linux-kernel - Re: [PATCH] kernel/cred: Fix bug on cred reference count

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Date:	Sat, 09 Jul 2011 10:13:47 +0100
From:	David Howells <dhowells@...hat.com>
To:	Edwin van Vliet <edwin@...atah.nl>
Cc:	dhowells@...hat.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/cred: Fix bug on cred reference count

Edwin van Vliet <edwin@...atah.nl> wrote:

> The comment above states that the old obj ref and subj ref should BOTH be
> released. However, the line is repeated so the old obj ref is released twice
> yet the subj ref is not. So either the comment is wrong, or this is simpy a
> bug. Releasing the same ref twice seems wrong in any case.

The comment is correct and the double release is correct.

task_struct has *two* refs to the cred being replaced and both need to be
updated:

	current->real_cred	- The objective cred
	current->cred		- The subjective cred

The subjective cred is the one used by a task to access other objects; the
objective cred details how the task is perceived by the outside world and
affects how other tasks act upon it.

Normally a task's two cred pointers point to the same cred, but under some
circumstances (such as in cachefiles), the subjective cred can be overridden.

David
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