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

Christian Brauner
2019-05-23T11:51:20Z

On Wed, May 22, 2019 at 06:57:37PM +0200, Oleg Nesterov wrote:
> On 05/22, Christian Brauner wrote:
> >
> > +static struct file *pick_file(struct files_struct *files, unsigned fd)
> > {
> > - struct file *file;
> > + struct file *file = NULL;
> > struct fdtable *fdt;
> >
> > spin_lock(&files->file_lock);
> > @@ -632,15 +629,65 @@ int __close_fd(struct files_struct *files, unsigned fd)
> > goto out_unlock;
> > rcu_assign_pointer(fdt->fd[fd], NULL);
> > __put_unused_fd(files, fd);
> > - spin_unlock(&files->file_lock);
> > - return filp_close(file, files);
> >
> > out_unlock:
> > spin_unlock(&files->file_lock);
> > - return -EBADF;
> > + return file;
>
> ...
>
> > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> > +{
> > + unsigned int cur_max;
> > +
> > + if (fd > max_fd)
> > + return -EINVAL;
> > +
> > + rcu_read_lock();
> > + cur_max = files_fdtable(files)->max_fds;
> > + rcu_read_unlock();
> > +
> > + /* cap to last valid index into fdtable */
> > + if (max_fd >= cur_max)
> > + max_fd = cur_max - 1;
> > +
> > + while (fd <= max_fd) {
> > + struct file *file;
> > +
> > + file = pick_file(files, fd++);
>
> Well, how about something like
>
> static unsigned int find_next_opened_fd(struct fdtable *fdt, unsigned start)
> {
> unsigned int maxfd = fdt->max_fds;
> unsigned int maxbit = maxfd / BITS_PER_LONG;
> unsigned int bitbit = start / BITS_PER_LONG;
>
> bitbit = find_next_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
> if (bitbit > maxfd)
> return maxfd;
> if (bitbit > start)
> start = bitbit;
> return find_next_bit(fdt->open_fds, maxfd, start);
> }

>
> unsigned close_next_fd(struct files_struct *files, unsigned start, unsigned maxfd)
> {
> unsigned fd;
> struct file *file;
> struct fdtable *fdt;
>
> spin_lock(&files->file_lock);
> fdt = files_fdtable(files);
> fd = find_next_opened_fd(fdt, start);
> if (fd >= fdt->max_fds || fd > maxfd) {
> fd = -1;
> goto out;
> }
>
> file = fdt->fd[fd];
> rcu_assign_pointer(fdt->fd[fd], NULL);
> __put_unused_fd(files, fd);
> out:
> spin_unlock(&files->file_lock);
>
> if (fd = -1u)
> return fd;
>
> filp_close(file, files);
> return fd + 1;
> }

Thanks, Oleg!

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?:

/**
* __close_next_open_fd() - Close the nearest open fd.
*
* @curfd: lowest file descriptor to consider
* @maxfd: highest file descriptor to consider
*
* This function will close the nearest open fd, i.e. it will either
* close @curfd if it is open or the closest open file descriptor
* c greater than @curfd that
* is smaller or equal to maxfd.
* If the function found a file descriptor to close it will return 0 and
* place the file descriptor it closed in @curfd. If it did not find a
* file descriptor to close it will return -EBADF.
*/
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);
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);

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) {
if (__close_next_fd(files, &fd, maxfd))
break;

cond_resched();
fd++;
}

return 0;
}

SYSCALL_DEFINE3(close_range, unsigned int, fd, unsigned int, max_fd,
unsigned int, flags)
{
if (flags)
return -EINVAL;

return __close_range(current->files, fd, max_fd);
}