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From: Sargun Dhillon <>
To: Tycho Andersen <>
Cc: LKML <>,
	Linux Containers <>,
	Linux API <>
Subject: Re: [RFC PATCH] ptrace: add PTRACE_GETFD request
Date: Fri, 6 Dec 2019 11:03:08 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20191206141045.GA22803@cisco>

On Fri, Dec 6, 2019 at 6:10 AM Tycho Andersen <> wrote:
> On Thu, Dec 05, 2019 at 11:44:53PM +0000, Sargun Dhillon wrote:
> > PTRACE_GETFD is a generic ptrace API that allows the tracer to
> > get file descriptors from the traceee.
> >
> > The primary reason to use this syscall is to allow sandboxers to
> I might change this to "one motivation to use this ptrace command",
> because I'm sure people will invent other crazy uses soon after it's
> added :)
Another use-case that's come up has been transparent proxy for
service meshes. Rather than doing intercept at L4 (iptables), or
DNS, just rewriting the connect is nicer. A side benefit is that
getpeername still works.

> > take action on an FD on behalf of the tracee. For example, this
> > can be combined with seccomp's user notification feature to extract
> > a file descriptor and call privileged syscalls, like binding
> > a socket to a privileged port.
> This can already be accomplished via injecting parasite code like CRIU
> does; adding a ptrace() command like this makes it much nicer to be
> sure, but it is redundant.
> Tycho
How can you do this if the tracee doesn't have privilege? For example,
if the tracee doesn't have CAP_SYS_BIND_SERVICE, how could you
get it to bind to a port that's privileged without taking the file descriptor
and doing it in a process that does have CAP_SYS_BIND_SERVICE?

The other aspect is that doing the parasitic code thing is kind of slow,
in that it requires quite a few operations.

  reply	other threads:[~2019-12-06 19:03 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-05 23:44 Sargun Dhillon
2019-12-06  2:38 ` Jann Horn
2019-12-06  6:16   ` Sargun Dhillon
2019-12-06  6:52     ` Aleksa Sarai
2019-12-06  8:25 ` Christian Brauner
2019-12-06 12:23   ` Oleg Nesterov
2019-12-06 14:10 ` Tycho Andersen
2019-12-06 19:03   ` Sargun Dhillon [this message]
2019-12-06 19:05     ` Jann Horn
2019-12-06 19:05       ` Jann Horn
2019-12-06 20:45 ` Andy Lutomirski

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