Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Jul 07, 2010 at 03:35:50PM +0200, Miklos Szeredi wrote:
> On Wed, 7 Jul 2010, J. Bruce Fields wrote:
> > > > If you use sys or proc, is it possible to get the uuid from a file
> > > > descriptor or pathname without races?
> > > 
> > > You can do stat/fstat to find out the device number (which is unique,
> > > but not persistent)
> > 
> > Is it really unique over time? (Can't a given st_dev value map to one
> > filesystem now, and another later?) 
> 
> It's unique at a single point in time.  But if you have a reference
> (e.g. open file descriptor) on the mount then that's not a problem.
> 
>    fd = open(path, ...);
>    fstat(fd, &st);
>    search st.st_dev in mountinfo
>    close(fd)
> 
> is effectively the same as an getuuid(path) syscall (lazy unmounted
> filesystems will not be found in mountinfo, but the reference is still
> there so st_dev will not be reused for other filesystems).

OK, cool.

That still leaves the problem that there isn't always an underlying
block device, and/or when there is it doesn't always uniquely specify
the filesystem.

--b.