linux-kernel - Re: [RFC v2] ptrace, pidfd: add pidfd

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Date:   Mon, 27 Apr 2020 19:08:26 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Hagen Paul Pfeifer <hagen@...u.net>
Cc:     linux-kernel@...r.kernel.org, Florian Weimer <fweimer@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Brian Gerst <brgerst@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Sargun Dhillon <sargun@...gun.me>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [RFC v2] ptrace, pidfd: add pidfd_ptrace syscall

On Sun, Apr 26, 2020 at 06:34:30PM +0200, Hagen Paul Pfeifer wrote:
> Working on a safety-critical stress testing tool, using ptrace in an
> rather uncommon way (stop, peeking memory, ...) for a bunch of
> applications in an automated way I realized that once opened processes
> where restarted and PIDs recycled.  Resulting in monitoring and
> manipulating the wrong processes.
> 
> With the advent of pidfd we are now able to stick with one stable handle
> to identifying processes exactly. We now have the ability to get this
> race free. Sending signals now works like a charm, next step is to
> extend the functionality also for ptrace.
> 
> API:
>          long pidfd_ptrace(int pidfd, enum __ptrace_request request,
>                            void *addr, void *data, unsigned flags);

I'm in general not opposed to this if there's a clear need for this and
users that are interested. But I think if people really prefer having
this a new syscall then we should probably try to improve on the old
one. Things that come to mind right away without doing a deep review are
replacing the void *addr pointer with a dedicated struct ptract_args or
union ptrace_args and a size argument. If we're not doing something
like this or something more fundamental we can equally well either just
duplicate all enums in the old ptrace syscall and append a _PIDFD to it
where it makes sense.

Christian