Re: [PATCH 1/6] tty: implement write_iter

Jiri Slaby
2021-01-21T09:41:45Z

On 21. 01. 21, 10:00, Greg Kroah-Hartman wrote:
> From: Linus Torvalds <torvalds@linux-foundation.org>
>
> This makes the tty layer use the .write_iter() function instead of the
> traditional .write() functionality.
>
> That allows writev(), but more importantly also makes it possible to
> enable .splice_write() for ttys, reinstating the "splice to tty"
> functionality that was lost in commit 36e2c7421f02 ("fs: don't allow
> splice read/write without explicit ops").
>
> Fixes: 36e2c7421f02 ("fs: don't allow splice read/write without explicit ops")
> Reported-by: Oliver Giles <ohw.giles@gmail.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> drivers/tty/tty_io.c | 48 ++++++++++++++++++++++++--------------------
> 1 file changed, 26 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 56ade99ef99f..338bc4ef5549 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -143,9 +143,8 @@ LIST_HEAD(tty_drivers); /* linked list of tty drivers */
> DEFINE_MUTEX(tty_mutex);
>
> static ssize_t tty_read(struct file *, char __user *, size_t, loff_t *);
> -static ssize_t tty_write(struct file *, const char __user *, size_t, loff_t *);
> -ssize_t redirected_tty_write(struct file *, const char __user *,
> - size_t, loff_t *);
> +static ssize_t tty_write(struct kiocb *, struct iov_iter *);
> +ssize_t redirected_tty_write(struct kiocb *, struct iov_iter *);
> static __poll_t tty_poll(struct file *, poll_table *);
> static int tty_open(struct inode *, struct file *);
> long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> @@ -478,7 +477,8 @@ static void tty_show_fdinfo(struct seq_file *m, struct file *file)
> static const struct file_operations tty_fops = {
> .llseek = no_llseek,
> .read = tty_read,
> - .write = tty_write,
> + .write_iter = tty_write,
> + .splice_write = iter_file_splice_write,
> .poll = tty_poll,
> .unlocked_ioctl = tty_ioctl,
> .compat_ioctl = tty_compat_ioctl,
> @@ -491,7 +491,8 @@ static const struct file_operations tty_fops = {
> static const struct file_operations console_fops = {
> .llseek = no_llseek,
> .read = tty_read,
> - .write = redirected_tty_write,
> + .write_iter = redirected_tty_write,
> + .splice_write = iter_file_splice_write,
> .poll = tty_poll,
> .unlocked_ioctl = tty_ioctl,
> .compat_ioctl = tty_compat_ioctl,
> @@ -607,9 +608,9 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
> /* This breaks for file handles being sent over AF_UNIX sockets ? */
> list_for_each_entry(priv, &tty->tty_files, list) {
> filp = priv->file;
> - if (filp->f_op->write == redirected_tty_write)
> + if (filp->f_op->write_iter == redirected_tty_write)
> cons_filp = filp;
> - if (filp->f_op->write != tty_write)
> + if (filp->f_op->write_iter != tty_write)

This now relies on implicit value of hung_up_tty_fops.write_iter (i.e.
NULL), okay.

> continue;
> closecount++;
> __tty_fasync(-1, filp, 0); /* can't block */
> filp->f_op = &hung_up_tty_fops;

Isn't this racy with VFS layer in vfs_write:
if (file->f_op->write)
ret = file->f_op->write(file, buf, count, pos);
else if (file->f_op->write_iter)
ret = new_sync_write(file, buf, count, pos);

? hung_up_tty_fops do not set iter_write and tty_fops do not set write.
When we switch from one to the other here, right after the 'if', but
before the call, what happens? Likely nothing for the ->write case
immediately as compilers cache the value, but for ->write_iter, I'm not
sure. Anyway, this looks broken to me. (Read on.)

> @@ -956,14 +957,20 @@ static inline ssize_t do_tty_write(
> size_t size = count;
> if (size > chunk)
> size = chunk;
> +
> ret = -EFAULT;
> - if (copy_from_user(tty->write_buf, buf, size))
> + if (copy_from_iter(tty->write_buf, size, from) != size)
> break;
> +
> ret = write(tty, file, tty->write_buf, size);
> if (ret <= 0)
> break;
> +
> + /* FIXME! Have Al check this! */
> + if (ret != size)
> + iov_iter_revert(from, size-ret);
> +
> written += ret;
> - buf += ret;
> count -= ret;
> if (!count)
> break;
> @@ -1023,9 +1030,9 @@ void tty_write_message(struct tty_struct *tty, char *msg)
> * write method will not be invoked in parallel for each device.
> */
>
> -static ssize_t tty_write(struct file *file, const char __user *buf,
> - size_t count, loff_t *ppos)
> +static ssize_t tty_write(struct kiocb *iocb, struct iov_iter *from)
> {
> + struct file *file = iocb->ki_filp;
> struct tty_struct *tty = file_tty(file);
> struct tty_ldisc *ld;
> ssize_t ret;
> @@ -1038,18 +1045,15 @@ static ssize_t tty_write(struct file *file, const char __user *buf,
> if (tty->ops->write_room == NULL)
> tty_err(tty, "missing write_room method\n");
> ld = tty_ldisc_ref_wait(tty);
> - if (!ld)
> - return hung_up_tty_write(file, buf, count, ppos);
> - if (!ld->ops->write)
> + if (!ld || !ld->ops->write)
> ret = -EIO;
> else
> - ret = do_tty_write(ld->ops->write, tty, file, buf, count);
> + ret = do_tty_write(ld->ops->write, tty, file, from);
> tty_ldisc_deref(ld);

Ok, here belongs my earlier note: "if ld == NULL => crash here." That is
if hangup happens during the ldisc wait, the kernel will crash in
tty_ldisc_deref.

Is there a reason not to convert hung_up_tty_fops too and leave the
return hung_up_tty_write here intact? This would also solve the comments
above.

> return ret;
> }
>
> -ssize_t redirected_tty_write(struct file *file, const char __user *buf,
> - size_t count, loff_t *ppos)
> +ssize_t redirected_tty_write(struct kiocb *iocb, struct iov_iter *iter)
> {
> struct file *p = NULL;
>

thanks,
--
js