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

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Message-ID: <20181204060354.bozjmjn24sw3axav@yavin>
Date:   Tue, 4 Dec 2018 17:03:54 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Florian Weimer <fweimer@...hat.com>, ebiederm@...ssion.com,
        linux-kernel@...r.kernel.org, serge@...lyn.com, jannh@...gle.com,
        luto@...nel.org, akpm@...ux-foundation.org, oleg@...hat.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, dancol@...gle.com, timmurray@...gle.com,
        linux-man@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall

On 2018-12-03, Christian Brauner <christian@...uner.io> wrote:
> > > As I pointed out in another mail my I is to make this work by using
> > > file descriptors for /proc/<pid>/task/<tid>.  I don't want this in the
> > > initial patchset though.  I prefer to slowly add those features once
> > > we have gotten the basic functionality in.
> > 
> > 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.

Personally, I'm +1 on -EOPNOTSUPP so we can get an MVP merged, and add
new features in later patches.

> > What happens if you use the new interface with an O_PATH descriptor?
> 
> You get EINVAL. When an O_PATH file descriptor is created the kernel
> will set file->f_op = &empty_fops at which point the check I added 
>         if (!proc_is_tgid_procfd(f.file))
>                 goto err;
> will fail. Imho this is correct behavior since technically signaling a
> struct pid is the equivalent of writing to a file and hence doesn't
> purely operate on the file descriptor level.

Not to mention that O_PATH file descriptors are a whole kettle of fish
when it comes to permission checking semantics.


-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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