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@xxxxxxxxxx> 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.
