Re: [PATCH] files: rcu free files_struct

[Al Viro]

On Wed, Dec 09, 2020 at 03:32:38PM -0600, Eric W. Biederman wrote:
> Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
> 
> > On Wed, Dec 09, 2020 at 11:13:38AM -0800, Linus Torvalds wrote:
> >> On Wed, Dec 9, 2020 at 10:05 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >> >
> >> > -                               struct file * file = xchg(&fdt->fd[i], NULL);
> >> > +                               struct file * file = fdt->fd[i];
> >> >                                 if (file) {
> >> > +                                       rcu_assign_pointer(fdt->fd[i], NULL);
> >> 
> >> This makes me nervous. Why did we use to do that xchg() there? That
> >> has atomicity guarantees that now are gone.
> >> 
> >> Now, this whole thing should be called for just the last ref of the fd
> >> table, so presumably that atomicity was never needed in the first
> >> place. But the fact that we did that very expensive xchg() then makes
> >> me go "there's some reason for it".
> >> 
> >> Is this xchg() just bogus historical leftover? It kind of looks that
> >> way. But maybe that change should be done separately?
> >
> > I'm still not convinced that exposing close_files() to parallel
> > 3rd-party accesses is safe in all cases, so this patch still needs
> > more analysis.
> 
> That is fine.  I just wanted to post the latest version so we could
> continue the discussion.  Especially with comments etc.

It's probably safe.  I've spent today digging through the mess in
fs/notify and kernel/bpf, and while I'm disgusted with both, at
that point I believe that close_files() exposure is not going to
create problems with either.  And xchg() in there _is_ useless.

Said that, BPF "file iterator" stuff is potentially very unpleasant -
it allows to pin a struct file found in any process' descriptor table
indefinitely long.  Temporary struct file references grabbed by procfs
code, while unfortunate, are at least short-lived; with this stuff sky's
the limit.

I'm not happy about having that available, especially if it's a user-visible
primitive we can't withdraw at zero notice ;-/

What are the users of that thing and is there any chance to replace it
with something saner?  IOW, what *is* realistically called for each
struct file by the users of that iterator?