Re: [patch -mm] update mq_notify to use a struct pid

[ebiederm@xmission.com (Eric W. Biederman)]

Cedric Le Goater <clg@fr.ibm.com> writes:

> Eric W. Biederman wrote:
>
> [ ... ]
>
>> There is also the case that should not come up with signals where
>> we have a pid from a child namespace, that we should also be able to
>> compute the pid for.
>
> I don't understand how a signal can come from a child pid namespace ?

SIG_CHLD is the only case where I think we will be sending a signal
from the child pid namespace.

Reading pids from the status files in /proc, from a parent namespace,
is another example where we need to deal with the pid of children.

>> In essence I intend to have a list of pid_namespace, pid_t pairs connected
>> to a struct pid that we can look through to find the appropriate pid.
>
> yes, that's the purpose of pid_nr() I guess.
>
> This list would contain in nearly all cases a single pair (current pid
> namespace, pid value). It will contain 2 pairs for a task that has unshared
> its pid namespace : a pair for the current pid namespace, that needs to
> allocated when unshare() is called, and one pair for the ancestor pid
> namespace which is already allocated.
>
> Do you see more ?

I don't see the list getting longer until we get into a nested pid namespaces.

As long as the interface is well defined for the container in a container
case I don't mind having additional restrictions.

I will note that you can get some extremely weird interactions if
you do things like open a file descriptor in the parent pid namespace.
Fork two children each child in a different  pid_namespaces.
fcntl(F_SETOWN) is called in one child, and fcntl(F_GETOWN) is called
in the other child.

So we can't just call BUG_ON, if we have can't find the namespace.
But returning 0 from pid_nr should be fine.

Eric
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