Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)

[Tetsuo Handa]

tty_open() is doing "filp->f_op = &tty_fops;".
__tty_hangup() is doing "filp->f_op = &hung_up_tty_fops;".
tty_hung_up_p() is doing "filp->f_op == &hung_up_tty_fops",
and most functions are checking tty_hung_up_p().

Therefore, if we can remember whether __tty_hangup() did
"filp->f_op = &hung_up_tty_fops;" without changing filp->f_op,
we can reuse tty_hung_up_p().

Since tty_open() allocates sizeof("struct tty_file_private") bytes
of memory associated with each "struct file", we can record whether
__tty_hangup() did "filp->f_op = &hung_up_tty_fops;" using this memory.

Therefore, diff will be something like this?
(Explanation and question follows this diff.)

 drivers/tty/tty_io.c |   36 +++++++++++++++++-------------------
 include/linux/tty.h  |    1 +
 2 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 093935e97f42..d7fa18f8c526 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -255,6 +255,7 @@ struct tty_file_private {
 	struct tty_struct *tty;
 	struct file *file;
 	struct list_head list;
+	bool hung;
 };
 
 /**
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 36fb945fdad4..bebd236679f0 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -182,7 +182,7 @@ int tty_alloc_file(struct file *file)
 {
 	struct tty_file_private *priv;
 
-	priv = kmalloc(sizeof(*priv), GFP_KERNEL);
+	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;
 
@@ -491,17 +491,6 @@ static const struct file_operations console_fops = {
 	.fasync		= tty_fasync,
 };
 
-static const struct file_operations hung_up_tty_fops = {
-	.llseek		= no_llseek,
-	.read_iter	= hung_up_tty_read,
-	.write_iter	= hung_up_tty_write,
-	.poll		= hung_up_tty_poll,
-	.unlocked_ioctl	= hung_up_tty_ioctl,
-	.compat_ioctl	= hung_up_tty_compat_ioctl,
-	.release	= tty_release,
-	.fasync		= hung_up_tty_fasync,
-};
-
 static DEFINE_SPINLOCK(redirect_lock);
 static struct file *redirect;
 
@@ -619,7 +608,8 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
 			continue;
 		closecount++;
 		__tty_fasync(-1, filp, 0);	/* can't block */
-		filp->f_op = &hung_up_tty_fops;
+		/* Accept race with tty_hung_up_p() test. */
+		data_race(priv->hung = true);
 	}
 	spin_unlock(&tty->files_lock);
 
@@ -743,7 +733,8 @@ void tty_vhangup_session(struct tty_struct *tty)
  */
 int tty_hung_up_p(struct file *filp)
 {
-	return (filp && filp->f_op == &hung_up_tty_fops);
+	/* Accept race with __tty_hangup(). */
+	return filp && data_race(((struct tty_file_private *) filp->private_data)->hung);
 }
 EXPORT_SYMBOL(tty_hung_up_p);
 
@@ -911,6 +902,8 @@ static ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)
 	struct tty_struct *tty = file_tty(file);
 	struct tty_ldisc *ld;
 
+	if (tty_hung_up_p(file))
+		return hung_up_tty_read(iocb, to);
 	if (tty_paranoia_check(tty, inode, "tty_read"))
 		return -EIO;
 	if (!tty || tty_io_error(tty))
@@ -1073,6 +1066,8 @@ static ssize_t file_tty_write(struct file *file, struct kiocb *iocb, struct iov_
 	struct tty_ldisc *ld;
 	ssize_t ret;
 
+	if (tty_hung_up_p(file))
+		return hung_up_tty_write(iocb, from);
 	if (tty_paranoia_check(tty, file_inode(file), "tty_write"))
 		return -EIO;
 	if (!tty || !tty->ops->write ||	tty_io_error(tty))
@@ -2159,11 +2154,6 @@ static int tty_open(struct inode *inode, struct file *filp)
 			return retval;
 
 		schedule();
-		/*
-		 * Need to reset f_op in case a hangup happened.
-		 */
-		if (tty_hung_up_p(filp))
-			filp->f_op = &tty_fops;
 		goto retry_open;
 	}
 	clear_bit(TTY_HUPPED, &tty->flags);
@@ -2197,6 +2187,8 @@ static __poll_t tty_poll(struct file *filp, poll_table *wait)
 	struct tty_ldisc *ld;
 	__poll_t ret = 0;
 
+	if (tty_hung_up_p(filp))
+		return hung_up_tty_poll(filp, wait);
 	if (tty_paranoia_check(tty, file_inode(filp), "tty_poll"))
 		return 0;
 
@@ -2249,6 +2241,8 @@ static int tty_fasync(int fd, struct file *filp, int on)
 	struct tty_struct *tty = file_tty(filp);
 	int retval = -ENOTTY;
 
+	if (tty_hung_up_p(filp))
+		return hung_up_tty_fasync(fd, filp, on);
 	tty_lock(tty);
 	if (!tty_hung_up_p(filp))
 		retval = __tty_fasync(fd, filp, on);
@@ -2658,6 +2652,8 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	int retval;
 	struct tty_ldisc *ld;
 
+	if (tty_hung_up_p(file))
+		return hung_up_tty_ioctl(file, cmd, arg);
 	if (tty_paranoia_check(tty, file_inode(file), "tty_ioctl"))
 		return -EINVAL;
 
@@ -2943,6 +2939,8 @@ static long tty_compat_ioctl(struct file *file, unsigned int cmd,
 		return tty_ioctl(file, cmd, arg);
 	}
 
+	if (tty_hung_up_p(file))
+		return hung_up_tty_compat_ioctl(file, cmd, arg);
 	if (tty_paranoia_check(tty, file_inode(file), "tty_ioctl"))
 		return -EINVAL;
 

Since tty_read() is called via file->f_op->read_iter() from call_read_iter() from
generic_file_splice_read(), no change is needed for tty_fops->splice_read.

Since tty_write() is called via file->f_op->write_iter() from call_write_iter()
 from do_iter_readv_writev() from do_iter_write() from vfs_iter_write() from
iter_file_splice_write(), no change is needed for tty_fops->splice_write.

Since both tty_fops and hung_up_tty_fops point to the same callback for
llseek/release, no change is needed for tty_fops->llseek and tty_fops->release.

Regarding tty_read()/tty_write()/tty_poll()/tty_ioctl()/tty_compat_ioctl(),
respectively embed hung_up_tty_read()/hung_up_tty_write()/hung_up_tty_poll()/
hung_up_tty_ioctl()/hung_up_tty_compat_ioctl() right before tty_paranoia_check().

Regarding tty_fasync(), embed hung_up_tty_fasync() right before tty_lock() in
tty_fasync() rather than tty_paranoia_check() in __tty_fasync(), for
tty_hung_up_p() is checked right after tty_lock() returned.

I don't know what changes are required for tty_open() and tty_show_fdinfo().
I assume that making no change for tty_show_fdinfo() is harmless.
But how does "hung_up_tty_fops does not call tty_open()" affects here?