Re: [PATCH V6 01/10] Use copy_process in vhost layer

[Mike Christie]

On 1/17/22 11:31 AM, Eric W. Biederman wrote:
> Mike Christie <michael.christie@xxxxxxxxxx> writes:
> 
>> On 12/22/21 12:24 PM, Eric W. Biederman wrote:
>>> All I am certain of is that you need to set
>>> "args->exit_signal = -1;".  This prevents having to play games with
>>> do_notify_parent.
>>
>> Hi Eric,
>>
>> I have all your review comments handled except this one. It's looking like it's
>> more difficult than just setting the exit_signal=-1, so I wanted to check that
>> I understood you.
> 
> [snip problems with exit_signal = -1]
> 
>>
>> What do you think?
> 
> I was wrong.  I appear to have confused the thread and the non-thread
> cases.
> 
> Perhaps I meant "args->exit_signal = 0".  That looks like
> do_notify_parent won't send it, and thread_group_leader continues to do
> the right thing.

That doesn't work too. exit_notify will call do_notify_parent but 
our parent, qemu, does not ignore SIGCHILD so we will not drop
down in into this chunk:

        psig = tsk->parent->sighand;
        spin_lock_irqsave(&psig->siglock, flags);
        if (!tsk->ptrace && sig == SIGCHLD &&
            (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
             (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT))) {

do_notify_parent will return false and so autoreap in exit_notify will
be false.



> 
> Baring any additional confusion on my part that cleanly solves the
> problem of how not to send a signal from a child process cleanly.
> 
> My apologies for sending you on a wild goose chase.
> 

It's no problem. I learned a lot about signal handling :)