Re: Issues when using pasta with bubblewrap

[Lisa Gnedt <lisa+passt-user@gnedt.at>]

Hi Stefano,

Thanks for you fast feedback!

On 2025-07-07 18:19, Stefano Brivio wrote:
> For context, "this" is: always join the user namespace owning a network
> namespace.

Yes, exactly.

> It looks reasonable (and desirable) to me, but I'm not sure how / why
> it breaks the --userns parameter.

In the case, where a PID is supplied, it misuses the userns variable
and sets it to the path of the network namespace and then always calls
the ioctl NS_GET_USERNS to get the owning user namespace.
However, the userns variable might also be manually set via the --userns
option, whereas we expect users to set this parameter to the path of a
user namespace. The ioctl NS_GET_USERNS returns the parent user namespace
when a user namespace is given. Therefore, we join the parent user
namespace with this patch instead of joining the given user namespace.

> We should probably never do this when --netns-only is given (that's
> Podman's case, for example).

I agree, it does not make sense to join a user namespace, when a user
explicitly only wants to join the network namespace.

> It would be good to have a way to "cleanly" exclude this new behaviour,
> but, once we add the NS_GET_USERNS trick, --netns-only doesn't exactly
> get us back to the previous behaviour. What about --userns-from-pid or
> something like that? That name isn't great though.

I agree that this would be one of the possible solutions. It would
enable the use either with PID or with the --netns option.
Maybe --userns-from-netns would be better? Somehow it would be cool to
include the relationship between userns and netns more concretely like
--join-owning-userns-from-netns, but on the other hand it is also a bit
too long.

However, I am not sure, if it is really necessary to have a separate
CLI option. Maybe it would also be a fine new default behavior just for
the case when a PID is specified, but no network or user namespace is
explicitly given.
If anyone really needs the old behavior, it is still possible to specify
the user namespace explicitly and, therefore, deactivate the new
behavior.
It seems that podman uses the --netns option, so it should be fully
unaffected by this proposed change of default behavior.

I am fine with both solutions. I thought a bit more about the current
and changed behavior by iterating trough the possible combinations of
options with my code changes in mind. I hope this is not too much, but
it also uncovered a few already existing strange edge cases.

 CLI options                      -> behavior
----------------------------------------------

PID                               -> new behavior (netns from PID, userns from netns from PID with fallback to userns from PID)
--netns-only PID                  -> new behavior (netns from PID, userns from netns from PID with fallback to userns from PID) ***2
                                     It looks like this is currently already a strange behavior, as it would get the netns and userns from PID.
--userns X PID                    -> existing behavior (netns from PID, userns from option)
--userns X --netns-only PID       -> new behavior (netns from PID, userns from netns from PID with fallback to userns from PID - strange) ***1
                                     It looks like this is currently already a strange behavior, as it would also get the userns from PID.
--netns-only --userns X PID       -> existing behavior (netns from PID, userns from option - strange) ***1
--netns X PID                     -> existing behavior (invalid)
(skipping further combination with --netns and PID)

COMMAND                           -> existing behavior (new netns, new userns)
--netns-only COMMAND              -> existing behavior (new netns, no userns)
--userns X COMMAND                -> existing behavior (new netns, userns from option)
--userns X --netns-only COMMAND   -> existing behavior (new netns, no userns - a bit strange) ***1
--netns-only --userns X COMMAND   -> existing behavior (new netns, userns from option - strange) ***1
--netns X COMMAND                 -> existing behavior (invalid)
(skipping further combination with --netns and COMMAND)

--netns X                         -> existing behavior (netns from option, no userns)
--netns X --netns-only            -> existing behavior (netns from option, no userns)
--netns X --userns Y              -> existing behavior (netns from option, userns from option)
--netns X --userns Y --netns-only -> existing behavior (netns from option, no userns - a bit strange) ***1
--netns X --netns-only --userns Y -> existing behavior (netns from option, userns from option - strange) ***1

Although it is not directly related to the change I am proposing, it 
might make sense to clean up the CLI option behavior a bit. I would
argue to forbid --userns in combination with --netns-only completely
(everything marked with ***1).

Furthermore, --netns-only PID seems to be currently broken (marked
with ***2). I think the netns_only variable (or use_userns how it is called
inside isolate.c) should most likely get higher priority than the userns
variable itself. This should fix the behavior to only use the netns
from PID and no userns.

> Now, 4.9 feels "old" enough, but pasta used to run on a 3.13 kernel a
> while ago, then a few things were (inadvertently) broken. But it
> "almost" does. Couldn't we just add a fallback for the case where
> NS_GET_USERNS fails? You're already handling the error. You could just
> print a warning and continue instead of calling die_perror()...

Yes, it makes sense to implement a fallback when changing the default
behavior. If it will become a separate option, it seems counter-intuitive
to have an automatic fallback.
I just thought it might be best to first discuss the wanted behavior before
starting to implement more complex changes.

Best regards,
Lisa