Re: [RFC] revoke(2) and generic handling of things like remove_proc_entry()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Apr 05, 2013 at 09:51:37PM +0100, Al Viro wrote:
> On Fri, Apr 05, 2013 at 12:56:09PM -0700, Greg Kroah-Hartman wrote:
> > > 4) nasty semantics issue - mmap() vs. revoke (of any sort, including
> > > remove_proc_entry(), etc.).  Suppose a revokable file had been mmapped;
> > > now it's going away.  What should we do to its VMAs?  Right now sysfs
> > > and procfs get away with that, but only because there's only one thing
> > > that has ->mmap() there - /proc/bus/pci and sysfs equivalents.  I've
> > > no idea how does pci_mmap_page_range() interact with PCI hotplug (and
> > > I'm not at all sure that whatever it does isn't racy wrt device removal),
> > 
> > The page range should just start returning 0xff all over the place, the
> > BIOS should have kept the mapping around, as it can't really assign it
> > anywhere else, so all _should_ be fine here.
> 
> Umm... 0xff or SIGSEGV?

I think, at first glance, 0xff, as the area is still "mapped" to the
device, and that never gets invaldated from what I can tell, despite the
device now being gone.

> > I think that's a reasonable constraint, although tearing down the VMAs
> > might be possible if we just invalidate the file handle "forcefully"
> > (i.e. manually tear them down and then further accesses should through a
> > SIGSEV fail, or am I missing something more basic here?)
> 
> The question is how to do that in a reasonably clean way; we would've done
> as part of ->kick(), I suppose, or right next to it.

I don't really know, sorry.

> > > 6) how do we get from revoke(2) to call of revoke_it() on the right object?
> > > Note that revoke(2) is done by pathname; we might want an ...at() variant,
> > > but all we'll have to play with will be inode, not an opened file.
> > 
> > Can we make revoke(2) require a valid file handle?  Is there a POSIX
> > spec for revoke(2) that we have to follow here, or given that we haven't
> > had one yet, are we free to define whatever we want without people
> > getting that upset?
> 
> BSD one takes a pathname and so do all derived ones...

Ugh, ok, they were there first, fair enough.

Hm, how do they solve this type of race condition?  Last time I looked
(middle of last year) at one of the revoke BSD implementations, I don't
recall anything special to try to prevent this.  Is it that they just
don't care as almost no one uses it, and it's only for tty devices?  Or
did I miss something?

thanks,

greg k-h