Re: [Containers] [PATCH 5/7] pid: Implement pid_nr

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

Dave Hansen <haveblue@us.ibm.com> writes:

> On Tue, 2006-08-15 at 13:00 -0600, Eric W. Biederman wrote:
>> Dave Hansen <haveblue@us.ibm.com> writes:
>> 
>> > On Tue, 2006-08-15 at 12:23 -0600, Eric W. Biederman wrote:
>> >> +static inline pid_t pid_nr(struct pid *pid)
>> >> +{
>> >> +       pid_t nr = 0;
>> >> +       if (pid)
>> >> +               nr = pid->nr;
>> >> +       return nr;
>> >> +} 
>> >
>> > When is it valid to be passing around a NULL 'struct pid *'?
>> 
>> When you don't have one at all.  Look at the fcntl case a few
>> patches later, or even the spawnpid case.
>
> Does the fcntl() one originate from anywhere other than find_pid() in
> f_setown()?  It seems like, perhaps, the error checking is being done at
> the wrong level.

Yes. It is normally NULL as file handles don't usually have a process
to send a signal to.

>> Then of course there is the later chaos when we get to pid spaces
>> where depending on the pid namespace you are in when you call this
>> on a given struct pid sometimes you will get a pid value and sometimes
>> you won't.
>
> OK, I think it is makes sense to me to say 'get_pid(tsk, pidspace)' and
> get back a NULL 'struct pid' if that task isn't visible in that
> namespace.  However, I don't get how it is handy to be able to defer the
> fact that the pid wasn't found until you go do a pid_nr() on that NULL.

If having a pid to signal is optional, which it almost always is, 
being able to handle that case easily and return a 0 to user space is helpful.

So far it is my assumption that everything that looks up a pid and converts
a pid to a number will do it with respect to the current processes pidspace.
A very reasonable assumption and only with siginfo have I found a case where
it looks like I might not be able to implement it that way.

Eric
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