[PATCH] tty: Correct tty buffer flush.

Ilya Zykov
2013-01-16T08:55:00Z

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(buf->head->read = 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 | 105 +++++++++++++---------------------------------
1 files changed, 29 insertions(+), 76 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 45d9161..8a3333d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -119,11 +119,14 @@ static void __tty_buffer_flush(struct tty_port *port)
struct tty_bufhead *buf = &port->buf;
struct tty_buffer *thead;

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

/**
@@ -193,20 +196,26 @@ static struct tty_buffer *tty_buffer_find(struct tty_port *port, 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 port->buf.lock
+ *
+ * Locking: Takes tty->port->buf.lock
*/
-static int __tty_buffer_request_room(struct tty_port *port, size_t size)
+int tty_buffer_request_room(struct tty_struct *tty, size_t size)
{
- struct tty_bufhead *buf = &port->buf;
+ struct tty_bufhead *buf = &tty->port->buf;
struct tty_buffer *b, *n;
int left;
+ unsigned long flags;
+
+ spin_lock_irqsave(&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 */
@@ -218,7 +227,7 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size)

if (left < size) {
/* This is the slow path - looking for new buffers to use */
- if ((n = tty_buffer_find(port, size)) != NULL) {
+ if ((n = tty_buffer_find(tty->port, size)) != NULL) {
if (b != NULL) {
b->next = n;
b->commit = b->used;
@@ -229,31 +238,9 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size)
size = left;
}

+ spin_unlock_irqrestore(&buf->lock, flags);
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 port->buf.lock
- */
-int tty_buffer_request_room(struct tty_struct *tty, size_t size)
-{
- struct tty_port *port = tty->port;
- unsigned long flags;
- int length;
-
- spin_lock_irqsave(&port->buf.lock, flags);
- length = __tty_buffer_request_room(port, size);
- spin_unlock_irqrestore(&port->buf.lock, flags);
- return length;
-}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);

/**
@@ -272,26 +259,17 @@ EXPORT_SYMBOL_GPL(tty_buffer_request_room);
int tty_insert_flip_string_fixed_flag(struct tty_struct *tty,
const unsigned char *chars, char flag, size_t size)
{
- struct tty_bufhead *buf = &tty->port->buf;
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(&buf->lock, flags);
- space = __tty_buffer_request_room(tty->port, goal);
- tb = buf->tail;
+ int space = tty_buffer_request_room(tty, goal);
+ struct tty_buffer *tb = tty->port->buf.tail;
/* If there is no space then tb may be NULL */
- if (unlikely(space == 0)) {
- spin_unlock_irqrestore(&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(&buf->lock, flags);
copied += space;
chars += space;
/* There is a small chance that we need to split the data over
@@ -318,26 +296,17 @@ EXPORT_SYMBOL(tty_insert_flip_string_fixed_flag);
int tty_insert_flip_string_flags(struct tty_struct *tty,
const unsigned char *chars, const char *flags, size_t size)
{
- struct tty_bufhead *buf = &tty->port->buf;
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(&buf->lock, __flags);
- space = __tty_buffer_request_room(tty->port, goal);
- tb = buf->tail;
+ int space = tty_buffer_request_room(tty, goal);
+ struct tty_buffer *tb = tty->port->buf.tail;
/* If there is no space then tb may be NULL */
- if (unlikely(space == 0)) {
- spin_unlock_irqrestore(&buf->lock, __flags);
+ if (unlikely(space == 0))
break;
- }
memcpy(tb->char_buf_ptr + tb->used, chars, space);
memcpy(tb->flag_buf_ptr + tb->used, flags, space);
tb->used += space;
- spin_unlock_irqrestore(&buf->lock, __flags);
copied += space;
chars += space;
flags += space;
@@ -393,21 +362,13 @@ EXPORT_SYMBOL(tty_schedule_flip);
int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars,
size_t size)
{
- struct tty_bufhead *buf = &tty->port->buf;
- int space;
- unsigned long flags;
- struct tty_buffer *tb;
-
- spin_lock_irqsave(&buf->lock, flags);
- space = __tty_buffer_request_room(tty->port, size);
-
- tb = buf->tail;
+ int space = tty_buffer_request_room(tty, size);
if (likely(space)) {
+ struct tty_buffer *tb = tty->port->buf.tail;
*chars = tb->char_buf_ptr + tb->used;
memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
tb->used += space;
}
- spin_unlock_irqrestore(&buf->lock, flags);
return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
@@ -431,21 +392,13 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
int tty_prepare_flip_string_flags(struct tty_struct *tty,
unsigned char **chars, char **flags, size_t size)
{
- struct tty_bufhead *buf = &tty->port->buf;
- int space;
- unsigned long __flags;
- struct tty_buffer *tb;
-
- spin_lock_irqsave(&buf->lock, __flags);
- space = __tty_buffer_request_room(tty->port, size);
-
- tb = buf->tail;
+ int space = tty_buffer_request_room(tty, size);
if (likely(space)) {
+ struct tty_buffer *tb = tty->port->buf.tail;
*chars = tb->char_buf_ptr + tb->used;
*flags = tb->flag_buf_ptr + tb->used;
tb->used += space;
}
- spin_unlock_irqrestore(&buf->lock, __flags);
return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);