Re: [PATCH v3 3/3] fs: store real path instead of fake path in backing file f_path

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

On Tue, Oct 10, 2023 at 08:14:15PM +0200, Miklos Szeredi wrote:
> On Tue, 10 Oct 2023 at 19:41, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Tue, Oct 10, 2023 at 05:55:04PM +0100, Al Viro wrote:
> > > On Tue, Oct 10, 2023 at 03:34:45PM +0200, Miklos Szeredi wrote:
> > > > On Tue, 10 Oct 2023 at 15:17, Amir Goldstein <amir73il@gmail.com> wrote:
> > > >
> > > > > Sorry, you asked about ovl mount.
> > > > > To me it makes sense that if users observe ovl paths in writable mapped
> > > > > memory, that ovl should not be remounted RO.
> > > > > Anyway, I don't see a good reason to allow remount RO for ovl in that case.
> > > > > Is there?
> > > >
> > > > Agreed.
> > > >
> > > > But is preventing remount RO important enough to warrant special
> > > > casing of backing file in generic code?  I'm not convinced either
> > > > way...
> > >
> > > You definitely want to guarantee that remounting filesystem r/o
> > > prevents the changes of visible contents; it's not just POSIX,
> > > it's a fairly basic common assumption about any local filesystems.
> >
> > Incidentally, could we simply keep a reference to original struct file
> > instead of messing with path?
> >
> > The only caller of backing_file_open() gets &file->f_path as user_path; how
> > about passing file instead, and having backing_file_open() do get_file()
> > on it and stash the sucker into your object?
> >
> > And have put_file_access() do
> >         if (unlikely(file->f_mode & FMODE_BACKING))
> >                 fput(backing_file(file)->file);
> > in the end.
> 
> That's much nicer, I like it.

Won't work, unfortunately ;-/  We have the damn thing created on open();
it really can't pin the original file, or we'll never get to closing it.

I don't think this approach could be salvaged - we could make that
reference non-counting outside of mmap(), but we have no good way to
catch the moment when it should be dropped; not without intercepting
vm_ops->close() (and thus vm_ops->open() as well)...

Oh, well..