices/mozglue/WindowsDllBlocklistDefs.in#396

We are hitting the same error as in bug 1826393, ERROR_MOD_NOT_FOUND is 126.

Similar to the utility process in bug 1826393, we are seeing
crashes in the plugin process due to certain anti-virus apps such
as Symantec. This causes us to fail to load the Widevine plugin
with ERROR_MOD_NOT_FOUND.

This patch adds support to add blocklist entries for the GMPlugin
process type, and mirrors the entries added for the utility
process. It also adds a test case to verify the blocklist
integration.

Similar to the utility process in bug 1826393, we are seeing
crashes in the plugin process due to certain anti-virus apps such
as Symantec. This causes us to fail to load the Widevine plugin
with ERROR_MOD_NOT_FOUND.

This patch adds support to add blocklist entries for the GMPlugin
process type, and mirrors the entries added for the utility
process. It also adds a test case to verify the blocklist
integration.

Backed out for causing build bustages

Backout link

Push with failures

Failure log

Interesting. It builds locally. I think it is because I messed up the directory name (lowercase vs uppercase). Sigh.

https://hg.mozilla.org/mozilla-central/rev/bfedc3d5625c