linux-kernel - Re: [PATCH v2] signal: add procfd

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Date:   Thu, 6 Dec 2018 10:54:02 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Daniel Colascione <dancol@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        linux-man <linux-man@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall

> On Dec 4, 2018, at 4:55 AM, Florian Weimer <fweimer@...hat.com> wrote:
>
> * Christian Brauner:
>
>>> On Mon, Dec 03, 2018 at 05:57:51PM +0100, Florian Weimer wrote:
>>> * Christian Brauner:
>>>
>>>> Ok, I finally have access to source code again. Scratch what I said above!
>>>> I looked at the code and tested it. If the process has exited but not
>>>> yet waited upon aka is a zombie procfd_send_signal() will return 0. This
>>>> is identical to kill(2) behavior. It should've been sort-of obvious
>>>> since when a process is in zombie state /proc/<pid> will still be around
>>>> which means that struct pid must still be around.
>>>
>>> Should we make this state more accessible, by providing a different
>>> error code?
>>
>> No, I don't think we want that. Imho, It's not really helpful. Signals
>> are still delivered to zombies. If zombie state were to always mean that
>> no-one is going to wait on this thread anymore then it would make sense
>> to me. But given that zombie can also mean that someone put a
>> sleep(1000) right before their wait() call in the parent it seems odd to
>> report back that it is a zombie.
>
> It allows for error checking that the recipient of a signal is still
> running.  It's obviously not reliable, but I think it could be helpful
> in the context of closely cooperating processes.
>
>>> Will the system call ever return ESRCH, given that you have a handle for
>>> the process?
>>
>> Yes, whenever you signal a process that has already been waited upon:
>> - get procfd handle referring to <proc>
>> - <proc> exits and is waited upon
>> - procfd_send_signal(procfd, ...) returns -1 with errno == ESRCH
>
> I see, thanks.
>
>>> Do you want to land all this in one kernel release?  I wonder how
>>> applications are supposed to discover kernel support if functionality is
>>> split across several kernel releases.  If you get EINVAL or EBADF, it
>>> may not be obvious what is going on.
>>
>> Sigh, I get that but I really don't want to have to land this in one big
>> chunk. I want this syscall to go in in a as soon as we can to fulfill
>> the most basic need: having a way that guarantees us that we signal the
>> process that we intended to signal.
>>
>> The thread case is easy to implement on top of it. But I suspect we will
>> quibble about the exact semantics for a long time. Even now we have been
>> on multiple - justified - detrous. That's all pefectly fine and
>> expected. But if we have the basic functionality in we have time to do
>> all of that. We might even land it in the same kernel release still. I
>> really don't want to come of as tea-party-kernel-conservative here but I
>> have time-and-time again seen that making something fancy and cover ever
>> interesting feature in one patchset takes a very very long time.
>>
>> If you care about userspace being able to detect that case I can return
>> EOPNOTSUPP when a tid descriptor is passed.
>
> I suppose that's fine.  Or alternatively, when thread group support is
> added, introduce a flag that applications have to use to enable it, so
> that they can probe for support by checking support for the flag.
>
> I wouldn't be opposed to a new system call like this either:
>
>  int procfd_open (pid_t thread_group, pid_t thread_id, unsigned flags);
>
> But I think this is frowned upon on the kernel side.

I have no problem with it, except that I think it shouldn’t return an
fd that can be used for proc filesystem access.