Re: [PATCH RFC v5] pidns: introduce syscall translate_pid

From: Michael Kerrisk (man-pages)
Date: Fri Apr 27 2018 - 08:15:12 EST

On 04/05/2018 09:02 AM, Konstantin Khlebnikov wrote:
> On 05.04.2018 01:29, Eric W. Biederman wrote:
>> Nagarathnam Muthusamy <nagarathnam.muthusamy@xxxxxxxxxx> 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.



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
.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;
Linux/UNIX System Programming Training: