Re: [PATCH v1 1/2] open: add close_range()

[Christian Brauner <christian@brauner.io>]

On Thu, May 23, 2019 at 04:32:14PM +0200, Jann Horn wrote:
> On Thu, May 23, 2019 at 1:51 PM Christian Brauner <christian@brauner.io> wrote:
> [...]
> > I kept it dumb and was about to reply that your solution introduces more
> > code when it seemed we wanted to keep this very simple for now.
> > But then I saw that find_next_opened_fd() already exists as
> > find_next_fd(). So it's actually not bad compared to what I sent in v1.
> > So - with some small tweaks (need to test it and all now) - how do we
> > feel about?:
> [...]
> > static int __close_next_open_fd(struct files_struct *files, unsigned *curfd, unsigned maxfd)
> > {
> >         struct file *file = NULL;
> >         unsigned fd;
> >         struct fdtable *fdt;
> >
> >         spin_lock(&files->file_lock);
> >         fdt = files_fdtable(files);
> >         fd = find_next_fd(fdt, *curfd);
> 
> find_next_fd() finds free fds, not used ones.
> 
> >         if (fd >= fdt->max_fds || fd > maxfd)
> >                 goto out_unlock;
> >
> >         file = fdt->fd[fd];
> >         rcu_assign_pointer(fdt->fd[fd], NULL);
> >         __put_unused_fd(files, fd);
> 
> You can't do __put_unused_fd() if the old pointer in fdt->fd[fd] was
> NULL - because that means that the fd has been reserved by another
> thread that is about to put a file pointer in there, and if you
> release the fd here, that messes up the refcounting (or hits the
> BUG_ON() in __fd_install()).
> 
> > out_unlock:
> >         spin_unlock(&files->file_lock);
> >
> >         if (!file)
> >                 return -EBADF;
> >
> >         *curfd = fd;
> >         filp_close(file, files);
> >         return 0;
> > }
> >
> > int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> > {
> >         if (fd > max_fd)
> >                 return -EINVAL;
> >
> >         while (fd <= max_fd) {
> 
> Note that with a pattern like this, you have to be careful about what
> happens if someone gives you max_fd==0xffffffff - then this condition
> is always true and the loop can not terminate this way.
> 
> >                 if (__close_next_fd(files, &fd, maxfd))
> >                         break;
> 
> (obviously it can still terminate this way)

Yup, this was only a quick draft.
I think the dumb simple thing that I did before was the best way to do
it for now.
I first thought that the find_next_open_fd() function already exists but
when I went to write a POC for testing realized it doesn't anyway.