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Subject: Re: [PATCH RFC v5] pidns: introduce syscall translate_pid
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <71d8d32b-0f59-d418-0ee4-fcc7782646ae@gmail.com>
Date:   Fri, 27 Apr 2018 14:15:01 +0200
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On 04/05/2018 09:02 AM, Konstantin Khlebnikov wrote:
> On 05.04.2018 01:29, Eric W. Biederman wrote:
>> Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com> writes:
>>
>>> On 04/04/2018 12:11 PM, Konstantin Khlebnikov wrote:
>>>> Each process have different pids, one for each pid namespace it belongs.
>>>> When interaction happens within single pid-ns translation isn't required.
>>>> More complicated scenarios needs special handling.
>>>>
>>>> For example:
>>>> - reading pid-files or logs written inside container with pid namespace
>>>> - attaching with ptrace to tasks from different pid namespace
>>>> - passing pids across pid namespaces in any kind of API
>>>>
>>>> Currently there are several interfaces that could be used here:
>>>>
>>>> Pid namespaces are identified by inode number of /proc/[pid]/ns/pid.
>>
>> Using the inode number in interfaces is not an option.  Especially not
>> withou referencing the device number for the filesystem as well.
> 
> This is supposed to be single-instance fs,
> not part of proc but referenced but its magic "symlinks".
> 
> Device numbers are not mentioned in "man namespaces".

Thanks for the heads-up!

That was a bug in the man-page. ioctl_ns(2) already says the right thing.
Now I patches namespaces(7), as below.

Cheers,

Michael

diff --git a/man7/namespaces.7 b/man7/namespaces.7
index 725ebaff6..3c155de7e 100644
--- a/man7/namespaces.7
+++ b/man7/namespaces.7
@@ -154,11 +154,14 @@ In Linux 3.7 and earlier, these files were visible as hard links.
 Since Linux 3.8,
 .\" commit bf056bfa80596a5d14b26b17276a56a0dcb080e5
 they appear as symbolic links.
-If two processes are in the same namespace, then the inode numbers of their
+If two processes are in the same namespace,
+then the device IDs and inode numbers of their
 .IR /proc/[pid]/ns/xxx
 symbolic links will be the same; an application can check this using the
+.I stat.st_dev
+and
 .I stat.st_ino
-field returned by
+fields returned by
 .BR stat (2).
 The content of this symbolic link is a string containing
 the namespace type and inode number as in the following example:


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