linux-kernel - Re: For review: user

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Date:	Tue, 09 Sep 2014 06:59:41 -0700
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	mtk.manpages@...il.com, lkml <linux-kernel@...r.kernel.org>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	containers@...ts.linux-foundation.org,
	Andy Lutomirski <luto@...capital.net>,
	richard.weinberger@...il.com, "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: For review: user_namespace(7) man page

On 08/30/2014 02:53 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
[...]


>>        The initial user namespace has no parent namespace, but, for con‐
>>        sistency, the kernel provides dummy user  and  group  ID  mapping
>>        files  for  this namespace.  Looking at the uid_map file (gid_map
>>        is the same) from a shell in the initial namespace shows:
>>
>>            $ cat /proc/$$/uid_map
>>                     0          0 4294967295
>>
>>        This mapping tells us that the range starting at  user  ID  0  in
>>        this namespace maps to a range starting at 0 in the (nonexistent)
>>        parent namespace, and the length of  the  range  is  the  largest
>>        32-bit unsigned integer.
> 
> Which deliberately leaves 4294967295 32bit (-1) unmapped.  (uid_t)-1 is
> used in several interfaces (like setreuid) as a way to specify no uid
> leaving it unmapped and unusuable guarantees that there will be no
> confusion when using those kernel methods.

So, I worked that piece into the text to give:

       This  mapping  tells us that the range starting at user ID 0 in
       this namespace maps to a range starting at 0 in  the  (nonexis‐
       tent)  parent  namespace,  and  the  length of the range is the
       largest 32-bit unsigned  integer.   (This  deliberately  leaves
       4294967295  (the  32-bit  signed  -1  value) unmapped.  This is
       deliberate: (uid_t) -1 is used  in  several  interfaces  (e.g.,
       setreuid(2))  as  a  way  to  specify  "no  user  ID".  Leaving
       setreuid(2)) unmapped and unusuable guarantees that there  will
       be no confusion when using these interfaces.

Okay?

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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