Prevent an O_NDELAY writer from blocking when a tty write is blocked by the tty atomic writer mutex

Without this a tty write could block if a previous blocking tty write was
in progress on the same tty and blocked by a line discipline or hardware
event.  Originally found and reported by Dave Johnson.

Signed-off-by: Alan Cox <alan@redhat.com>
Acked-by: Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 28 insertions, 12 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index a96f26a63fa..a37e6330db8 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1726,6 +1726,23 @@ static ssize_t tty_read(struct file * file, char __user * buf, size_t count,
 	return i;
 }
 
+void tty_write_unlock(struct tty_struct *tty)
+{
+	mutex_unlock(&tty->atomic_write_lock);
+	wake_up_interruptible(&tty->write_wait);
+}
+
+int tty_write_lock(struct tty_struct *tty, int ndelay)
+{
+	if (!mutex_trylock(&tty->atomic_write_lock)) {
+		if (ndelay)
+			return -EAGAIN;
+		if (mutex_lock_interruptible(&tty->atomic_write_lock))
+			return -ERESTARTSYS;
+	}
+	return 0;
+}
+
 /*
  * Split writes up in sane blocksizes to avoid
  * denial-of-service type attacks
@@ -1737,13 +1754,12 @@ static inline ssize_t do_tty_write(
 	const char __user *buf,
 	size_t count)
 {
-	ssize_t ret = 0, written = 0;
+	ssize_t ret, written = 0;
 	unsigned int chunk;
 	
-	/* FIXME: O_NDELAY ... */
-	if (mutex_lock_interruptible(&tty->atomic_write_lock)) {
-		return -ERESTARTSYS;
-	}
+	ret = tty_write_lock(tty, file->f_flags & O_NDELAY);
+	if (ret < 0)
+		return ret;
 
 	/*
 	 * We chunk up writes into a temporary buffer. This
@@ -1776,8 +1792,8 @@ static inline ssize_t do_tty_write(
 
 		buf = kmalloc(chunk, GFP_KERNEL);
 		if (!buf) {
-			mutex_unlock(&tty->atomic_write_lock);
-			return -ENOMEM;
+			ret = -ENOMEM;
+			goto out;
 		}
 		kfree(tty->write_buf);
 		tty->write_cnt = chunk;
@@ -1812,7 +1828,8 @@ static inline ssize_t do_tty_write(
 		inode->i_mtime = current_fs_time(inode->i_sb);
 		ret = written;
 	}
-	mutex_unlock(&tty->atomic_write_lock);
+out:
+	tty_write_unlock(tty);
 	return ret;
 }
 
@@ -3163,14 +3180,13 @@ static int tiocsetd(struct tty_struct *tty, int __user *p)
 
 static int send_break(struct tty_struct *tty, unsigned int duration)
 {
-	if (mutex_lock_interruptible(&tty->atomic_write_lock))
+	if (tty_write_lock(tty, 0) < 0)
 		return -EINTR;
 	tty->driver->break_ctl(tty, -1);
-	if (!signal_pending(current)) {
+	if (!signal_pending(current))
 		msleep_interruptible(duration);
-	}
 	tty->driver->break_ctl(tty, 0);
-	mutex_unlock(&tty->atomic_write_lock);
+	tty_write_unlock(tty);
 	if (signal_pending(current))
 		return -EINTR;
 	return 0;