Re: [PATCH 2/4] nfs: check for NULL vfsmount in nfs_getattr

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

On Tue, Jul 31, 2018 at 10:51:57PM +0000, Mark Fasheh wrote:
> Hi Al,
> 
> On Tue, Jul 31, 2018 at 11:16:05PM +0100, Al Viro wrote:
> > On Tue, Jul 31, 2018 at 02:10:43PM -0700, Mark Fasheh wrote:
> > > ->getattr from inside the kernel won't always have a vfsmount, check for
> > > this.
> > 
> > Really?  Where would that happen?
> 
> It happens in my first patch. FWIW, I'm not tied to that particular bit of
> code, or even this (latest) approach. I would actually very much appreciate
> your input into how we might fix the problem we have where we are sometimes
> not exporting a correct ino/dev pair to user space.

Which callers would those be?  I don't see anything in your series that
wouldn't have vfsmount at hand...

> I have a good break-down of the problem and possible solutions here:
> 
> https://www.spinics.net/lists/linux-fsdevel/msg128003.html

btrfs pulling odd crap with device numbers is a problem, but this is far from
the most obvious unpleasantness caused by that - e.g. userland code expecting
that unequal st_dev for directory and subdirectory means that we have
a mountpoint there.  Or assuming it can compare that to st_dev of mountpoints
(or, indeed, the values in /proc/self/mountinfo) to find which fs it's on...

/proc/*/maps is unfortunate, but it does contain the pathnames, doesn't it?
What _real_ problems are there - the ones where we don't have a saner solution?
Note that /proc/locks is (a) FPOS interface and (b) unsuitable for your
approach anyway.