Re: [PATCH v2 resend] vfs: new O_NODE open flag

From: Eric W. Biederman
Date: Sat Nov 07 2009 - 06:09:47 EST

Miklos Szeredi <miklos@xxxxxxxxxx> writes:

> On Fri, 06 Nov 2009, ebiederm@xxxxxxxxxxxx (Eric W. Biederman wrote:
>> So far no one who believes this to be a security hole has found it
>> worth their while to look at nd-> in proc_pid_follow_link
>> and write a patch.
> A rather disgusting patch that would be. The fact is, checking
> permissions on follow_link makes little to no sense. Consider
> truncate(2), for example. Will we add another intent for that? I
> really hope not

No. I was just thinking we have the open intent that is there for
combining lookup and open. We can look test for LOOKUP_OPEN and do
exactly what we need.

> I'm more and more convinced, that the current behavior is the right
> one.

I think the 15 or so years we have had the current behavior without
problems is persuasive.

I think it is an interesting puzzle on how to get dup instead of
reopen as there are cases where that could be useful behavior as well.

The usefulness of an O_NONE flag increases significantly if you can
open the reference file later with more permissions. Essentially
making a hardlink into a running program. Hmm. Weird cases do seem
to show up when the last dir entry is removed.

I wonder if we want a rule that you can't open a file with link count
of 0. Reasoning may get truly strange otherwise.

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