On 8 Jun 2023, at 21:16, Ilya Maximets wrote:
> The signal_fds pipe and wevent are a mechanism to wake up the process
> after it received a signal and stored the number for the future
> processing. They are not intended for inter-process communication.
> However, in the current code, descriptors are not closed on fork().
>
> The main scenario where we use fork() is a monitor process. Monitor
> doesn't actually use poll loops and doesn't wait on the descriptor.
> But when a child process is killed, it (child) sends a byte to itself,
> then it wakes up due to POLLIN on the pipe and terminates itself after
> processing all the callbacks. The byte stays unread. And the pipe is
> still open in the monitor process. When child dies, the monitor wakes
> up and forks again. New child inherits the same pipe that still
> contains unread data. This data is never read, so the child will
> constantly wake itself up for no reason.
>
> Interestingly enough raise(SIGSEGV) doesn't immediately kill the
> process. The execution continues til the end of a signal handler,
> so we're still able to write a byte to a pipe even in this case.
> Presumably because we don't have SA_NODEFER.
>
> Fix the issue by re-creating the pipe/event on fork. This way
> every new child will have its own notification channel and will
> not wake up any other processes.
>
> There was already an attempt to fix the issue, but it didn't get a
> follow up (see the reported-at tag). This is an alternative solution.
>
> Fixes: ff8decf1a318 ("daemon: Add support for process monitoring and
> restart.")
> Reported-at:
> https://patchwork.ozlabs.org/project/openvswitch/patch/[email protected]/
> Signed-off-by: Ilya Maximets <[email protected]>
The changes look good to me.
Only tested on linux, and looking up the APIs on Windows, that part seems fine.
Acked-by: Eelco Chaudron <[email protected]>
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