Re: [PATCH v3 07/11] tty: improve tty_insert_flip_char() fast path

On Thu, Jun 22, 2017 at 07:13:51PM +0200, Arnd Bergmann wrote:

kernelci.org reports a crazy stack usage for the VT code when CONFIG_KASAN
is enabled:

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=]

The problem is that tty_insert_flip_char() gets inlined many times into
kbd_keycode(), and also into other functions, and each copy requires 128
bytes for stack redzone to check for a possible out-of-bounds access on
the 'ch' and 'flags' arguments that are passed into
tty_insert_flip_string_flags as a variable-length string.

This introduces a new __tty_insert_flip_char() function for the slow
path, which receives the two arguments by value. This completely avoids
the problem and the stack usage goes back down to around 100 bytes.

Without KASAN, this is also slightly better, as we don't have to
spill the arguments to the stack but can simply pass 'ch' and 'flag'
in registers, saving a few bytes in .text for each call site.

This should be backported to linux-4.0 or later, which first introduced
the stack sanitizer in the kernel.

Cc: stable@vger.kernel.org
Fixes: c420f167db8c ("kasan: enable stack instrumentation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
I already submitted this separately to Greg, but he hasn't replied
yet. I assume that it's fine if Andrew picks it up along with the
other patches and drops it again in case Greg applies it to linux-next.

I've been traveling in China this week, give me a chance to catch up
please.

And no, I don't like this patch either, I think kasan needs to be fixed
here, not work around it in odd ways in code that is completly
acceptable to "sane" compilers.  But give me a week to catch up on my
pending stuff first...

thanks,

greg k-h