linux-kernel - Re: [take 3] Use pid in inotify events.

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Date:	Thu, 20 Nov 2008 14:34:11 -0800
From:	"John McCutchan" <john@...nmccutchan.com>
To:	"Evgeniy Polyakov" <zbr@...emap.net>
Cc:	mtk.manpages@...il.com, "Christoph Hellwig" <hch@....de>,
	"Robert Love" <rlove@...ve.org>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>
Subject: Re: [take 3] Use pid in inotify events.

On Wed, Nov 19, 2008 at 6:53 AM, Evgeniy Polyakov <zbr@...emap.net> wrote:
> Hi Michael.
>
> On Wed, Nov 19, 2008 at 09:34:46AM -0500, Michael Kerrisk (mtk.manpages@...glemail.com) wrote:
>> > So effectively you propose to have second generation of the inotify
>> > which will have additional pid field, which will be unused by all but
>> > the same uid events?
>>
>> I susepect that Christoph wants the same thing as I do: some thinking
>> towards a future-proof design, rather than a quick hack to address the needs
>> of a single application.
>
> So far the only real need is a pid. That will solve the cases I'm
> working on and it may be interesting for other applications. It is
> possible to extend read/write IO with offset and size parameters though.
>
> Do you see any other possible extensions?
>
>> > If you want to return -EPERM, than it will be _always_ returned for non
>> > sysadmin capable user, which effectively makes it unusable.
>> >
>> Again, appropriate flags in inotify_init1() could fix this -- e.g., only
>> fill the field (and give an error if no perms) if a flag is set.
>
> Um, hmm... Permission is _always_ denied for 'alien' IO, as it was
> pointed by Robert, at init time there is no way to know, will there be
> alien IO (i.e. originated by the process with different uid) or not.
> More on this: inotify initialization is just a memory allocation in
> the kernel, nothing more.
>
> We can argue about object insertion into inotify queue though. But
> again, we check already that it has read permissions, and if so, we are
> allowed to receive notificatons about IO against given target, since if
> new code will return for whatever reason -EPERM, people will use old
> code.
>
> So, putting PID/whatever else into event can be flag-driven, but there
> is no way to return EPERM anywhere in the call chain not breaking
> backward compatibility of the whole idea.


I really don't like the idea of overloading the cookie field to store
the pid for only the events that don't already use the cookie field.

Coming into this late, maybe I missed it but can you explain why you
need the pid that caused the event?

-- 
John McCutchan <john@...nmccutchan.com>
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