[Containers] [PATCH 7/7] file: Modify struct fown_struct to use a struct pid.

[Oleg Nesterov]

On 08/15, Eric W. Biederman wrote:
>
> File handles can be requested to send sigio and sigurg
> to processes.   By tracking the destination processes
> using struct pid instead of pid_t we make the interface
> safe from all potential pid wrap around problems.

As I can see, this patch adds 2 user visible changes. I am
not arguing, but probably it makes sense to document this.

Before this patch, fcntl(F_GETOWN) returns the same pid that
was given to fcntl(F_SETOWN). Now it returns 0 if there were
no process/group with such a pid when F_SETOWN was called.

The second change is good (I'd say this is bugfix). It is
not possible anymore to send the signal to not yet created
processes via fcntl(F_SETOWN, last_pid + a_little), or hit
a problem with pid re-use.

Oleg.