read

https://elixir.bootlin.com/linux/v5.7/source/fs/read_write.c#L596

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
{
	return ksys_read(fd, buf, count);
}

https://elixir.bootlin.com/linux/v5.7/source/fs/read_write.c#L577

ssize_t ksys_read(unsigned int fd, char __user *buf, size_t count)
{
	struct fd f = fdget_pos(fd);
	ssize_t ret = -EBADF;

	if (f.file) {
		loff_t pos, *ppos = file_ppos(f.file);
		if (ppos) {
			pos = *ppos;
			ppos = &pos;
		}
		ret = vfs_read(f.file, buf, count, ppos);
		if (ret >= 0 && ppos)
			f.file->f_pos = pos;
		fdput_pos(f);
	}
	return ret;
}

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;
	}
}

https://elixir.bootlin.com/linux/v5.7/source/fs/read_write.c#L447

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
	ssize_t ret;

	if (!(file->f_mode & FMODE_READ))
		return -EBADF;
	if (!(file->f_mode & FMODE_CAN_READ))
		return -EINVAL;
	if (unlikely(!access_ok(buf, count)))
		return -EFAULT;

	ret = rw_verify_area(READ, file, pos, count);
	if (!ret) {
		if (count > MAX_RW_COUNT)
			count =  MAX_RW_COUNT;
		ret = __vfs_read(file, buf, count, pos);
		if (ret > 0) {
			fsnotify_access(file);
			add_rchar(current, ret);
		}
		inc_syscr(current);
	}

	return ret;
}

https://elixir.bootlin.com/linux/v5.7/source/fs/read_write.c#L422

ssize_t __vfs_read(struct file *file, char __user *buf, size_t count,
		   loff_t *pos)
{
	if (file->f_op->read)
		return file->f_op->read(file, buf, count, pos);
	else if (file->f_op->read_iter)
		return new_sync_read(file, buf, count, pos);
	else
		return -EINVAL;
}

Links

• struct file