On Wed, Jun 14, 2017 at 11:28 PM, Samuel Thibault <samuel.thiba...@ens-lyon.org> wrote: > Hello, > > Arnd Bergmann, on mer. 14 juin 2017 23:15:38 +0200, wrote: >> 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. > > >> --- 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); >> } > > I'm surprised that this be able generate so much stack use: the > tty_insert_flip_char inline only uses a pointer and an int. > > And I'm surprised that multiple inlines can accumulate stack usage.
The reason is that CONFIG_KASAN forces each local variable to have a separate location on the stack whenever it gets passed into an external function (tty_insert_flip_string_flags in this case), so the sanitizer is able to report exactly which instance caused the problem. > I however agree that it's a bad idea to inline it in functions where > it's called so many times (and we're talking about the keyboard anyway). > >> -static void puts_queue(struct vc_data *vc, char *cp) >> +static noinline_if_stackbloat void puts_queue(struct vc_data *vc, char *cp) > > I don't see why, it's only called once in the callers. k_fn, however, is > called several times in k_pad, so that could be why, but then it's > rather be the inlining of k_fn which is a bad idea. It's called by applkey, which in turn is called by k_pad(), and this all gets inlined by default. >> -static void fn_send_intr(struct vc_data *vc) >> +static noinline_if_stackbloat void fn_send_intr(struct vc_data *vc) > > This one is only referenced, not called, I don't see how that could pose > problem. I was surprised by this as well, but it seems that gcc these days is smart enough to turn the indirect function calls for k_handler[type] and/or f_handler[value] into inlines again when it has already determined the index to be constant. It's been a while since I looked at the patch, and I'd have to disassemble it again to figure out the details, but I'm pretty sure I needed this to get the stack usage down to normal levels. Arnd