Re: [PATCH 15/39] ovl: add helper to return real file

From: Al Viro
Date: Sun Jun 10 2018 - 01:42:45 EST

On Tue, May 29, 2018 at 04:43:15PM +0200, Miklos Szeredi wrote:
> In the common case we can just use the real file cached in
> file->private_data. There are two exceptions:
> 1) File has been copied up since open: in this unlikely corner case just
> use a throwaway real file for the operation. If ever this becomes a
> perfomance problem (very unlikely, since overlayfs has been doing most fine
> without correctly handling this case at all), then we can deal with that by
> updating the cached real file.

See the ovl_mmap() problem. FWIW, I would probably suggest something along
the lines of
->private_data either points to struct file, or is 1 | address of
2-element array of struct file *
odd value => mask bit 0 away, cast to struct file ** and dereference
even value and it's still in the right layer => use that
even value and it is in the wrong layer =>
allocate a two-pointer array
open in the right layer
stick that into array[0] and original - into array[1]
cmpxchg array | 1 into ->private_data
if that succeeds
return array[0]
fput array[0], free array, then use the value returned
by cmpxchg - mask bit 0 away, cast and dereference