[PATCH] tty: Correct tty buffer flush.

Ilya Zykov
2012-12-04T13:11:16Z

The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
when another thread can use it. It can be cause of "NULL pointer dereference".
Main idea of the patch, this is never free last (struct tty_buffer) in the active buffer.
Only flush the data for ldisc(tty->buf.head->read = tty->buf.head->commit).
At that moment driver can collect(write) data in buffer without conflict.
It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.

Also revert:
commit c56a00a165712fd73081f40044b1e64407bb1875
tty: hold lock across tty buffer finding and buffer filling
In order to delete the unneeded locks any more.

Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
---
drivers/tty/tty_buffer.c | 96 +++++++++++++---------------------------------
1 files changed, 27 insertions(+), 69 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 91e326f..4f02f9c 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -114,11 +114,14 @@ static void __tty_buffer_flush(struct tty_struct *tty)
{
struct tty_buffer *thead;

- while ((thead = tty->buf.head) != NULL) {
- tty->buf.head = thead->next;
- tty_buffer_free(tty, thead);
+ if (tty->buf.head == NULL)
+ return;
+ while ((thead = tty->buf.head->next) != NULL) {
+ tty_buffer_free(tty, tty->buf.head);
+ tty->buf.head = thead;
}
- tty->buf.tail = NULL;
+ WARN_ON(tty->buf.head != tty->buf.tail);
+ tty->buf.head->read = tty->buf.head->commit;
}

/**
@@ -185,19 +188,25 @@ static struct tty_buffer *tty_buffer_find(struct tty_struct *tty, size_t size)
/* Should possibly check if this fails for the largest buffer we
have queued and recycle that ? */
}
+
/**
- * __tty_buffer_request_room - grow tty buffer if needed
+ * tty_buffer_request_room - grow tty buffer if needed
* @tty: tty structure
* @size: size desired
*
* Make at least size bytes of linear space available for the tty
* buffer. If we fail return the size we managed to find.
- * Locking: Caller must hold tty->buf.lock
+ *
+ * Locking: Takes tty->buf.lock
*/
-static int __tty_buffer_request_room(struct tty_struct *tty, size_t size)
+int tty_buffer_request_room(struct tty_struct *tty, size_t size)
{
struct tty_buffer *b, *n;
int left;
+ unsigned long flags;
+
+ spin_lock_irqsave(&tty->buf.lock, flags);
+
/* OPTIMISATION: We could keep a per tty "zero" sized buffer to
remove this conditional if its worth it. This would be invisible
to the callers */
@@ -219,29 +228,8 @@ static int __tty_buffer_request_room(struct tty_struct *tty, size_t size)
size = left;
}

- return size;
-}
-
-
-/**
- * tty_buffer_request_room - grow tty buffer if needed
- * @tty: tty structure
- * @size: size desired
- *
- * Make at least size bytes of linear space available for the tty
- * buffer. If we fail return the size we managed to find.
- *
- * Locking: Takes tty->buf.lock
- */
-int tty_buffer_request_room(struct tty_struct *tty, size_t size)
-{
- unsigned long flags;
- int length;
-
- spin_lock_irqsave(&tty->buf.lock, flags);
- length = __tty_buffer_request_room(tty, size);
spin_unlock_irqrestore(&tty->buf.lock, flags);
- return length;
+ return size;
}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);

@@ -264,22 +252,14 @@ int tty_insert_flip_string_fixed_flag(struct tty_struct *tty,
int copied = 0;
do {
int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
- int space;
- unsigned long flags;
- struct tty_buffer *tb;
-
- spin_lock_irqsave(&tty->buf.lock, flags);
- space = __tty_buffer_request_room(tty, goal);
- tb = tty->buf.tail;
+ int space = tty_buffer_request_room(tty, goal);
+ struct tty_buffer *tb = tty->buf.tail;
/* If there is no space then tb may be NULL */
- if (unlikely(space == 0)) {
- spin_unlock_irqrestore(&tty->buf.lock, flags);
+ if (unlikely(space == 0))
break;
- }
memcpy(tb->char_buf_ptr + tb->used, chars, space);
memset(tb->flag_buf_ptr + tb->used, flag, space);
tb->used += space;
- spin_unlock_irqrestore(&tty->buf.lock, flags);
copied += space;
chars += space;
/* There is a small chance that we need to split the data over
@@ -309,22 +289,14 @@ int tty_insert_flip_string_flags(struct tty_struct *tty,
int copied = 0;
do {
int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
- int space;
- unsigned long __flags;
- struct tty_buffer *tb;
-