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

[Al Viro]

On Tue, Oct 10, 2023 at 08:57:21PM +0300, Amir Goldstein wrote:
> On Tue, Oct 10, 2023 at 8:41 PM Al Viro <viro at 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 at 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.
> >
> > No need to mess with write access in any special way and it's closer
> > to the semantics we have for normal mmap(), after all - it keeps the
> > file we'd passed to it open as long as mapping is there.
> >
> > Comments?
> 
> Seems good to me.
> It also shrinks backing_file by one pointer.
> 
> I think this patch can be an extra one after
> "fs: store real path instead of fake path in backing file f_path"
> 
> Instead of changing storing of real_path to storing orig file in
> one change?
> 
> If there are no objections, I will write it up.

Actually, now that I'd looked at it a bit more...  Look:
we don't need to do *anything* in put_file_access(); just
make file_free()
        if (unlikely(f->f_mode & FMODE_BACKING))
		fput(backing_file(f)->user_file);
instead of conditional path_put().  That + change of open_backing_file()
prototype + get_file() in there pretty much eliminates the work done
in 1/3 - you don't need to mess with {get,put}_file_write_access()
at all.

I'd start with this:

struct file *vm_user_file(struct vm_area_struct *vma)
{
	return vma->vm_file;
}
+ replace file = vma->vm_file; with file = vm_user_file(vma) in
the places affected by your 2/3.  That's the first (obviously
safe) commit.  Then the change of backing_file_open() combined
with making vm_user_file() do this:
	file = vma->vm_file;
	if (file && unlikely(file->f_mode & FMODE_BACKING))
		file = backing_file(file)->user_file;
	return file;

Voila.  Two-commit series, considerably smaller than your
variant...