Leveraging pidfs for process creation without fork

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]



I was excited to learn about about pidfds the other day, precisely in hopes that it would open the door to such a "sane process creation API". I searched the LKML, found this thread, and now hope to rekindle the discussion; my apologies if there has been more discussion since that I missed and I am making redundant noise.


On Tue, Feb 2, 2021, at 4:23 AM, David Laight wrote:
From: Andy Lutomirski
> Sent: 01 February 2021 18:30
> 2. A sane process creation API.  It would be delightful to be able to
> create a fully-specified process without forking.  This might end up
> being a fairly complicated project, though -- there are a lot of
> inherited process properties to be enumerated.

Since you are going to (eventually) load in a program image
have to do several system calls to create the process isn't
likely to be a problem.
So using separate calls for each property isn't really an issue
and solves the horrid problem of the API structure.

I definitely concur creating an embryonic process and then setting the properties sounds separately like the right approach. I'm no expert, but I gather from afar that between BPF and io_uring, plenty of people are investigating general methods of batched/pipelined communication with the kernel, and so there's little reason to go around making more ad-hoc mammoth syscalls for specific sets of tasks.


So you could create an embryonic process that inherits a lot
of stuff from the current process, the do actions that
sort out the fds, argv, namespace etc.
Finally running the new program.

All that sounds good, but I wonder if it would be possible to have a flag such that inheritance (where practical) would *not* be the default for new processes. I'm convinced that better security will always be an uphill battle until privileges/capabilities/resources are *not* shared by default. Only when more sharing requires monotonically more programmer effort will productivity/laziness align with the principle of least privilege.

With fork/exec, there's no good way to achieve this, I think it's safe to say. But with the embryonic processes method, where one has the ability to e.g. set/unset file descriptors on the embryo under construction, it seems quite natural.

This is one wrinkle of interface evolution --- as new sandboxing mechanisms / namespaces are created, we would either need to create yet-new "no really, default no-share" flags, or arguably be causing API breakage as previously "leaking" privileges are patched up. I am hopeful that either having versioned flags, or thoroughly documenting up-front that the exact behavior is subject to change as "leaks are plugged" is OK, but I recognize that the former might be too much complexity and the latter to weasel-wordy, and therefore the whole idea of "opt-in sharing only" will have to wait.


The security <-> ergonomics aspect is the main point of interest for me, but there a few random ideas:

1. I originally thought an fd to an embryonic process should in fact point to the task_struct rather than pid, since there is no risk of the data becoming useless asynchronously --- an embryonic process is never scheduled and cannot do anything like exiting on it's own. But there is no reason an embryonic process need start with just one thread, so allowing entire embryonic thread groups might actually be virtuous. I don't know for sure, but I figure in that case it is simpler to just stick with the pid indirection.

2. Embryonic processes can be "forked at rest" (i.e. just duplicated), which would allow a regime where they are used as templates for process creation, duplicated ("forked at rest"), and sent around for other tasks to spawn processes themselves. If my idea for "opt-in sharing only" fails per the above, sending around an "as isolated as possible" embryo template could be a decent fallback.

That's all I got. I hope continuing this design process is of interest to others.



[Index of Archives]     [Linux Samsung SoC]     [Linux Rockchip SoC]     [Linux Actions SoC]     [Linux for Synopsys ARC Processors]     [Linux NFS]     [Linux NILFS]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]

  Powered by Linux