Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

[Andrew Vagin <avagin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On Tue, Jul 26, 2016 at 10:03:25AM +0200, Michael Kerrisk (man-pages) wrote:
> On 07/26/2016 04:54 AM, Andrew Vagin wrote:
> > On Mon, Jul 25, 2016 at 09:59:43AM -0500, Eric W. Biederman wrote:
> > > "Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> > 
> > [snip]
> > 
> > > [snip]
> > > > > > So, from my point of view, the important piece that was missing from
> > > > > > your commit message was the note to use readlink("/proc/self/fd/%d")
> > > > > > on the returned FDs. I think that detail needs to be part of the
> > > > > > commit message (and also the man page text). I think it even be
> > > > > > helpful to include the above program as part of the commit message:
> > > > > > it helps people more quickly grasp the API.
> > > > > 
> > > > > Please, please make the standard way to compare these things fstat.
> > > > > That is much less magic than a symlink, and a little more future proof.
> > > > > Possibly even kcmp.
> > 
> > I like the idea to use kcmp to compare namespaces. I am going to add this
> > functionality to kcmp and describe all these in the man page.
> 
> Hi Andrey,
> 
> Can you briefly sketch out the proposed API and how it would be used?
> I'd find it useful to see that even before the implementation.

Sure. If a process wants to compare two namespaces, it needs to get file
descriptors for them (open /proc/PID/ns/XXX, use new ioctl-s, find a
process which has them),
and then it calls kcmp(pid1, pid2, KCMP_NSFD, ns_fd1, ns_fd2)

For example, if we want to compare pid namespaces for 1 and 2 processes:

pid = getpid();
ns_fd1 = open("/proc/1/ns/pid")
ns_fd2 = open("/proc/2/ns/pid")

if (!kcmp(pid, pid, KCMP_NSFD, ns_fd1, ns_fd2))
	printf("Both processes live in the same pid namespace\n");

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