Firefox freezes after starting recording profile

[david-tejada]

I am trying to use the profiler. I have added the button, but when I hit Start Recording Firefox completely freezes and I have to quit it. I'm using Firefox 99 on Ubuntu 20.04. I have tried with version 91 in a virtual machine and it works fine. Have also tried with version 93 in a virtual machine with Fedora 35 and even though it lets me start recording, the moment I hit stop it freezes. I'm not sure if this is a profiler issue or a Firefox issue. I have also asked in the Firefox support forums. Feel free to close this issue if it doesn't belong here

[julienw]

Hey David, thanks for filing the bug!
Indeed it's a known issue, that's already filed on the bugzilla side of the profiler in Bug 1759196. Therefore I'll close this one.

TL;DR: it looks like Ubuntu updated something in how they build some packages (maybe a compiler change, maybe compiler option changes) that trigger a bug in our sandbox, when handling syscalls with 32 bits arguments and checking their values. We're calling such a syscall in the profiler.
This doesn't happen in all distributions, for example it doesn't seem to happen on Debian stable.

In the mean time there are 2 ways to make it work, both being imperfect:

• you could allow this syscall unconditionally with a preference:
  Set security.sandbox.content.syscall_whitelist to 228, then restart Firefox.
  Upsides: the sandbox still works for other syscalls, so you don't decrease your security much
  Downsides: with a pref it's easy to set it and forget it, and then keep it even after the bug is fixed.
• you could disable the complete sandbox temporary by launching Firefox with MOZ_DISABLE_CONTENT_SANDBOX=1.
  Downsides: the sandbox being completely disabled you're more vulnerable to security issues in websites resulting in bugs in Firefox.
  Upsides: this is temporary because this is an environment variable, so you're unlikely to keep it when the fix is in Firefox.

We're committed in fixing the bug soon because it's blocking a lot of our users.

[david-tejada]

Thanks for the quick reply! This is very useful. For now I'm going to use the first option and follow that bug so I don't forget to change the preference back.