Re: parallel lookups on NFS

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

On Sat, Apr 30, 2016 at 06:33:36PM -0400, Jeff Layton wrote:
> I'll do the same (re: KASAN).
> 
> Also FWIW, a few months ago I hit some oopses in the same inline
> function (get_freepointer). It turned out to be a double-free due to my
> own misuse of the fsnotify API. I wonder though if this might also be a
> double free somewhere?

It is a double-free somewhere, all right...  What happens there is that
nfs_readdir really relies upon being the only thread to manipulate the
page cache of that directory.  We get nfs_revalidate_mapping() called
and if it ends up evicting a page currently in use by nfs_do_filldir(),
you get nfs_readdir_clear_array() called _twice_ - once on kicking it
out of page cache (and those kfree of the names are obviously Not Good(tm)
for nfs_do_filldir() copying those names to userland) and then when
nfs_do_filldir() gets to cache_page_release().

Sigh...