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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Apr 05, 2013 at 12:56:09PM -0700, Greg Kroah-Hartman wrote:

> Which methods do you mean here?

file->f_op->some_method()

> The vfs core would call start_using(), or would filesystems / drivers
> need to do this?

The former; we have relatively few places that call file_operations
members directly and we'd turn each of those into
	if (likely(start_using(file)) {
		res = file->f_op->foo(....);
		stop_using(file);
	} else {
		res = error_value_appropriate_for_foo;
	}
 
> > 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 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.

> > 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...