New subject: [Bug 53314] ptrace attach timeout freezes processes to stopped state

5 Jul 2022

      https://bugs.winehq.org/show_bug.cgi?id=53314

            Bug ID: 53314
           Summary: ptrace attach timeout freezes processes to stopped
                    state
           Product: Wine
           Version: 7.12
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: wineserver
          Assignee: wine-bugs(a)winehq.org
          Reporter: superemppu+winehq(a)live.fi
      Distribution: ---

Created attachment 72691
  --> https://bugs.winehq.org/attachment.cgi?id=72691
patch remove timeout

When wineserver wants to attach ptrace to a process, ptrace() is first called
with PTRACE_ATTACH and then wineserver waits on waitpid() until the process has
received a SIGSTOP. This is correct behavior and described on ptrace manpage.
https://linux.die.net/man/2/ptrace

However, there is a timeout: If the SIGSTOP hasn't reached the process in 3
seconds, wineserver will give up on the PTRACE_ATTACH and immediately do a
PTRACE_DETACH. According to the manpage, PTRACE_DETACH takes effect only when
the process is already stopped, so that PTRACE_DETACH does nothing. When
eventually the SIGSTOP reaches the process, it will be stopped forever and
wineserver thinks that the ptrace session is already over.

This bug only occurs when receiving a SIGSTOP takes over 3 seconds for a
process. On most setups that never happens, but on slow running fuse
filesystems the kernel may be in uninterruptible disk sleep for over 3 seconds.
In our setup this happens consistently when the process runs on fuse-overlayfs
on top of a slow NFS, and it probably happens on sshfs too. As a result, games
such as Subnautica and Arma Cold War Assault freeze to stopped state.

How can we fix this?

Method 1: Just remove the timeout entirely (patch attached). The only case
where that timeout matters is if the kernel is doing something important, in
which case we should probably wait, or a kernel thread is stuck, in which case
we have bigger problems. The fact that this bug has been in Wine for 20 years
and not reported proves that the timeout is not triggered much.

Disadvantages: If a kernel thread of one wine process is stuck, wineserver will
also be stuck for other wine processes even if their kernel threads are fine.

Method 2: If we want to keep the timeout, we should not do PTRACE_DETACH
immediately, but rather set a flag that PTRACE_ATTACH failed. When the SIGSTOP
eventually comes, a SIGCHLD will be sent to wineserver and we can do the
PTRACE_DETACH in the SIGCHLD handler if the flag was set.

Disadvantages: We need an arbitrary timeout. 3 seconds is not enough for slow
fuse filesystems. If the timeout is triggered, processes may still crash
because they could not use whatever windows API wineserver was emulating with
ptrace.

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[Bug 53314] New: ptrace attach timeout freezes processes to stopped state

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