fdget_pos

https://elixir.bootlin.com/linux/v5.7/source/include/linux/file.h#L72

static inline struct fd fdget_pos(int fd)
{
	return __to_fd(__fdget_pos(fd));
}

https://elixir.bootlin.com/linux/v5.7/source/fs/file.c#L818

unsigned long __fdget_pos(unsigned int fd)
{
	unsigned long v = __fdget(fd);
	struct file *file = (struct file *)(v & ~3);

	if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
		if (file_count(file) > 1) {
			v |= FDPUT_POS_UNLOCK;
			mutex_lock(&file->f_pos_lock);
		}
	}
	return v;
}

https://elixir.bootlin.com/linux/v5.7/source/fs/file.c#L807

unsigned long __fdget(unsigned int fd)
{
	return __fget_light(fd, FMODE_PATH);
}

https://elixir.bootlin.com/linux/v5.7/source/fs/file.c#L774

/*
 * Lightweight file lookup - no refcnt increment if fd table isn't shared.
 *
 * You can use this instead of fget if you satisfy all of the following
 * conditions:
 * 1) You must call fput_light before exiting the syscall and returning control
 *    to userspace (i.e. you cannot remember the returned struct file * after
 *    returning to userspace).
 * 2) You must not call filp_close on the returned struct file * in between
 *    calls to fget_light and fput_light.
 * 3) You must not clone the current task in between the calls to fget_light
 *    and fput_light.
 *
 * The fput_needed flag returned by fget_light should be passed to the
 * corresponding fput_light.
 */

static unsigned long __fget_light(unsigned int fd, fmode_t mask)
{
	struct files_struct *files = current->files;
	struct file *file;

	if (atomic_read(&files->count) == 1) {
		file = __fcheck_files(files, fd);
		if (!file || unlikely(file->f_mode & mask))
			return 0;
		return (unsigned long)file;
	} else {
		file = __fget(fd, mask, 1);
		if (!file)
			return 0;
		return FDPUT_FPUT | (unsigned long)file;
	}
}