From: Arnd Bergmann <a...@arndb.de>
Date: Wed, 8 Feb 2017 22:18:26 +0100
> When CONFIG_KASAN is enabled, the "--param asan-stack=1" causes rather large
> stack frames in some functions. This goes unnoticed normally because
> CONFIG_FRAME_WARN is disabled with CONFIG_KASAN by default as of commit
> 3f181b4d8652 ("lib/Kconfig.debug: disable -Wframe-larger-than warnings with
> KASAN=y").
>
> The kernelci.org build bot however has the warning enabled and that led
> me to investigate it a little further, as every build produces these warnings:
>
> net/wireless/nl80211.c:4389:1: warning: the frame size of 2240 bytes is
> larger than 2048 bytes [-Wframe-larger-than=]
> net/wireless/nl80211.c:1895:1: warning: the frame size of 3776 bytes is
> larger than 2048 bytes [-Wframe-larger-than=]
> net/wireless/nl80211.c:1410:1: warning: the frame size of 2208 bytes is
> larger than 2048 bytes [-Wframe-larger-than=]
> net/bridge/br_netlink.c:1282:1: warning: the frame size of 2544 bytes is
> larger than 2048 bytes [-Wframe-larger-than=]
>
> It turns out that there is a relatively simple workaround for the netlink
> users that currently use a local variable in order to do the type conversion:
> Moving the three functions (for each of the typical sizes) to lib/nlattr.c
> avoids using local variables in the caller, which drastically reduces the
> stack usage for nl80211 and br_netlink.
>
> It would be good if we could enable the frame size check after that again,
> but that should be a separate patch and it requires some more testing
> to see which the largest acceptable frame size should be.
>
> Cc: Andrey Ryabinin <aryabi...@virtuozzo.com>
> Cc: Alexander Potapenko <gli...@google.com>
> Cc: Dmitry Vyukov <dvyu...@google.com>
> Cc: kasan-...@googlegroups.com
> Signed-off-by: Arnd Bergmann <a...@arndb.de>
You should only extern these things when KASAN is enabled.
The reason is that uninlining these routines makes attribute emission
more expensive and for some applications performance of this matters.
