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Date:   Tue, 28 Apr 2020 10:45:46 +1000
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, Hagen Paul Pfeifer <hagen@jauu.net>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Brian Gerst <brgerst@gmail.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        David Howells <dhowells@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [RFC v2] ptrace, pidfd: add pidfd_ptrace syscall
Message-ID: <20200428004546.mlpwixgms2ekpfdm@yavin.dot.cyphar.com>
References: <20200426130100.306246-1-hagen@jauu.net>
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On 2020-04-27, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> On Mon, Apr 27, 2020 at 10:08:03PM +0200, Arnd Bergmann wrote:
> > The way I understood Jann was that instead of a new syscall that duplic=
ates
> > everything in ptrace(), there would only need to be a new ptrace request
> > such as PTRACE_ATTACH_PIDFD that behaves like PTRACE_ATTACH
> > but takes a pidfd as the second argument, perhaps setting the return va=
lue
> > to the pid on success. Same for PTRACE_SEIZE.
>=20
> That was my initial suggestion, yes. Any enum that identifies a target
> by a pid will get a new _PIDFD version and the pidfd is passed as pid_t
> argument. That should work and is similar to what I did for waitid()
> P_PIDFD. Realistically, there shouldn't be any system where pid_t is
> smaller than an int that we care about.
>=20
> > In effect this is not much different from your a), just a variation on =
the
> > calling conventions. The main upside is that it avoids adding another
> > ugly interface, the flip side is that it makes the existing one slightl=
y worse
> > by adding complexity.
>=20
> Basically, if a new syscall than please a proper re-design with real
> benefits.
>=20
> In the meantime we could make due with the _PIDFD variant. And then if
> someone wants to do the nitty gritty work of adding a ptrace variant
> purely based on pidfds and with a better api and features that e.g. Jann
> pointed out then by all means, please do so. I'm sure we would all
> welcome this as well.

I agree. It would be a shame to add a new ptrace syscall and not take
the opportunity to fix the multitude of problems with the existing API.
But that's a Pandora's box which we shouldn't open unless we want to
wait a long time to get an API everyone is okay with -- a pretty high
price to just get pidfds support in ptrace.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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