Re: [PATCH] nbd: show nbd client pid in sysfs

[Wouter Verhelst <wouter@grep.be>]

On Sun, Dec 10, 2006 at 08:58:19PM +0100, Pavel Machek wrote:
> Hi!

Hi

> > > > This simple patch allows nbd to expose the nbd-client 
> > > > daemon's PID in /sys/block/nbd<x>/pid. This is helpful 
> > > > for tracking connection status of a device and for 
> > > > determining which nbd devices are currently in use.
> > > 
> > > Actually is it needed at all? Perhaps nbd clients should be modified
> > > to put nbdX in their process nam?
> > 
> > I don't think that's the right approach; only the kernel can guarantee
> > that a given process is actually managing a given nbd device (I could
> > have some rogue process running around announcing that it's managing
> > nbd2, and then what?)
> 
> I'd say "do not run rogue processes as root" :-).
> 
> nbd-client should run as root -- I do not think interface was
> carefully audited to run it as a user -- so rogue process should not
> really be a problem.

IOW, you're suggesting I walk the process list from userspace to find a
process for which a) it claims it's running for a given nbd device, and
b) I can verify that it actually has the permissions to do so. That's a
whole lot of code in comparison to

f=open("/sys/block/nbd2/pid", O_RDONLY);
read(f,buf,len);

I think I very much prefer the above two lines, not only for simplicity.

Also, your suggestion relies on users /only/ using the official
nbd-client, and is fragile in cases where that assumption is false
(i.e., it's susceptible to false negatives). The suggested patch does
not have that problem.

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