Re: [RFC v2] ptrace, pidfd: add pidfd_ptrace syscall

[Aleksa Sarai <cyphar@cyphar.com>]

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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 duplicates
> > 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 value
> > to the pid on success. Same for PTRACE_SEIZE.
> 
> 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.
> 
> > 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 slightly worse
> > by adding complexity.
> 
> Basically, if a new syscall than please a proper re-design with real
> benefits.
> 
> 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.

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

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