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

On Wed, May 22, 2019 at 06:57:37PM +0200, Oleg Nesterov wrote:

On 05/22, Christian Brauner wrote:

quoted

+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;

...

quoted

+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);
}