On Thu, Jun 15, 2017 at 6:53 AM, Greg Kroah-Hartman
[off-list ref] wrote:

On Thu, Jun 15, 2017 at 06:52:21AM +0200, Greg Kroah-Hartman wrote:

quoted

On Wed, Jun 14, 2017 at 11:15:38PM +0200, Arnd Bergmann wrote:

quoted

As reported by kernelci, some functions in the VT code use significant
amounts of kernel stack when local variables get inlined into the caller
multiple times:

drivers/tty/vt/keyboard.c: In function 'kbd_keycode':
drivers/tty/vt/keyboard.c:1452:1: error: the frame size of 2240 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Annotating those functions as noinline_if_stackbloat prevents the inlining
and reduces the overall stack usage in this driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/tty/vt/keyboard.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index f4166263bb3a..c0d111444a0e 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c

@@ -301,13 +301,13 @@ int kbd_rate(struct kbd_repeat *rpt)
 /*
  * Helper Functions.
  */
-static void put_queue(struct vc_data *vc, int ch)
+static noinline_if_stackbloat void put_queue(struct vc_data *vc, int ch)
 {
    tty_insert_flip_char(&vc->port, ch, 0);
    tty_schedule_flip(&vc->port);
 }

Ugh, really?  We have to start telling gcc not to be stupid here?
That's not going to be easy, and will just entail us doing this all over
the place, right?

The code isn't asking to be inlined, so why is gcc allowing it to be
done that way?  Doesn't that imply gcc is the problem here?

Wait, you are now, in this patch, _asking_ for it to be inlined.  How is
that solving anything?

The three functions that gain the attribute are all those that gcc decided
to inline for itself. Usually gcc makes reasonable inlining decisions, so
I left the existing behavior my marking them as 'inline' without
CONFIG_KASAN and 'noinline' when KASAN is enabled.

Would you rather see this patch instead?

diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h
index c28dd523f96e..25348c5ffcb7 100644
--- a/include/linux/tty_flip.h
+++ b/include/linux/tty_flip.h

@@ -13,8 +13,8 @@ extern int tty_prepare_flip_string(struct tty_port *port,
 extern void tty_flip_buffer_push(struct tty_port *port);
 void tty_schedule_flip(struct tty_port *port);

-static inline int tty_insert_flip_char(struct tty_port *port,
-                                       unsigned char ch, char flag)
+static noinline_if_stackbloat int
+tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag)
 {
        struct tty_buffer *tb = port->buf.tail;
        int change;

This is just as good at eliminating the crazy stack usage in vt/keyboard.o,
but it will also impact all other users of that function.

        Arnd