On Tue, Oct 10, 2023 at 08:14:15PM +0200, Miklos Szeredi wrote: > On Tue, 10 Oct 2023 at 19:41, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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..
