linux-kernel - Re: [PATCH RFC 04/12] userns: Convert cifs to use kuid/kgid where appropriate

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Date:	Tue, 20 Nov 2012 09:22:42 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Steve French <smfrench@...il.com>
Cc:	linux-fsdevel@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, "Serge E. Hallyn" <serge@...lyn.com>,
	Steve French <sfrench@...ba.org>
Subject: Re: [PATCH RFC 04/12] userns: Convert cifs to use kuid/kgid where appropriate

Steve French <smfrench@...il.com> writes:

> Do you have a pointer to the background on kuid/kgid

You might try this lwn article
https://lwn.net/Articles/491310/

In a nutshell each user namespace has it's own view of uid_t and gid_t
values.  Those values are mapped into kernel internal kuid_t and kgid_t
values.  Then user space interfaces convert from kuid_t and kgid_t when
read from userspace.

The initial user namespace as a 1-1 identity mapping between kuid_t and
uid_t values.

kuid_t and kgid_t are not assignment compatible with uid_t and gid_t so
that if the conversions are left out a compile error results.

My strategy is to push kuid_t and kgid_t values as deeply into the
kernel data structures as possible so that there is a high propbability
that when we get it wrong a compile error will happen.

Eric
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